Terrestrial Hydrometeorology E-Book

W. James Shuttleworth

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 696

Veröffentlichungsjahr: 2012

Beschreibung

Both hydrologists and meteorologists need to speak a common scientific language, and this has given rise to the new scientific discipline of hydrometeorology, which deals with the transfer of water and energy across the land/atmosphere interface.

Terrestrial Hydrometeorology is the first graduate-level text with sufficient breadth and depth to be used in hydrology departments to teach relevant aspects of meteorology, and in meteorological departments to teach relevant aspects of hydrology, and to serve as an introductory text to teach the emerging discipline of hydrometeorology.

The book will be essential reading for graduate students studying surface water hydrology, meteorology, and hydrometeorology. It can also be used in advanced undergraduate courses, and will be welcomed by academic and professional hydrologists and meteorologists worldwide.

Additional resources for this book can be found at: http://www.wiley.com/go/shuttleworth/hydrometeorology.

Leseprobe

Contents

Foreword

Preface

Acknowledgments

1 Terrestrial Hydrometeorology and the Global Water Cycle

Introduction

Water in the Earth system

Components of the global hydroclimate system

Important points in this chapter

2 Water Vapor in the Atmosphere

Introduction

Latent heat

Atmospheric water vapor content

Ideal Gas Law

Virtual temperature

Saturated vapor pressure

Measures of saturation

Measuring the vapor pressure of air

Important points in this chapter

3 Vertical Gradients in the Atmosphere

Introduction

Hydrostatic pressure law

Adiabatic lapse rates

Vertical pressure and temperature gradients

Potential temperature

Virtual potential temperature

Atmospheric stability

Important points in this chapter

4 Surface Energy Fluxes

Introduction

Latent and sensible heat fluxes

Energy balance of an ideal surface

Evaporative fraction and Bowen ratio

Energy budget of open water

Important points in this chapter

5 Terrestrial Radiation

Introduction

Blackbody radiation laws

Radiation exchange for ‘gray’ surfaces

Integrated radiation parameters for natural surfaces

Maximum solar radiation at the top of atmosphere

Maximum solar radiation at the ground

Atmospheric attenuation of solar radiation

Actual solar radiation at the ground

Longwave radiation

Net radiation at the surface

Height dependence of net radiation

Important points in this chapter

6 Soil Temperature and Heat Flux

Introduction

Soil surface temperature

Subsurface soil temperatures

Thermal properties of soil

Formal description of soil heat flow

Thermal waves in homogeneous soil

Important points in this chapter

7 Measuring Surface Heat Fluxes

Introduction

Measuring solar radiation

Measuring net radiation

Measuring soil heat flux

Measuring latent and sensible heat

Comparison of evaporation measuring methods

Important points in this chapter

8 General Circulation Models

Introduction

What are General Circulation Models?

How are General Circulation Models used?

How do General Circulation Models work?

Intergovernmental Panel on Climate Change (IPCC)

Important points in this chapter

9 Global Scale Influences on Hydrometeorology

Introduction

Global scale influences on atmospheric circulation

Latitudinal imbalance in radiant energy

Lower atmosphere circulation

Ocean circulation

Oceanic influences on continental hydroclimate

Water vapor in the atmosphere

Important points in this chapter

10 Formation of Clouds

Introduction

Cloud generating mechanisms

Cloud condensation nuclei

Saturated vapor pressure of curved surfaces

Cloud droplet size, concentration and terminal velocity

Ice in clouds

Cloud formation processes

Cloud genera

Important points in this chapter

11 Formation of Precipitation

Introduction

Precipitation formation in warm clouds

Precipitation formation in other clouds

Which clouds produce rain?

