Hydrometeorology
Advancing Weather and Climate Science Series

Hydrometeorology presents an introduction to relevant topics in the interdisciplinary fields of hydrology and meteorology. This book is one of the few books aiming to provide a balance between aspects of meteorological and hydrological processes. The transfer of energy and water between the land surface and lower atmosphere within the hydrological cycle is addressed followed by a description of the nature of precipitation, and how it is formed. Forecasting precipitation is reviewed on all scales, and the range of rainfall-runoff models and coastal surge models and forecasts (including tsunamis) which have been, and are being, used are discussed.

The mechanisms of snow, ice (glacier, sea and tundra), evaporation and transpiration, how drought occurs and the representation of wind are described. How rainfall (including radar measurements) and river flow information is gathered and analysed (including, frequency analysis, Probable Maximum Precipitation and Flood) are presented. Satellite measurements of precipitation are discussed. Examples of major past floods and droughts are given.

Past and future climate change, which is included, underpins the importance of hydro-meteorological processes. The structure of the general circulation of the atmosphere and how it influences weather and climate including the Hadley, Ferrel and Polar cells, the Trade winds and the El Nino, is outlined. Finally, the influence of urban areas on rainfall formation, dealing with urban drainage and air quality are described.

Each chapter ends with one or two specific points as appendices, elements discussed in the chapter and a list of sample problems to aid understanding.

Readership: This book is aimed at 3rd year undergraduate and postgraduate students on hydrology/hydrometeorology, environmental science and geography courses. Professionals in environmental protection agencies and consultancies will also find the book of great interest. It contains a balance of both the physics and mathematics which underpin such courses and activities.

Preface xv

Acknowledgements xvii

About the Companion Website xviii

1 The Hydrological Cycle 1

1.1 Overview 1

1.2 Processes comprising the hydrological cycle 3

1.3 Global influences on the hydrological cycle 4

1.4 Water balance 6

1.5 Impact of aerosols on the hydrological cycle 6

1.6 Coupled models for the hydrological cycle 7

1.7 Global Energy and Water Cycle Exchanges Project (GEWEX) 8

1.8 Flooding 8

Summary of key points in this chapter 9

Problems 10

References 10

2 Precipitation 11

2.1 Introduction 11

2.2 Equation of state for a perfect gas 11

2.3 Hydrostatic pressure law 12

2.4 First law of thermodynamics 12

2.5 Atmospheric processes: dry adiabatic lapse rate 13

2.6 Water vapour in the atmosphere 15

2.7 Atmospheric processes: saturated adiabatic lapse rate 16

2.8 Stability and convection in the atmosphere 16

2.9 The growth of precipitation particles 18

2.10 Precipitation systems 21

2.10.1 Localized convection 22

2.10.2 Mesoscale precipitation systems 23

2.10.3 Mid-latitude depressions 26

2.10.4 Tropical storms 30

2.10.5 Orographic effects on precipitation distribution 31

2.10.6 Topographical effects on precipitation distribution 33

2.11 Global atmospheric circulation 33

Appendix 2.1 Growth of a raindrop 33

Summary of key points in this chapter 35

Problems 36

References 37

3 Evaporation and Transpiration 41

3.1 Introduction 41

3.2 Modelling potential evaporation based upon observations 41

3.3 Aerodynamic approach 42

3.4 Energy balance 44

3.5 The Penman equation 44

3.6 Sensible and water vapour fluxes 45

3.7 Evaporation of water from wet vegetation surfaces: the interception process 47

3.8 Measuring evaporation and transpiration 47

3.9 Water circulation in the soil–plant–atmosphere continuum 48

3.10 Water circulation and transpiration 50

3.11 Water flux in plants 50

3.12 Modelling land surface temperatures and fluxes 51

3.13 Soil–vegetation–atmosphere transfer schemes 54

3.14 Estimation of large scale evapotranspiration and total water storage in a river basin 56

Appendix 3.1 Combination of aerodynamic and energy balance methods of computing lake evaporation 57

