Atmospheric moist convection Peter Bechtold Research Department March 2009 – last revised 4 March 2019 Atmospheric Moist Convection Table of Contents PREFACE ................................................................................................................................................................. 4 1 THE NATURE OF MOIST CONVECTION ................................................................................................... 5 1.1 INTRODUCTION .................................................................................................................................................. 5 1.2 TROPICAL METEOROLOGY AND CLIMATE .......................................................................................................... 6 1.2.1 Precipitation and radiative convective equilibrium ...................................................................................... 7 1.2.2 Cloud distributions ....................................................................................................................................... 8 1.3 TROPICAL CIRCULATIONS ................................................................................................................................ 10 1.3.1 The Hadley and Walker circulation ............................................................................................................ 10 1.3.2 Tropical Waves ........................................................................................................................................... 11 1.3.3 The equatorial Kelvin and Rossby wave ..................................................................................................... 11 1.3.4 African easterly waves ................................................................................................................................ 13 1.3.5 Tropical waves and the MJO ...................................................................................................................... 14 1.3.6 Summary of tropical motions and scales .................................................................................................... 15 1.4 MIDLATITUDE CONVECTION AND ITS SYNOPTIC AND OROGRAPHIC FORCING ................................................... 16 1.5 THE DIURNAL CYCLE OF CONVECTION ............................................................................................................. 20 1.6 BUOYANCY AND THE PARCEL OR PLUME METHOD ........................................................................................... 23 1.6.1 Buoyancy ..................................................................................................................................................... 23 1.6.2 Convective Available Potential Energy (CAPE) ......................................................................................... 25 1.6.3 Mixing ......................................................................................................................................................... 27 1.7 LARGE-SCALE EFFECTS OF CONVECTION .......................................................................................................... 29 1.7.1 Q1,Q2 and Q3 ............................................................................................................................................. 29 1.7.2 Mesoscale convective systems ..................................................................................................................... 33 1.7.3 Quasi-equilibrium ....................................................................................................................................... 35 1.8 SUMMARY ........................................................................................................................................................ 35 2 PARAMETRIZATION OF CONVECTION ..................................................................................................36 2.1 AIMS OF CONVECTIVE PARAMETRIZATION ....................................................................................................... 37 2.2 TYPES OF CONVECTION SCHEMES ..................................................................................................................... 37 2.3 THE “KUO” SCHEME ........................................................................................................................................ 38 2.4 ADJUSTMENT SCHEMES: THE BETTS-MILLER SCHEME ..................................................................................... 38 2.5 THE MASS FLUX APPROACH .............................................................................................................................. 39 2.5.1 Derivation of the eddy fluxes ...................................................................................................................... 40 2.5.2 Mass flux and Q1-Q3 ................................................................................................................................... 42 2.5.3 Mass flux entraining detraining plume model ............................................................................................ 42 2.5.4 Large scale cumulus effects ........................................................................................................................ 44 2.5.5 Convective closure ...................................................................................................................................... 44 2.6 SUMMARY ........................................................................................................................................................ 45 3 THE IFS CONVECTION PARAMETERIZATION ......................................................................................46 3.1 BASIC LARGE-SCALE CONVECTIVE TENDENCIES .............................................................................................. 47 3.2 TRIGGERING OF CONVECTION .......................................................................................................................... 48 3.3 CLOUD MODEL EQUATIONS: UPDRAUGHTS ....................................................................................................... 49 3.4 CLOUD MODEL EQUATIONS: DOWNDRAUGHTS ................................................................................................. 50 3.5 ENTRAINMENT AND DETRAINMENT .................................................................................................................. 50 3.6 MICROPHYSICS AND PRECIPITATION ................................................................................................................ 52 2 Meteorological Training Course Lecture Series Atmospheric Moist Convection 3.7 CLOSURE .......................................................................................................................................................... 52 3.7.1 Deep convection .......................................................................................................................................... 52 3.7.2 Shallow convection ..................................................................................................................................... 54 3.7.3 Mid-level convection ................................................................................................................................... 55 3.8 NUMERICS ........................................................................................................................................................ 55 3.8.1 Vertical discretization and conservation .................................................................................................... 55 3.8.2 Explicit solution .......................................................................................................................................... 56 3.8.3 Implicit solution .......................................................................................................................................... 56 3.8.4 The semi-lagrangien solution ..................................................................................................................... 57 3.9 TRACER TRANSPORT EXPERIMENTS .................................................................................................................. 57 3.9.1 Numerical stability ...................................................................................................................................... 58 3.9.2 Comparison SCM, CRM and global simulations ........................................................................................ 59 4 FORECASTING - CASE STUDIES ................................................................................................................61 4.1 1-4 DECEMBER 2003 FRENCH FLOOD ............................................................................................................... 61 4.2 CONVECTIVE ADJUSTMENT .............................................................................................................................. 65 4.3 PARTLY OROGRAPHICALLY FORCED CONVECTION OVER IBERIAN PENINSULA ................................................. 67 4.4 FORECASTED SATELLITE IMAGERY................................................................................................................... 69 ACKNOWLEDGEMENTS .....................................................................................................................................70 5 APPENDIX: SIMPLE