Precipitation form

Raindrop size distribution

Rainfall rates and kinetic energy

Forms of frozen precipitation

Other forms of precipitation

Important points in this chapter

12 Precipitation Measurement and Observation

Introduction

Precipitation measurement using gauges

Snowfall measurement

Precipitation measurement using ground-based radar

Precipitation measurement using satellite systems

Important points in this chapter

13 Precipitation Analysis in Time

Introduction

Precipitation climatology

Trends in precipitation

Oscillations in precipitation

System signatures

Intensity-duration relationships

Statistics of extremes

Conditional probabilities

Important points in this chapter

14 Precipitation Analysis in Space

Introduction

Mapping precipitation

Areal mean precipitation

Spatial organization of precipitation

Design storms and areal reduction factors

Probable maximum precipitation

Spatial correlation of precipitation

Important points in this chapter

15 Mathematical and Conceptual Tools of Turbulence

Introduction

Signature and spectrum of atmospheric turbulence

Mean and fluctuating components

Rules of averaging for decomposed variables

Variance and standard deviation

Measures of the strength of turbulence

Linear correlation coefficient

Kinematic flux

Advective and turbulent fluxes

Important points in this chapter

16 Equations of Atmospheric Flow in the ABL

Introduction

Time rate of change in a fluid

Conservation of momentum in the atmosphere

Conservation of mass of air

Conservation of atmospheric moisture

Conservation of energy

Conservation of a scalar quantity

Summary of equations of atmospheric flow

Important points in this chapter

17 Equations of Turbulent Flow in the ABL

Introduction

Fluctuations in the ideal gas law

The Boussinesq approximation

Neglecting subsidence

Geostrophic wind

Divergence equation for turbulent fluctuations

Conservation of momentum in the turbulent ABL

Conservation of moisture, heat, and scalars in the turbulent ABL

Neglecting molecular diffusion

Important points in this chapter

18 Observed ABL Profiles: Higher Order Moments

Introduction

Nature and evolution of the ABL

Daytime ABL profiles

Nighttime ABL profiles

Higher order moments

Important points in this chapter

19 Turbulent Closure, K Theory, and Mixing Length

Introduction

Richardson number

Turbulent closure

Low order closure schemes

Local, first order closure

Mixing length theory

Important points in this chapter

20 Surface Layer Scaling and Aerodynamic Resistance

Introduction

Dimensionless gradients

Obukhov length

Flux-gradient relationships

Returning fluxes to natural units

Resistance analogues and aerodynamic resistance

Important points in this chapter

21 Canopy Processes and Canopy Resistances

Introduction

Boundary layer exchange processes

Shelter factors

Stomatal resistance

Energy budget of a dry leaf

Energy budget of a dry canopy

Important points in this chapter

22 Whole-Canopy Interactions

Introduction

Whole-canopy aerodynamics and canopy structure

Excess resistance

Roughness sublayer

Wet canopies

Equilibrium evaporation

Evaporation into an unsaturated atmosphere

Important points in this chapter

23 Daily Estimates of Evaporation

Introduction

Daily average values of weather variables

Open water evaporation

Reference crop evapotranspiration

Evaporation from unstressed vegetation: the Matt-Shuttleworth approach

Evaporation from water stressed vegetation

Important points in this chapter

24 Soil Vegetation Atmosphere Transfer Schemes

Introduction

Basis and origin of land-surface sub-models

Developing realism in SVATS

Important points in this chapter

25 Sensitivity to Land Surface Exchanges

Introduction

Influence of land surfaces on weather and climate

Important points in this chapter

26 Example Questions and Answers

Introduction

Example questions

Example Answers

Plates

Index

COMPANION WEBSITE

This book has a companion website:

    www.wiley.com/go/shuttleworth/hydrometeorology

with Figures and Tables from the book for downloading

This edition first published 2012© 2012 by John Wiley & Sons, Ltd

Wiley-Blackwell is an imprint of John Wiley & Sons, formed by the merger of Wiley’s global Scientific, Technical and Medical business with Blackwell Publishing.

Registered officeJohn Wiley & Sons, Ltd, The Atrium, Southern Gate, Chichester, West Sussex, PO19 8SQ, UK

Editorial offices9600 Garsington Road, Oxford, OX4 2DQ, UKThe Atrium, Southern Gate, Chichester, West Sussex, PO19 8SQ, UK111 River Street, Hoboken, NJ 07030-5774, USA

For details of our global editorial offices, for customer services and for information about how to apply for permission to reuse the copyright material in this book please see our website at www.wiley.com/wiley-blackwell.

The right of the author to be identified as the author of this work has been asserted in accordance with the UK Copyright, Designs and Patents Act 1988.

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by the UK Copyright, Designs and Patents Act 1988, without the prior permission of the publisher.

Designations used by companies to distinguish their products are often claimed as trademarks. All brand names and product names used in this book are trade names, service marks, trademarks or registered trademarks of their respective owners. The publisher is not associated with any product or vendor mentioned in this book. This publication is designed to provide accurate and authoritative information in regard to the subject matter covered. It is sold on the understanding that the publisher is not engaged in rendering professional services. If professional advice or other expert assistance is required, the services of a competent professional should be sought.

Library of Congress Cataloging-in-Publication Data

Shuttleworth, W. James.Terrestrial hydrometeorology / W. James Shuttleworth.p. cm.

ISBN 978-0-470-65938-0 (hardback) – ISBN 978-0-470-65937-3 (paper) 1. Hydrometeorology–Textbooks. I. Title.GB2803.2.S58 2012551.57–dc23

2011041765

A catalogue record for this book is available from the British Library.

This book is dedicated with love and gratitude to Hazel,Craig, Matthew, Nicholas, Jonathan and Amy for all the good and worthwhile things they have brought into my life.

Foreword

As a doctoral student of hydrology in the 1970s my only exposure to the meteorological aspects of the hydrologic cycle was a few introductory chapters in hydrology textbooks. These were limited in scope because class emphasis was on the surface and subsurface water flow. Coverage of precipitation and evaporation were also limited to single brief chapters, and there was no exposure to the interface between meteorology and hydrology. Since then there has been a complete transformation. The discipline of hydrometeorology has evolved rapidly due to advances in observational technologies and large scale modeling, both stimulated by the scientific need to address emerging issues such as climate change and the requirement to provide water resources to a growing global population. International programs such as the Global Energy and Water Cycle EXperiment (GEWEX) initiated by the World Climate Research Programme and the Biospheric Aspects of the Hydrologic Cycle (BAHC) initiated by the International Geosphere Biosphere Program heightened interest in the coupling between atmospheric and the terrestrial systems. But the many scientists and students involved in this new interdisciplinary research had to gain their knowledge of the two fields in piecemeal fashion. A textbook was obviously needed to bridge the two disciplines.