Appendix 3.2 Modelling soil moisture wetness 57

Summary of key points in this chapter 58

Problems 59

References 60

4 Snow and Ice 63

4.1 Introduction 63

4.2 Basic processes 63

4.2.1 Formation of snow 63

4.2.2 Formation of snow cover and its effects on the atmosphere 65

4.2.3 Formation of ice 67

4.3 Characteristics of snow cover 68

4.4 Glaciers 70

4.5 Sea ice 71

4.6 Permafrost 71

4.7 The physics of melting and water movement through snow 71

4.8 Water equivalent of snow 74

4.9 Modelling snowmelt and stream flow 76

4.10 Snow avalanches 80

4.11 Worldwide distribution and extremes of snow cover 81

Appendix 4.1 Estimates of catchment snowmelt inflow rates 83

Summary of key points in this chapter 84

Problems 86

References 87

5 Measurements and Instrumentation 90

5.1 Measurement, resolution, precision and accuracy 90

5.2 Point measurements of precipitation 90

5.2.1 Raingauge types 90

5.2.2 Measuring snow and hail 92

5.2.3 Errors in measurement 94

5.3 Areal measurements of precipitation using raingauge networks 96

5.4 Radar measurements of rainfall 96

5.4.1 Basics 96

5.4.2 Errors in radar measurements 97

5.4.3 Adjustment using raingauges 101

5.4.4 Summary of problem areas associated with radar measurements of precipitation 102

5.4.5 The use of multi]parameter radar 103

5.4.6 Drop size distributions 104

5.4.7 Rainfall estimation using parametric variables 104

5.4.8 Measurement of snow 106

5.4.9 Measurement of hail 107

5.4.10 Precipitation type 108

5.5 Soil moisture 109

5.5.1 Approaches 109

5.5.2 Gravimetric method 109

5.5.3 Electrical resistance method 110

5.5.4 Neutron method 110

5.5.5 Gamma ray attenuation method 110

5.5.6 COSMOS-UK 111

5.5.7 Dielectric methods 111

5.5.8 Tensiometric method 113

5.5.9 Satellite remote sensing 113

5.6 Evaporation and evapotranspiration 113

5.7 Flow measurement: basic hydrometry 113

5.8 Measuring stream discharge 115

5.8.1 The stage-discharge curve 115

5.8.2 Automated moving boat methods 117

5.9 Brief overview of modern telemetry 117

5.9.1 Ground-based telemetry links 117

5.9.2 VHF and UHF radio links 117

5.9.3 Satellite links 118

Appendix 5.1 Combining dissimilar estimates by the method of least squares 118

Summary of key points in this chapter 119

Problems 121

References 121

6 Satellite-Based Remote Sensing 125

6.1 Overview of satellite remote sensing 125

6.2 Surface scattering of electromagnetic radiation 129

6.3 Interaction of electromagnetic radiation with the atmosphere 131

6.4 Visible and infrared data 132

6.4.1 Precipitation 134

6.4.2 Snow depth 135

6.4.3 Soil moisture and evapotranspiration 136

6.5 Multispectral data 137

6.5.1 Precipitation 137

6.5.2 Cloud recognition 137

6.5.3 Snow 138

6.6 Passive microwave techniques 138

6.6.1 Precipitation 141

6.6.2 Global Precipitation Climatology Project (GPCP) 143

6.6.3 Global Precipitation Measurement mission (GPM) 143

6.6.4 Snow depth 143

6.6.5 Sea ice and sea surface temperature 145

6.6.6 Soil moisture and evapotranspiration 145

6.7 Active (radar) microwave techniques 147

6.7.1 Synthetic aperture radar 147

6.7.2 Radar systems 149

6.7.3 Tropical Rainfall Measuring Mission (TRMM) 150

6.8 The surface energy balance system (SEBS) 150

6.9 Summary of satellite measurement issues 151

Appendix 6.1 Radiation balance 154

Summary of key points in this chapter 155

Problems 157

References 157

7 Analysis of Precipitation Fields and Flood Frequency 163

7.1 Introduction 163

7.2 Areal mean precipitation 163

7.3 Spatial and temporal storm analysis 165

7.3.1 Spatial statistical analyses 165

7.3.2 Temporal analyses 167

7.3.3 Oscillations in precipitation 168

7.3.4 Conditional probabilities 169

7.3.5 Kriging 169

7.3.6 Accuracy of the precipitation products 171

7.4 Model storms for design 172

7.5 Approaches to estimating flood frequency 173

7.6 Probable maximum precipitation (PMP) 175

7.7 Probable maximum flood (PMF) 177

7.8 Flood Studies Report (FSR) 177

7.9 Flood Estimation Handbook (FEH) 180

Appendix 7.1 Three-dimensional description of a rainfall surface 182

Appendix 7.2 Gumbel distribution 183

Summary of key points in this chapter 183

Problems 185

References 185

8 Precipitation Forecasting 188

8.1 Introduction 188

8.2 Nowcasting 188

8.2.1 Definition 188

8.2.2 Impact of errors in precipitation measurements 189

8.2.3 Extrapolation of radar data 189

8.3 Probabilistic radar nowcasting 192

8.4 Numerical models: structure, data requirements, data assimilation 194

8.4.1 Probabilistic quantitative precipitation forecasting 194

8.4.2 Mesoscale models 197

8.4.3 Data assimilation 197

8.4.4 Performance of high resolution mesoscale model-based nowcasting systems 198

8.5 Medium range forecasting 198

8.6 Seasonal forecasting 201

Appendix 8.1 Brier skill score 203

Summary of key points in this chapter 203

Problems 205

References 205

9 Flow Forecasting 209

9.1 Basic flood forecasting techniques 209

