Weather Analysis and Forecasting: Applying Satellite Water Vapor Imagery and Potential Vorticity Analysis

By Christo Georgiev and Patrick Santurette

Weather Analysis and Forecasting is a practical guide to using potential vorticity fields and water vapor imagery from satellites to elucidate complex weather patterns and train meteorologists to improve operational forecasting. In particular, it details the use of the close relationship between satellite imagery and the potential vorticity fields in the upper troposphere and lower stratosphere. It shows how to interpret water vapor patterns in terms of dynamical processes in the atmosphere and their relation to diagnostics available from weather prediction models.

The book explores topics including: a dynamical view of synoptic development; the interpretation problem of satellite water vapor imagery; practical use of water vapor imagery and dynamical fields; significant water vapor imagery features associated with synoptic dynamical structures; and use of water vapor imagery for assessing NWP model behavior and improving forecasts. Applications are illustrated with color images based on real meteorological situations.

The book's step-by-step pedagogy makes this an essential training manual for forecasters in meteorological services worldwide, and a valuable text for graduate students in atmospheric physics and satellite meteorology.

* Shows how to analyze current satellite images for assessing weather models' behavior and improving forecasts * Provides step-by-step pedagogy for understanding and interpreting meteorological processes * Includes full-color throughout to highlight "real-world" models, patterns, and examples

• Language: English
• Publisher: Academic Press
• Release date: Jul 5, 2005
• ISBN: 9780080455266

Christo Georgiev

Christo G. Georgiev has worked in the Forecasting Department of the National Meteorological Service (NMS) of Bulgaria since 1993 as a satellite meteorology researcher, Associate Professor since 2004 and as Professor since 2012. He is well acquainted with the meteorological satellites, weather analysis and forecasting matters, having worked for the NMS of Bulgaria in a position of Programme Manager of Forecasting Technology (2008-2011), Head of Operational Weather Forecasting (2011-2014) and Head of Remote Sensing since 1 December 2015. He has taught at various national and international training courses for using satellite data in weather forecasting.

Book preview

Weather Analysis and Forecasting

Applying Satellite Water Vapor Imagery and Potential Vorticity Analysis

Patrick Santurette

Forecast Laboratory Météo-France

Christo G. Georgiev

National Institute of Meteorology and Hydrology Bulgarian Academy of Sciences

Academic Press

Table of Contents

Cover image

Title page

Preface

Acknowledgments

Introduction

Part I: Fundamentals

Chapter 1: A Dynamical View of Synoptic Development

Publisher Summary

1.1 VORTICITY AND POTENTIAL VORTICITY

1.2 THE CONCEPT OF PV THINKING

1.3 OPERATIONAL USE OF PV FIELDS FOR MONITORING SYNOPTIC DEVELOPMENT

Chapter 2: The Interpretation Problem of Satellite Water Vapor Imagery

Publisher Summary

2.1 RADIATION MEASUREMENTS IN WATER VAPOR ABSORPTION BANDS

2.2 INFORMATION CONTENT OF WATER VAPOR IMAGE GRAY SHADES

Part II: Practical Use of Water Vapor Imagery and Dynamical Fields

Chapter 3: Significant Water Vapor Imagery Features Associated with Synoptic Dynamical Structures

Publisher Summary

3.1 INTERPRETATION OF SYNOPTIC-SCALE LIGHT AND DARK IMAGERY FEATURES

3.2 MID- TO UPPER-TROPOSPHERE WIND FIELD

3.3 BLOCKING REGIMES

3.4 CYCLOGENESIS

3.5 WV IMAGERY ANALYSIS OF MAIN INGREDIENTS OF A SEVERE WEATHER SITUATION

3.6 SUMMARY

Chapter 4: Use of Water Vapor Imagery for Assessing NWP Model Behavior and Improving Forecasts

Publisher Summary

4.1 OPERATIONAL USE OF THE RELATIONSHIP BETWEEN PV FIELDS AND WV IMAGERY

4.2 SYNTHETIC (PSEUDO) WATER VAPOR IMAGES

4.3 COMPARING PV FIELDS, WV IMAGERY, AND SYNTHETIC WV IMAGES

4.4 AGREEMENT AMONG THE WV IMAGE, THE PV FIELD, AND THE SYNTHETIC WV IMAGE/NWP MOISTURE DISTRIBUTION

4.5 INSTANCES OF MISMATCH BETWEEN THE SYNTHETIC WV IMAGE/NWP MOISTURE DISTRIBUTION AND THE PV FIELD

4.6 MISMATCH BETWEEN THE WV IMAGE AND THE PV FIELD AND AGREEMENT BETWEEN THE PV FIELD AND THE SYNTHETIC IMAGE/NWP MOISTURE DISTRIBUTION