Being a visionary in the field, Professor Jim Shuttleworth recognized this void and accepted the challenge. For over a decade he devoted himself to developing courses and writing a book, Terrestrial Hydrometeorology, that specifically addresses the topic of hydrometeorology as a unified component within the Earth system, with appropriate emphasis on hydrometeorology in the terrestrial environment where people live.

The resulting book contains twenty-six chapters which provide excellent coverage of key elements of the hydrologic cycle associated with the coupling of the atmosphere with land surfaces. Coverage of the energy cycle and its role, including the feedback via the water cycle, are extensively and clearly addressed in the first ten chapters. A thorough discussion of precipitation formation, measurement and analysis follows in chapters ten through fourteen. The latter sections of the book provide in-depth coverage of the atmospheric boundary layer dynamics and turbulent transfer that play a primary role in feedbacks in the exchange of water and energy between land and near surface atmosphere. This section of the book demonstrates Shuttleworth’s creative contribution to thinking in the theory of soil moisture and evapotranspiration processes. The final chapter rounds off this ideal textbook by providing example questions and answers that students can use to test their understanding.

It is exciting to finally have a single textbook on terrestrial hydrometeorology that is balanced, timely and elegant, and that will be appropriate for use in graduate courses for many years to come. Terrestrial Hydrometeorology will provide opportunity for atmospheric science and hydrology programs to develop courses that satisfy their cross-disciplinary educational requirements and, in this way, play an important role in the education of the new generation of interdisciplinary scientists who investigate the complex role of the hydrologic cycle in the climate system. For many academics such a book would be the capstone publication of their career but, knowing Jim Shuttleworth as I do, I am certain that we can expect more such creative contributions in the future!

Soroosh SorooshianDirector of the Center for Hydrometeorology and Remote SensingDistinguished Professor of Civil and Environmental Engineeringand Earth System Science at the University of California – Irvine

Preface

Water is the medium through which the atmosphere has most influence on human wellbeing and terrestrial surfaces have significant influence on the atmosphere. Hitherto atmospheric and hydrologic science and practice have largely developed separately. To hydrologists, meteorological variables were monitored and used as independent forcing in models of hydrological responses. But hydrologists now understand that near-surface meteorology is itself in part determined by how surface water moves and how much water the land surface returns to the atmosphere as evaporation. Consequently, graduate-level hydrological training must now include relevant aspects of meteorological science. To meteorologists, atmospheric exchanges with the land surface were regarded as boundary conditions that could be calculated simply, with corrections to weather forecast models then made by assimilating meteorological observations. But because about half the energy that drives the atmosphere enters from below, meteorological forecasts beyond a few days, and climate predictions in particular, require models that include adequately realistic sub-models of surface hydrology and associated energy exchanges. Consequently, graduate-level meteorological training must now include relevant aspects of hydrological science. In fact the relationship between hydrologists and meteorologists and their need to speak a common scientific language is such that it is now recognized that a new science discipline that overlies the land-atmosphere interface is needed, and courses that teach this new discipline of Hydrometeorology are now being created at universities.

Hitherto there has been no single graduate-level text with sufficient breadth across the hydrological and meteorological sciences that provides understanding with adequate depth in both disciplines for use in hydrology departments to teach relevant aspects of meteorology, in meteorological departments to teach relevant aspects of surface hydrology, and to serve as an introductory text to teach the emerging discipline of hydrometeorology. The primary intended readership of this book is, therefore, graduate students studying surface water hydrology, meteorology, and hydrometeorology. However this book could be used in relevant advanced undergraduate courses and it will likely also find broader readership among scientists seeking to broaden their education.

Acknowledgments

This book was written in response to a need for an appropriate text for use in teaching a core course in the University of Arizona Hydrometeorology Program. That course was based on an existing course taught by the author in the Hydrology and Water Resources Program that had evolved over the years in response to students’ needs and students’ input. The resulting syllabus ultimately determined the content of this book. I would, therefore, like to thank all the many students who contributed to that evolution and who have in this way participated in the definition of Terrestrial Hydrometeorology, both the subject and the text book. The manuscript was largely written while the author was on sabbatical leave as a Fellow of the Joint Centre for Hydro-Meteorological Research (JCHMR) which is located in the Centre for Ecology and Hydrology (CEH), Wallingford, UK. I am grateful for the support and the friendship I received from everyone at the CEH, and in particular I would like to thank Richard Harding, Eleanor Blyth, Bob Moore, Martin Best, and Alan Jenkins for facilitating my pleasant and rewarding time at JCHMR. The NSF Science and Technology Center for Sustainability of semi-Arid and Riparian Areas (SAHRA) provided partial financial support during that period and also subsequently supported the refinement of this text through the patience and precision of Annisa Tangreen who provided copy editing support for the manuscript. I am pleased to acknowledge the support of SAHRA, and Annisa in particular. I would also like to thank my good friend and ex-colleague, John Gash, who carefully checked for typographic errors in equations and made a final review of the manuscript, and I am also happy to acknowledge Xu Liang of University of Pittsburg for advice and her input to the review of SVATS given in Chapter 24. Finally, I would like to give my wholehearted thanks to my wife, Hazel, for her immense patience through the many exacting hours it took to prepare this text, for her cheerfulness when the writing was difficult, and the gin-and-tonics we shared when it was just too difficult!

