Processes controlling the evolution of deep moist convection in the summertime West African Sahel Miroslav Provod University of Leeds School of Earth and Environment Submitted in accordance with the requirements for the degree of Doctor of Philosophy March 2016 The University of Leeds - ii - Declaration of Authorship The candidate confirms that the work submitted is his/her own, except where work which has formed part of jointly-authored publications has been included. The contribution of the candidate and the other authors to this work has been explicitly indicated below. The candidate confirms that appropriate credit has been given within the thesis where reference has been made to the work of others. Chapter 3 includes characterization of cold pools which has been published, with co-authors, in Monthly Weather Review. The reference is Provod, M., J.H. Marsham, D.J. Parker, C.E. Birch, A Characterization of Cold Pools in the West African Sahel, MWR, 2016. J. H. Marsham and D.J. Parker supervised the student. C.E. Birch gave advice on methods and ran model simulations (these not used in the publication, but are used throughout the thesis). This copy has been supplied on the understanding that it is copyright material and that no quotation from the thesis may be published without proper acknowledgement. The right of Miroslav Provod to be identified as Author of this work has been asserted by him in accordance with the Copyright, Designs and Patents Act 1988. © 2016 The University of Leeds and Miroslav Provod - iii - Acknowledgements Firstly, I would like to express my thanks to my academic supervisors John Marsham and Doug Parker. Their advice, patience and help have been invaluable during the completion of this thesis. John has always been there to answer my questions, give advice and provide prompt feedback on results and gave invaluable guidance during the completion of this thesis. Both John and Doug have provided some great ideas that led to results in this thesis. I would also like to thank Cathryn Birch who ran some of the Cascade simulations, the output of which has been used throughout the thesis. Cathryn also promptly and fully answered all the questions I had regarding model output. Several other members of the School of Earth and Environment provided a great help throughout the PhD. A special thanks goes to Richard Pope, who gave a great help during struggles with code and provided hours of his time to explain and teach me some complex IDL coding. Richard Rigby, in IT support, has always provided great and very prompt help during IT issues. A great thanks also goes to all the current and past people in the office, both for help when it was needed as well as for the great office environment, including: Richard Pope, Becky Jansen, Nsikanabasi Silas Umo, Sarah Dennis, Hannah Walker, Seshagirirao Kolusu, Lindsay Lee, John Prytherch, Peggy Achtert, Mark Holden, Kirsty Pringle and Tamsin Farmer. Apologies if I missed out someone. We are a team and I will really miss you! My family also deserves to be mentioned. My wife, Lucie Provodova, my daughter, Sofia Provodova, my parents, Miroslav Provod and Dana Provodova, and sister, Marketa Provodova, have been all vital to me, not only during this PhD, but throughout my life, providing support of every form. Without your help, this thesis would not be here. Thank you all!!! - iv - Abstract Deep moist convection and mesoscale convective systems (MCSs) are integral to the West African monsoon and, as the main rain-producing mechanism in the Sahel, are essential to the livelihoods of millions. Current operational forecasting models struggle to predict rainfall with a good precision. It is therefore necessary to study the processes controlling deep moist convection in detail, in order to understand them better and to be able to evaluate simulations to identify errors for future model development. In this thesis properties of cold pools from Sahelian MCSs were characterised from surface observations. It was observed that early season cold pools were stronger and drier, likely due to drier mid-levels before the monsoon onset. The properties of observed cold pools were used to evaluate a Unified Model (UM) convection-permitting simulation. The comparison showed that simulated cold pools are generally weaker than observed. Cold pools and MCS structure were further investigated in two case studies. This enabled an analysis of MCS vertical structure. Processes controlling the diurnal cycle of convection were analysed using observations and UM simulations. This showed that while surface CAPE follows a diurnal cycle with a maximum in the afternoon and minimum in the early morning, elevated CAPE was found to have a nearly opposite cycle, due to advection of high equivalent potential temperature air overnight by the nocturnal low- level jet. In addition, the low-level jet provides low-level wind shear which balances the cold-pool related vorticity and helps to maintain the MCS until morning, when the jet decays and MCSs tend to dissipate. The jet also creates moisture flux convergence overnight, supporting MCSs. Finally, mechanisms underlying storm initiation and regeneration were analysed in UM simulations, showing the roles of soil moisture boundaries, pre-existing cold pools and bookend vortices. - v - Table of Contents Declaration of Authorship ......................................................................... ii Acknowledgements .................................................................................. iii Abstract ..................................................................................................... iv Table of Contents ....................................................................................... v List of Tables ...........................................................................................viiii List of Figures ........................................................................................... ix Abbreviations ...........................................................................................xix 1. Introduction ............................................................................................ 1 1.1 Motivations and aims ..................................................................... 1 1.1.1 Thesis structure .................................................................. 4 1.2 Literature review ............................................................................ 5 1.2.1. The West African Monsoon ................................................ 8 1.2.2. Mesoscale convective systems (MCSs) in the West Afrcian Monsoon ................................................................ 12 1.2.3. Simulations of MCSs in the West African Monsoon .......... 30 2. Methods ................................................................................................ 33 2.1 Observational datasets ................................................................ 27 2.2 Met-Office Unified Model convection-permitting simulations from the Cascade project ........................................................... 30 3. A Characterization of cold pools in the West African Sahel compared with a convection-permitting model ............................. 33 3.1 Introduction ................................................................................. 33 3.2 Methods ...................................................................................... 33 3.2.1 Cold pool detection in observed surface time-series and verification using remote-sensing data ....................................... 33 3.2.2 Selection of monsoon season sub-periods ........................... 35 3.2.3 Atmosphric tide and diurnal pressure variation ..................... 36 3.2.4 Statistical evaluation of observed cold-pool properties ......... 38 3.2.5 Evaluating cold pools in the 4 km covection-permitting Cascade simultions .................................................................... 39 3.3 Characterization of observed and simulated cold pools ...........40 3.3.1 Composite cold pools .............................................................. 39 3.3.2 Thermodynamic properties of cold pools ............................ 41 3.3.3 Role of mid-level dryness ................................................... 57 3.3.4 Cold-pool winds .................................................................. 61 - vi - 3.4 Conclusions ................................................................................ 69 4. Case studies of two squall-line mesoscale convective systems ...... 73 4.1 Introduction ........................................................................ 73 4.2 Methods ............................................................................. 78 4.2.1 Data ............................................................................ 78 4.2.2 TIme-to-space conversion .......................................... 79 4.2.3 Vertical profiles ........................................................... 81 4.3 Results ............................................................................... 83 4.3.1 Case study of 22nd July 2006 squall-line MCS ............ 83 4.3.2 Case study of 31st July 2006 squall-line MCS ............. 95 4.3.3 Preliminary analysis of UM simulation of the 31st July 2006 squall-line MCS .......................................................106 4.4 Discussion & Conclusions .................................................109 4.4.1 Future work ................................................................111