4.7 USING SATELLITE AND SYNTHETIC WV IMAGES AND PV CONCEPTS TO GET AN ALTERNATIVE NUMERICAL FORECAST

4.8 SUMMARY

CONCLUSION

APPENDIX A: RADIATIVE TRANSFER THEORY AND SOME RADIATION EFFECTS FOR THE WV CHANNELS OF METEOSAT, GOES, AND MSG

APPENDIX B: SYNTHETIC (PSEUDO) WATER VAPOR IMAGES

APPENDIX C: PV MODIFICATION TECHNIQUE AND PV INVERSION TO CORRECT THE INITIAL STATE OF THE NUMERICAL MODEL

APPENDIX D: GLOSSARY OF ACRONYMS

REFERENCES

INDEX

Preface

The main purpose of this book is to provide weather forecasters and operational meteorologists with a practical guide for interpreting water vapor channel imagery in combination with dynamical fields to enable weather analysis and forecasting.

Recent developments in dynamic meteorology have indicated the relevancy of using potential vorticity fields in operational meteorology. This guide illustrates this approach by presenting the reader with current techniques for interpreting water vapor imagery in association with the characteristics of the synoptic situation. The book focuses on numerous examples showing superimpositions between operational model fields and satellite images and includes brief explanations, where appropriate, of the role of imagery in a forecasting environment. Conceived as a practical training manual for weather forecasters, the book will be of interest and value to university students as well.

Acknowledgments

This manual has been developed in Météo-France in the framework of cooperation with the National Institute of Meteorology and Hydrology of Bulgaria. The authors are grateful to Jean Coiffier for many helpful suggestions and valuable discussions during the studies as well as for his assistance in the reproduction of many of the drawings. Computer and software support was provided by Fabienne Dupont. The process of potential vorticity modification and inversion for developing the material in Chapter 4 was performed with the assistance and collaboration of Philippe Arbogast. The material in Sections 4.6.2 and 4.6.3 was developed in cooperation with Francisco Martín León from Servicio de Técnicas de Análisis y Predicción (STAP) of Instituto Nacional de Meteorología (INM), Madrid. The illustrations in Sections 3.3.2, 4.6.2, and 4.6.3, and some of those in Section 4.1.3 used output fields from the Spanish version of the HIRLAM model, which were kindly provided by STAP department of INM. All other illustrations made by using satellite imagery (Meteosat 7 satellite of EUMETSAT) and numerical model fields are the property of Météo-France, which has funded the work on the manual. Special thanks are also due to the anonymous reviewers, as well as to the developmental editor David Couzens, for their generous contributions of time and insight.

Introduction

Water vapor (WV) channel images from Meteosat and other geostationary satellites serve operational forecasters as a valuable tool for synoptic-scale analysis. Since WV images represent radiation emitted by water vapor in the middle and upper troposphere, they provide useful information on the flow patterns at these altitudes. Obtaining a better understanding of important large-scale atmospheric processes calls for diagnosing imagery jointly with meteorological fields showing atmospheric circulation at mid- and upper levels. Such dynamical fields include absolute vorticity or potential vorticity owing to their close relationship with WV channel radiance, and these fields can be displayed in several ways.

Circulation and vorticity have been recognized as helpful quantities since the beginning of the 20th century and, on this basis, potential vorticity theory was first developed by Rossby and Ertel in the late 1930s. Although potential vorticity (PV) was introduced as a dynamic atmospheric parameter in the early 1940s, its application was limited, mainly because of the complexity involved in calculating PV fields. With the advent of modern computer technology and its application to meteorology, various computer-generated PV fields started appearing in 1964. Hoskins et al. (1985) acknowledged the analysis of isentropic PV maps as a crucial diagnostic tool for understanding dynamical processes in the atmosphere. As a consequence, there has been enormously increased interest in using potential vorticity for diagnosing atmospheric behavior, especially of cyclogenesis, for research and operational forecasting purposes.