1

Terrestrial Hydrometeorology and the Global Water Cycle

Introduction

Water is not the most common molecule on planet Earth, but it is the most important. Life started in water and cannot survive long without it; it makes up approximately 60% of animal tissue and 90% of plant tissue. The most important greenhouse gas in the atmosphere is water vapor. If it were not present the Earth’s surface temperature would be several tens of degrees cooler, and predicting the effect of changing atmospheric water content is arguably the greatest challenge facing those who seek to predict future changes in climate. It is the continuous cycling of water between oceans and continents that sustains the water flows over land which in large measure determine the evolution of landscapes. The ability of water to store energy in the form of latent heat or because of its high thermal capacity means that moving water as vapor or fluid transports large quantities of energy around the globe. The presence of frozen water on land as snow also has a major impact on whether energy from the Sun is captured at the Earth’s surface or is reflected back to space. In fact, it is hard to think of a process or phenomenon important to the way our Earth behaves in which the presence of water is not significant.

Hydrologists originally considered hydroclimatology to be ‘the study of the influence of climate upon the waters of the land’ (Langbein 1967). This definition is now outdated because it implies too passive a role for land surface influences on the overlying atmosphere. The atmosphere is driven by energy from the Sun, but about half of this energy enters from below, via the Earth’s surface. Whether that surface is ocean or land matters and, if land, the nature of the land surface also matters because this affects the total energy input to the atmosphere and the form in which it enters. In practice, the science of hydroclimatology is often concerned with understanding the movements of energy and water between stores within the Earth system. Because climate is the time-average of weather, strictly speaking hydroclimatology emphasizes the time-average movement of energy and water. Such movement occurs in two directions, both out of and into the atmosphere. Consequently, the present text is motivated not only by the need to understand the global and regional scale atmospheric features that affect the weather in a specific catchment, but also to understand how the surface-atmosphere exchanges that operate inside a catchment contribute along with those from nearby catchments to determine the subsequent state of the atmosphere downwind.

Broadly speaking, hydrometeorology differs from hydroclimatology in much the same way that meteorology differs from climatology. Hydrometeorologists therefore tend to be more interested in activity at shorter time scales than hydroclimatologists. They are particularly concerned with the physics, mathematics, and statistics of the processes and phenomena involved in exchanges between the atmosphere and ground that typically occur over hours or days. Sometimes these short-term features are described statistically. Hydrometeorologists may, for example, analyze precipitation data to compute the historical statistics of intense storms and flood hazards. However, hydrometeorologists are also interested in seeking basic physical understanding of surface exchanges of water and energy. This commonly involves the study of processes that act in the vegetation covering the ground, or the soil and rock beneath the ground, or in lower levels of the atmosphere where most atmospheric water vapor is found. The present text includes some description of the statistical approaches used in hydrometeorology but gives greater prominence to providing an understanding of fundamental hydrometeorological processes.

Water in the Earth system

Although there have been several studies which have attempted to quantify where water is to be found across the globe, the magnitude of the Earth’s water reservoirs and how much water flows between these reservoirs still remains poorly defined. Table 1.1 gives estimates of the size of the eight main reservoirs together with the approximate proportion of the entire world’s water stored in each reservoir and an estimate of the turnover time for the water. The magnitude of the groundwater reservoir and the associated residence time is complicated by the fact that a large proportion of the water in this reservoir is ‘fossil water’ stored in deep aquifers which were created over thousands of years by slow geo-climatic processes. The amount of such fossil water stored is very difficult to estimate globally. Defining a residence time for oceans is also complicated. This is because oceans usually have a fairly shallow layer of surface water on the order of 100 m deep that interacts comparatively readily with the atmospheric and terrestrial reservoirs, but this layer overlies a much deeper, slower moving, and more isolated reservoir of saline water.

Table 1.1 Estimated sizes of the main water reservoirs in the Earth system, the approximate percentage of water stored in them and turnover time of each reservoir (Data from Shiklomanov, 1993).

Clearly oceans are by far the largest reservoir of water on Earth, which means that a vast proportion of water on the Earth is salt water. The majority of Earth’s freshwater supply is currently stored in the polar ice caps, as glaciers or permafrost, or as groundwater. Freshwater lakes, rivers, and marshes contain only about 0.01% of Earth’s total water. The water present in the atmosphere is very small indeed, only about 0.001%. However, the water exchanged between this atmospheric reservoir and the oceanic and land reservoirs is comparatively large, on the order of 100 km3 per year for land and 400 km3 per year for oceans. Consequently, there is a rapid turnover in atmospheric water and the atmospheric residence time is low.