9.2 Model calibration and equifinality 210

9.3 Flood forecasting model development 210

9.4 Conversion of detailed hydrodynamic models to simplified models suitable for real]time flood forecasting 213

9.5 Probabilistic flood forecasting and decision support methods 215

9.6 Derivation of station rating (stage]discharge) curves 216

9.7 Performance testing of forecasting models and updating procedures 216

9.8 Configuration of models on to national and international forecasting platforms 218

9.9 Flood warnings and levels of service 222

9.9.1 United Kingdom 222

9.9.2 United States and Canada 222

9.10 Case studies worldwide: river and urban 224

Appendix 9.1 St Venant equations 224

Appendix 9.2 Flow in unsaturated and saturated zones 226

Summary of key points in this chapter 227

Problems 228

References 229

10 Coastal Flood Forecasting 233

10.1 Types of coastal flooding 233

10.2 Models used to predict storm surge flooding 233

10.2.1 Empirical models 234

10.2.2 First-generation models 235

10.2.3 Second-generation models 235

10.2.4 Third-generation models 235

10.2.5 Wave, tide and surge models 235

10.3 Probabilistic surge forecasting 238

10.4 Tsunamis 239

10.5 Examples of coastal flooding in the United Kingdom 241

10.5.1 The surge of 1953 241

10.5.2 Wirral floods 2013 241

10.5.3 Surges along the east coast of England, December 2013 241

10.5.4 Aberystwyth floods January 2014 242

10.6 Some examples of coastal flooding worldwide 243

Appendix 10.1 Wave overtopping at the coast 244

Summary of key points in this chapter 245

Problems 247

References 247

11 Drought 249

11.1 Definitions 249

11.2 Drought indices 250

11.3 The physics of drought 253

11.4 Frequency analysis: predictability 254

11.5 Modelling the occurrence of drought 256

11.6 Major drought worldwide 258

11.7 Examples of the consequences of drought 258

11.8 Strategies for drought protection, mitigation or relief 260

Appendix 11.1 Defining aridity 261

Summary of key points in this chapter 261

Problems 263

References 263

12 Wind and the Global Circulation 266

12.1 Equations of motion 266

12.2 Atmospheric Ekman layer 268

12.3 Fronts 269

12.4 Jet streams 270

12.5 Hurricanes 271

12.6 Lee waves 272

12.7 Land and sea breezes 272

12.8 The wind structure of the atmospheric circulation 273

12.9 Hadley cell 273

12.10 Polar cell 274

12.11 Ferrel cell 275

12.12 Walker circulation 275

12.13 El Niño/Southern Oscillation 276

12.14 Monsoons 276

Appendix 12.1 Large scale air motion 278

Appendix 12.2 Ageostrophic motion 278

Summary of key points in this chapter 279

Problems 281

References 282

13 Climatic Variations and the Hydrological Cycle 284

13.1 An introduction to climate 284

13.2 Evidence of climate change 286

13.2.1 Climatology of the last ice age 292

13.2.2 Intergovernmental Panel on Climate Change (IPCC) 295

13.3 Causes of climatic change 297

13.3.1 The natural energy system 298

13.3.2 The hydrological cycle 299

13.3.3 The carbon cycle 301

13.3.4 Other biochemical cycles 301

13.4 Modelling climatic change 303

13.5 Possible effects of climate change upon the hydrological cycle and water resources 307

Appendix 13.1 Estimating return times for events in a long term climate record 310

Summary of key points in this chapter 310

Problems 313

References 314

14 Hydrometeorology in the Urban Environment 318

14.1 Introduction 318

14.2 Urban boundary layer and the water cycle 318

14.3 Urban development and rainfall 320

14.4 Sewer flooding 322

14.5 Surface runoff from urban areas 324

14.6 Floodplain development 326

14.7 Acid rain 327

14.7.1 Basics 327

14.7.2 Modelling wet deposition 328

14.8 Urban air and water pollution 329

Appendix 14.1 Number of runoff events from an urban drainage system 330

Summary of key points in this chapter 331

Problems 332

References 333

Glossary 336

Index 347

Christopher G. Collier received a BSc in Physics and ARCS in Science at Imperial College, London in 1968. Subsequently he received a PhD (1999) and a DSc (2008) from the University of Salford. He joined the Meteorological Office in 1968, and later chaired the European Union International Weather Radar Networking project, and served on numerous World Meteorological Organisation (WMO), BNSC, EUMETSAT, ESA and NERC committees. He is a Chartered Meteorologist of the Royal Meteorological Society, and was President of that Society 2004-2006 being elected an Honorary Fellow in 2012, and served on the committees of the British Hydrological Society. He is a member of the American Meteorological Society. He left the Met Office in 1995 becoming a Professor of Environmental Remote Sensing at the University of Salford, and joined the National Centre for Atmospheric Science based at the University of Leeds becoming Professor of Atmospheric Science and Head of Strategic Partnerships in 2009. He was awarded the First Vaisala Prize for radar measurements of precipitation in 1986.