A superposition of potential vorticity fields and satellite water vapor channel images shows a close relationship in the circulation systems of extratropical cyclones. The relationship facilitates image interpretation and helps to validate numerical weather prediction (NWP) output. A mismatch between the vorticity fields and the imagery can indicate a model analysis or forecasting error. The relationship has also been applied to the adjustment of initial fields in NWP (e.g., Pankiewicz et al., 1999; Swarbrick, 2001).

Part I presents the fundamentals essential for understanding the more specific material presented in Part II. Chapter 1 presents basic points of atmospheric dynamics. Chapter 2 describes the information content of radiances measured by satellites in water vapor channels and illustrates the approach for interpreting imagery gray shades.

Part II focuses on operational applications. Water vapor images are matched with various fields to provide operational forecasters with knowledge about the relationship between the potential vorticity distribution and the satellite images. Chapter 3 illustrates the dynamical insight offered by WV imagery for interpreting the evolution of significant synoptic-scale circulation patterns. Chapter 4 is the core of the book. It is focused on the problem of validating NWP fields from analyses and early forecasts. A methodology is presented for helping to improve operational forecasts by comparing PV fields, satellite WV imagery, and pseudo WV images, which are synthetic products of the numerical model.

Although much of the material in Chapters 2 and 3 has appeared elsewhere, it was necessary to integrate it here to enable a better understanding of the new material discussed in Chapter 4.

Both Chapters 3 and 4 conclude with summaries, which will let you refer easily to any of the specific interpretation problems discussed in the book.

Part I

Fundamentals

Outline

Chapter 1: A Dynamical View of Synoptic Development

Chapter 2: The Interpretation Problem of Satellite Water Vapor Imagery

CHAPTER ONE

A Dynamical View of Synoptic Development

Publisher Summary

This chapter discusses the dynamic structures of atmosphere at the synoptic scale. In mid-latitudes, at synoptic scale, the important dynamic properties are those related to the rotation of air particles. This rotation is linked both to the motion of the Earth and to the rotation component of the wind. The rotation of fluid particles is described by the variable vorticity. Vorticity is a measure of the local rotation or spin of the atmosphere. It is the key variable of synoptic dynamics. Synoptic development can also be viewed in terms of vertical velocity associated with the evolution of vorticity in the middle and upper troposphere. Such a quasi-geostrophic approach can be used for the purposes of subjective analysis to diagnose the vertical circulation associated with a large-scale distribution of pressure and temperature. Together with quasi-geostrophic theory, potential vorticity (PV) thinking has proven to be useful for viewing and understanding synoptic development in mid-latitudes.

1.1. VORTICITY AND POTENTIAL VORTICITY

1.2. THE CONCEPT OF PV THINKING

1.3. OPERATIONAL USE OF PV FIELDS FOR MONITORING SYNOPTIC DEVELOPMENT

1.1 VORTICITY AND POTENTIAL VORTICITY

Some meteorological parameters are more effective than others for studying the appearance and evolution of dynamical structures at the synoptic scale. The conservative fields—those that remain unchanged when one follows a particle of fluid in motion—are best suited for detecting and monitoring the structures that play various key roles in a meteorological scenario. With the assumption of adiabatic motions, the potential temperature θ and wet-bulb potential temperature θw are thermodynamics tracers for the air particles. They allow us to compare the thermal properties of air particles without taking into account the effects due to thermal advection and pressure changes. However, they represent only a few of the important properties that determine the evolution of the atmosphere. To better understand the observed phenomena, dynamic properties also must be taken into account.

In mid-latitudes, at synoptic scale, the important dynamic properties are those related to the rotation of air particles. This rotation is linked both to the motion of the Earth and to the rotation component of the wind. The rotation of fluid particles is described by the variable vorticity. Vorticity is a measure of the local rotation or spin of the atmosphere: It is the key variable of synoptic dynamics. As illustrated in Figure 1.1, the vorticity vector gives the direction of the spin axis, and its magnitude is proportional to the local angular velocity about this axis. The fluid particles turn around their vorticity vector and the absolute vorticity is equal to the relative spin around a local cylinder plus the rotation of the coordinate system.

FIGURE 1.1 A vorticity vector and the local rotation in the atmosphere indicated by the circulation around a cylinder of air oriented along the vorticity vector. (Adapted from Hoskins, 1997.)