Figure 1.1 illustrates the annual average hydrological cycle for the Earth as a whole, together with an alternative set of estimates of water stores made by combining observations with model-calculated data. It is clear that the simple concept of a hydrological cycle that merely involves water evaporating from the ocean, falling as precipitation over land then running back to the ocean is a poor representation of the truth. There are also substantial hydrological cycles over the oceans which cover about 70% of the globe, and over the continents which cover the remainder, as well as water exchanged in atmospheric and river flows between these two.

Figure 1.1 The global annual average hydrological cycle including estimates of the main water reservoirs (in plain font in units of 103 km3) and of the flow of moisture between stores (in italics in units of 103 km3 yr−1). (From Trenberth et al., 2007, published with permission.) See Plate 1 for a colour version of this image.

On average there is a net transfer from oceanic to continental surfaces because the oceans evaporate about 413 × 103 km3 yr-1 of water, which is equivalent to about 1200 mm of evaporation, but they receive back only about 90% of this as precipitation. Some of the water evaporated from the ocean is therefore transported over land and falls as precipitation, but on average about 65% of this terrestrial precipitation is then re-evaporated and this provides some of the water subsequently falling as precipitation elsewhere over land. On average about 35% of terrestrial precipitation returns to the ocean as surface runoff, but the proportion of terrestrial precipitation that is re-evaporated and the proportion leaving as surface runoff varies significantly both regionally and with season. Area-average runoff in the semi-arid south western USA is, for example, commonly just a few percent. When averaged over large continents and over a full year, variations in the fraction of precipitation leaving as runoff are less. Table 1.2 gives an example of the estimated annual water balance for the continents (Korzun 1978). Runoff ratios in the range of 35 to 45% are the norm, but the extensive arid and semi-arid regions of Africa reduce average runoff for that continent. Fractional runoff in the form of icebergs from Antarctica is hard to quantify but may be 80% because sublimation from the snow and ice covered surface is low.

Table 1.2 Estimated continental water balance (Data from Korzun, 1978).

Components of the global hydroclimate system

Understanding the hydroclimate of the Earth does not merely require knowledge of hydrometeorological process in the atmosphere. Several different components of the Earth system interact to control the way near-surface weather variables vary in time and space. It is helpful to recognize the nature of these components from the outset and to appreciate in general terms how they influence global hydroclimatology. For this reason we next consider salient features of the atmosphere, hydrosphere, cryosphere, and lithosphere, biosphere, and anthroposphere.

Atmosphere

The air surrounding the Earth is a mixture of gases, mainly (~80%) nitrogen and (~20%) oxygen, but also other minority gases such as carbon dioxide, ozone, and water vapor which have an importance to hydroclimatology not adequately reflected by their low concentration. Compared to the diameter of the Earth (~20,000 km), the depth of the atmosphere is small. The density of air changes with height but about 90% of the mass of the atmosphere is within 30 km and 99.9% within 80 km of the ground.

The atmosphere is (almost) in a state of hydrostatic equilibrium in the vertical, with dense air at the surface and less dense air above; there is an associated change in pressure. The temperature of the air changes with height in a very distinctive way and this can be used to classify different layers or ‘spheres’. Figure 1.2 shows the vertical profile of air temperature in the US Standard Atmosphere (US Standard Atmosphere, 1976) as a function of height and atmospheric pressure. Starting from the surface, the main layers are the troposphere, stratosphere, mesosphere, and thermosphere, separated by points of inflection in the vertical temperature profile that are called ‘pauses’. Near the ground, air temperature falls quickly with height for reasons which are discussed in more detail later. Higher in the atmosphere the air is warmed by the release of latent heat when water is condensed in clouds and, in the upper stratosphere, it is also warmed by the absorption of a portion of incoming solar radiation. There is then further cooling through the mesosphere, but some further warming at the very top of the atmosphere where most of the Sun’s gamma rays are absorbed.

Figure 1.2 Idealized vertical temperature profile for the US Standard Atmosphere showing the most important layers, ‘spheres’, and the ‘pauses’ that separate them.

The relative concentration of atmospheric nitrogen, oxygen and other inert gases is uniform with height, but most ozone is found in the middle atmosphere where it absorbs ultraviolet radiation to warm the air. The concentration of carbon dioxide falls away in the mesosphere and the vast majority of atmospheric water vapor is found within 10 km of the ground, mainly in the lower levels of the troposphere. The fact that water vapor content falls quickly with height is strongly related to the fall in temperature with height. The amount of water vapor that air can hold before becoming saturated is less at lower temperatures and water is precipitated out as water droplets or ice particles in clouds. The concentrations of liquid and solid water in clouds and that of other atmospheric constituents, including solid particles such as dust particles, sulfate aerosols, and volcanic ash, all vary substantially both in space and with time.

As previously mentioned, the residence time for water in the atmosphere is short, about 10 days. In fact, a comparatively short response time is a general feature of the atmosphere that distinguishes it from the other components of the climatic system. Air has a relatively large compressibility and low specific heat and density compared to the fluids and solids that make up the hydrosphere, cryosphere, lithosphere and biosphere. Because air is more fluid and unstable, any perturbations generated by changes in the inputs that drive the atmosphere typically decay with time scales on the order of days to weeks.

Differential heating by the Sun causes movement in the atmosphere that is complicated by the rotation of the Earth, the Earth’s orbit around the Sun, and inhomogeneous surface conditions. Consequently, the air in the troposphere undergoes large-scale circulation which, on average, is organized at the global scale. There are substantial perturbations within this circulation associated with weather systems at mid-latitudes, and also pseudo-random turbulent motion in the atmospheric boundary layer and near ‘jet streams’ higher in the atmosphere. Figure 1.3 shows how contributions to the variance of atmospheric movements in the atmospheric boundary layer change as a function of frequency. Most movement occurs at low frequencies. The first peak in this figure is associated with movement linked to the annual cycle and is in response to seasonal changes in solar heating, while the third peak is linked to the daily cycle of heating. The large contribution to variance at the time scales of days to weeks is the result of the large-scale disturbances associated with transient weather systems. At lower frequencies atmospheric variance is therefore mainly associated with horizontal features within the atmospheric circulation. The fourth maximum in variance, which occurs at timescales of an hour or less, is different because it is due to small-scale turbulent motions. Such turbulent variations occur in all directions, but their influence on the vertical movement of atmospheric properties and constituents is particularly important. Understanding this influence on the vertical movement is a critical aspect of hydrometeorology.

Figure 1.3 Approximate spectrum of the contributions to the variance in the atmosphere for frequencies between 1 second and 1000 days.

Hydrosphere

The liquid water in oceans, interior seas, lakes, rivers, and subterranean waters constitute the Earth’s hydrosphere. Oceans cover about 70% of the Earth’s surface and therefore intercept more total solar energy than land surfaces. Most of the energy leaves oceanic surfaces in the form of latent heat in water vapor, but this is not necessarily the case for land surfaces. Consequently, maritime air masses are very different to continental air masses. The atmosphere and oceans are strongly coupled by the exchange of energy, matter (water vapor), and momentum at their interface, and precipitation strongly influences ocean salinity. The mass and specific heat of the water in oceans is much greater than for air and understanding this difference is very important in the context of seasonal changes in the atmosphere. The oceans represent an enormous reservoir for stored energy. As a result, changes in the sea surface temperature happen fairly slowly and this moderates the rate of change of associated features in the atmosphere, thereby greatly aiding seasonal climate prediction.

The oceans are also denser than the atmosphere and have a larger mechanical inertia, so ocean currents are much slower than atmospheric flows, and oceanic movement at depth is particularly slow. The atmosphere is heated from below by the Sun’s energy intercepted by the underlying surface, but oceanic heating is from above. Consequently, there is a profound difference in the way buoyancy acts in these two fluid media. The higher temperature at the surface of the sea means oceanic mixing by surface winds tends to be suppressed, and such mixing is limited to the active surface layer that has a thickness on the order of 100 m. A strong gradient of temperature below this surface layer separates it from the deep ocean. The response time for oceanic movement in the upper mixed layer is weeks to months to seasons. In the deep ocean, however, movement due to density variations associated with changes in temperature and salinity occur over time scales from centuries to millennia. There are eddies in the upper ocean but turbulence is in general much less pronounced than in the atmosphere. Ocean currents are important because they move heat from the tropical regions, where incidental solar radiation is greatest, toward colder mid-latitude and polar regions where radiation is least. Currents in the upper layer of the ocean are driven by the prevailing wind patterns in the atmosphere. Ocean flow is from east to west in the tropics (in response to the trade winds), poleward on the eastern side of continents, then back toward the equator on the western side of continents.

Lakes, rivers, and subterranean waters make up the remainder of the hydrosphere. They can have significant hydrometeorological and hydroclimatological significance in continental regions, particularly at regional and local scale. The contrast between the influence on the atmosphere of open water on the one hand and land surfaces on the other is significant. This is responsible for ‘lake effect’ snowfall in the US Great Lakes and ‘river breeze’ effects near the Amazon River, for example. River flow into oceans also has an important influence on ocean salinity near coasts.

Cryosphere

The areas of snow and ice, including the extended ice fields of Greenland and Antarctica, other continental glaciers and snowfields, sea ice, and areas of permafrost, are the Earth’s cryosphere. The cryosphere has an important influence on climate because of its high reflectivity to solar radiation. Continental snow cover and sea ice have a market seasonally, and this can give rise to significant intra-annual and perhaps interannual variations in the surface energy budgets of frozen polar oceans and continents with seasonal snow cover. Gradual warming in polar regions has the potential to give rise to similar changes in surface energy balance over longer time periods. The low thermal diffusivity of ice can also influence the surface energy balance at high latitudes, because ice acts as an insulator inhibiting loss of heat to the atmosphere from the underlying water and land. Near-surface cooling also gives rise to stable atmospheres, which inhibit convection and contribute to cooler climates locally.

The large continental ice sheets do not change quickly enough to influence seasonal or interannual climate much, but historical changes in ice sheet extent and potential changes in the future extent of ice sheets are important because they are associated with changes in sea level. If substantial melting of the continental ice sheets occurs, altered sea level could change the boundaries of islands and continents. Since many inhabited areas are close to such boundaries, sea level change will likely have serious consequences for human welfare that are disproportionate to the fractional area of land affected. The effect of global warming on ice sheets is considered a major threat for this reason.