To interpret a process in terms of quasi-geostrophic theory, only the vertical component of the vorticity equation is explicitly considered. The vertical component of absolute vorticity is ζ = f + ξ, where f is the Coriolis parameter and the relative vorticity is given by

It is also supposed that, at synoptic scale, the Earth’s rotation dominates (i.e., ζ ≥ f), in which case the relative vorticity equation contains only stretching and shrinking of this basic rotation (Hoskins, 1997). Two examples are presented in Figure 1.2. Along the zero vertical motion at the ground, we can make the following observations:

FIGURE 1.2 Tropospheric (a) ascent and (b) descent that leads to, respectively (a) stretching and (b) shrinking of vorticity associated with (a) an increase and (b) a decrease of vorticity and circulation. (Adapted from Hoskins, 1997.)

• Tropospheric ascent implies stretching and the creation of absolute vorticity greater than f; that is, cyclonic relative vorticity, in the lower troposphere.

• Similarly, tropospheric descent implies shrinking and creation of relative anticyclonic vorticity in the lower troposphere.

• If the initial relative vorticity is zero, the two situations in Figures 1.2a and b correspond to cyclonic and anticyclonic surface development.

Consistent with this discussion, synoptic development can be viewed in terms of vertical velocity (derived in the framework of the quasi-geostrophic theory) associated with the evolution of vorticity in the middle and upper troposphere. Pedder (1997) shows that such a quasi-geostrophic approach can be used for the purposes of subjective analysis to diagnose the vertical circulation associated with a large-scale distribution of pressure and temperature.

Together with quasi-geostrophic theory, PV thinking has proven to be quite useful for viewing and understanding synoptic development in mid-latitudes (for theoretical background and references see Hoskins et al., 1985, which contains an exhaustive review of the use of potential vorticity). A simple isentropic coordinate version of PV is given by the expression

(1)

where

(2)

is the air mass density in xyθ space, θ is the potential temperature, p is the pressure, g is the acceleration due to the gravity, and

(3)

is the absolute isentropic vorticity.

Equation (1) says that potential vorticity is a product of the absolute vorticity and the static stability. The units commonly used for the presentation of PV are 10−6 m² s−1 K kg–¹, termed the PV unit (PVU).

Three properties underlie the use of potential vorticity for representing dynamical processes in the atmosphere:

1. The familiar Lagrangian conservation principle for potential vorticity, which states that if one neglects the contributions from diabatic and turbulent mixing processes, then the PV of an air parcel is conserved along its three-dimensional trajectory of motion.

2. The principle of invertibility of the PV distribution, which holds whether or not diabatic and frictional processes are important. Given the PV everywhere and suitable boundary conditions, then Equation (1) can be solved to obtain, diagnostically, geopotential heights, wind fields, vertical velocities, θ, and so on under a suitable balance condition, depending on access to sufficient information about diabatic and frictional processes.

3. Together with the two principles, another property of PV that allows its use as a concept for describing and understanding atmospheric dynamics is the specific climatological distribution of PV.

1.2 THE CONCEPT OF PV THINKING

1.2.1 The Conservation Principle

Conservation of PV enables us to identify and follow significant features in space and time. In Figure 1.3, we consider a small vorticity tube whose lower section is at a potential temperature θ and whose upper section is at the potential temperature θ + dθ. In a dry atmosphere moving adiabatically, this small cylindrical element with a constant mass necessarily moves between these constant potential temperature surfaces (iso-θ), with each particle preserving its potential temperature. Since the vorticity tube follows the two iso-θ surfaces, the quantity dθ remains constant. At the same time, the PV should be preserved for the fluid element during evolution of the tube. Thus, when h increases (decreasing the θ gradient), the vorticity also increases, and conversely, when h decreases, the vorticity decreases. The stretching/shrinking effect on the vorticity tube bounded by the two isentropic surfaces therefore coincides with the variation of the θ gradient.

FIGURE 1.3 Conservation of the potential vorticity during the descent of a vorticity tube along two iso-θ surfaces.

So, the conservation of potential vorticity in the atmosphere induces changes by the stretching/shrinking effect. The transport of a maximum of PV affects the synoptic flow and, as a consequence, produces vertical motion. From an operational point of view, PV thereby provides a very powerful and succinct view of atmospheric dynamics. Superimposing various PV fields onto a satellite image is a natural diagnostic tool, well suited to making dynamical processes directly visible to the human eye. In particular, a joint interpretation of upper level PV fields and water vapor imagery provides valuable information because PV structures