Lithosphere

The lithosphere, which includes the continents and the ocean floor, has an almost permanent influence on the climatic system. There is large-scale transfer of angular momentum through the action of torques between the oceans and the continents. Continental topography affects air motion and global circulation through the transfer of mass, angular momentum, and sensible heat, and the dissipation of kinetic energy by friction in the atmospheric boundary layer. Because the atmosphere is comparatively thin, organized topography in the form of extended mountain ranges that lie roughly perpendicular to the preferred atmospheric circulation, such as the Rocky Mountains in North America and the Andes Mountains in South America, can inhibit how far maritime air penetrates into continents and thus affect where clouds and precipitation occur.

The transfer of mass between the atmosphere and lithosphere is mainly as water vapor, rain, and snow. However, this may sometimes also occur as dust when volcanoes throw matter into the atmosphere and increase the turbidity of the air. The ejected sulfur-bearing gases and particulate matter may modify the aerosol load and radiation balance of the atmosphere and influence climate over extended areas. The soil moisture in the active layer of the continental lithosphere that is accessible to the atmosphere via plants can have a marked influence on the local energy balance at the land surface. Soil moisture content affects the rate of evaporation, the reflection by soil of solar radiation, and the thermal conductivity of the soil. Because soil moisture tends to change fairly slowly it can provide a land-based ‘memory’ with an effect on the atmosphere broadly equivalent to that of slowly changing sea surface temperature.

Biosphere

Terrestrial vegetation, continental fauna, and the flora and fauna of the oceans make up the biosphere. It is now recognized that the nature of vegetation covering the ground is not only influenced by the regional hydroclimate, but also itself influences the hydroclimate of a region. This is because the type of vegetation present affects the aerodynamic roughness and solar reflectivity of the surface, and whether water falling as precipitation leaves as evaporation or runoff. The rooting depth of vegetation matters because it determines the size of the moisture store available to the atmosphere. Changes in the type of vegetation present may occur in response to changes in climate, and modern climate prediction models attempt to represent such evolution. Imposed changes caused by human intervention through, for example, large-scale deforestation or irrigation also occur and these can be extensive and alter surface inputs to the atmosphere of continental regions.

The behavior of the biosphere influences the carbon dioxide present in the atmosphere and oceans through photosynthesis and respiration. It is essential to include description of these influences when models are used to simulate global warming. For this reason, advanced sub-models describing the biosphere in meteorological models seek to represent the energy, water, and carbon exchanges of the biosphere simultaneously. Water and carbon exchanges are linked by the fact that the water transpired and the carbon assimilated by vegetation occurs by molecular diffusion through the same plant stomata. Models of the biosphere are often referred to as Land Surface Parameterization Schemes (LSPs) or Soil Vegetation Atmosphere Transfer Schemes (SVATs); see Chapter 24 for greater detail. Figure 1.4 shows an example of the Simple Biosphere Model (SiB; Sellers et al., 1986), an example of a SVAT that is currently widely used.

Figure 1.4 A schematic diagram of the physical and physiological processes represented in the second generation Simple Biosphere (SiB2) soil vegetation atmosphere transfer scheme. (From Colello et al., 1998, published with permission.) See Plate 2 for a colour version of this image.

Because the biosphere is sensitive to changes in climate, detailed study of past changes in its nature and behavior as revealed in fossils and tree rings and in pollen and coral records is important as a means of documenting the prevailing climate in previous eras.

Anthroposphere

The word anthroposphere is used to describe the effect of human beings on the Earth system. For much of our existence human impact on the environment was small, but as our numbers grew our impact on the atmosphere and landscape expanded. With the start of the industrial revolution in the late eighteenth century, humans developed the ability to harness power from fossil fuels and transitioned from mostly observers to participants in global change. We have significantly altered the biogeochemical cycles of carbon, nitrogen, sulfur, and phosphorus. Alteration of the carbon cycle has changed the acidity of the oceans and is changing the climate of Earth. Chemical inventions such as chlorinated fluorocarbons (CFCs) have altered the ozone in the stratosphere and the amount of ultraviolet light reaching the Earth’s surface. The footprint of our chemical activities is now found in the air, water, land, and biota of Earth in the form of naturally occurring and human-created molecules.

The anthroposphere has expanded to occupy land for dwellings and agriculture. Human dwellings now occupy about 8% of ice-free land and about three-quarters of the land surface has been altered by humans in some way. As mentioned above and discussed in more detail in later chapters, changing the nature of the land surface is important for hydrometeorology because it alters the way energy from the Sun enters the atmosphere from below and, if sufficiently extensive, land-use change has impact on regional and perhaps global climate and weather. Examples of such change include urban heat islands and changes to regional evaporation due to the building of large dams, extensive irrigation, and land-cover change such as deforestation.

When compared with most natural changes in other spheres of the globe, change in the anthroposphere is happening very rapidly. This is partly because human population has increased quickly over the past few centuries and still is today, but also because strides in technology have empowered humans to directly and indirectly effect change to the environment in new and different ways, and because as society develops the per capita demand for energy increases hugely.

Important points in this chapter

Hydrometeorology: hydrometeorology (and this text) concerns the physics, mathematics, and statistics of processes and phenomena involved in exchanges between the atmosphere and ground that typically occur over hours or days, and how the time average of these exchanges combine to define hydroclimatology.Water reservoirs: the size of the Earth’s water reservoirs are poorly defined but include the oceans (~ 95.6%), groundwater (~2.4%), frozen water (1.9%), and water bodies, soil moisture, atmospheric water, rivers and biological water (in total ~ 0.01%).Water cycle: as a global average about 90% of oceanic evaporation falls back to the oceans as precipitation, the remainder being transported over land; and about 55–65% of the precipitation falling over land re-evaporates (depending on the continent) leaving 35–45% to runoff back to the ocean in rivers and icebergs. Atmosphere:

— Constituents and structure: About 80% N2 and 20% O2 and other minority gases (CO2, O3, H2O, etc.), 99.9% of which are within 80 km of the ground in the troposphere, stratosphere, mesosphere, thermosphere, with most water vapor in the lower troposphere within 10 km of the ground.

— Circulation: Differential heating by the Sun causes global circulation in the troposphere which moves energy toward the poles and which is complicated by the Coriolis force but is, on average, organized.

— Variance: Contributions to the variance of the atmosphere arise at frequencies linked to the seasonal cycle, transient weather systems, and the daily cycle, with these contributions separated by a distinct spectral gap from those at higher frequencies that are associated with turbulence.

Hydrosphere:

— Extent and importance: Oceans cover ~70% of the Earth and the solar energy they intercept is mainly used to evaporate water vapor into the atmosphere; they have a large thermal capacity and act as ‘memory’ in the Earth system that influences season climate.

— Structure: Oceans have a surface layer 10s–100s m deep warmed by the Sun’s energy in which there are wind-driven ocean currents, this layer being separated by a strong thermal gradient from the deep ocean which moves very slowly in response to changes in temperature and salinity.

— Currents: Upper ocean currents move heat from the tropics to polar regions: ocean flow is east to west in the tropics, poleward on the eastern side of continents, then back toward the equator on the western side of continents.

Cryosphere: comprises the polar ice fields and glaciers that change slowly and transient continental snow/ice fields with a strong seasonal influence on climate.Lithosphere: organized topography perpendicular to atmospheric flow can inhibit penetration of maritime air into continents, and aerosols from volcanoes can alter the radiation balance over extensive areas.Biosphere: vegetation cover affects aerodynamic roughness and reflection of solar energy and by intercepting rainfall and accessing water in the soil through roots, also whether precipitation leaves as evaporation or runoff.Anthroposphere: human population is now large enough to influence climate, mainly by changing the concentrations of gases in the atmosphere and by modifying land cover over large areas.

References

Colello, G.D., Grivet, C., Sellers, P.J., & Berry, J.A. (1998) Modeling of energy, water, and CO2 flux in a temperate grassland ecosystem with SiB2: May–October 1987. Journal of Atmospheric Sciences, 55 (7), 1141–69.

Langbein, W.G. (1967) Hydroclimate. In: The Encyclopaedia of Atmospheric Sciences and Astrogeology (ed. R.W. Fairbridge), pp. 447–51. Reinhold, New York.

Korzun, V.I. (1978) World Water Balance of the Earth. Studies and Reports in Hydrology, 25. UNESCO, Paris.

Sellers, P.J., Mintz, Y., Sud, Y.C. & Dalcher, A. (1986) A simple biosphere model (SiB) for use within general circulation models. Journal of Atmospheric Sciences, 43, 505–531.

Shiklomanov, J.A. (1993) World fresh water resources. In: Water in Crisis: A Guide to the World’s Fresh Water Resources (ed. P.H. Gleick), pp. 13–24. Oxford University Press, New York.

Trenberth, K.E., Smith, L., Qian, T., Dai, A. & Fasullo, J. (2007) Estimates of the global water budget and its annual cycle using observational and model data. Journal of Hydrometeorology, 8, 758–69.

US Standard Atmosphere (1976) US Government Printing Office, Washington, DC.

2

Water Vapor in the Atmosphere

Introduction

Hydrometeorologists are commonly concerned with quantifying the amount of water in the atmosphere in vapor, liquid, and solid form and with seeking to describe the way energy and water move vertically in the atmosphere toward and away from the ground. In this chapter we consider the basic definitions and important concepts needed for this.

Latent heat

The molecules that make up ice are held rigidly together in close proximity by intermolecular forces. In liquid water the molecules are also close together but, because they are at a higher temperature, they move around and their average separation is therefore somewhat greater. In water vapor, molecular separation is very much larger: molecules in water vapor are typically separated by about ten molecular diameters. As water molecules move farther apart, the forces that bind them reduce quickly with distance and at ten molecular diameters these forces are much smaller than when the molecules are in near contact. Viewed in this way, the transition from ice to liquid water and then to water vapor can be viewed as a temperature related increase in the separation of molecules in the face of the attractive intermolecular forces acting between them.