Remote Sensing Tropical Cyclone Rainfall over the Southwest Pacific Region Anil Deo Submitted in total fulfilment of the requirements for the degree of Doctor of Philosophy School of Earth Sciences The University of Melbourne Melbourne, Australia 18 April 2018 This thesis is dedicated to my wife Madhu Malti Devi and daughter Adria Vaneesha Deo. Without their support and patience, the successful completion of this work would not have been possible. ii Abstract The south-west Pacific (SWP) region is susceptible to the catastrophic effects of tropical cyclones (TCs). The region, therefore, has received adequate attention in terms of scientific research pertaining to TC genesis, tracks and intensity. It (especially the island countries), nonetheless, has not received much attention, in comparison to studies elsewhere (e.g. the Atlantic and western North Pacific region), on additional important aspects of TCs such as rainfall estimation and characteristics of the TC-related rainfall drop size distribution (DSD). The latter has implications for radar rainfall estimation and cloud modelling studies. In this thesis, we first validate the Tropical Rainfall Measuring Mission (TRMM) Multi- satellite Precipitation Analysis (TMPA) 3B42 quantitative precipitation estimates (QPEs) during the passage of TCs over New Caledonia and Fiji. It is shown that TMPA has skill in representing the rainfall during the passage of TCs over New Caledonia and Fiji. TMPA overestimates light rainfall and underestimates moderate to heavy daily rainfall. The skill deteriorates with increasing elevation, as underestimation is greater at large altitudes. The ability of TMPA also varies with TC intensity and distance from the TC centre, whereby it is more skilful for less intense TCs (category 1-2) and near the TC centre than in the outer rainbands. The rainfall DSD during the passage of TCs over Darwin, Australia, is evaluated next and this is compared with the DSD associated with non-tropical cyclone (non-TC) events. It is shown that the TC-related DSD is statistically different from the non-TC related DSD, the former encompassing a larger concentration of small to moderate size drops. The TC related drop diameter is lower than the non-TC values at all rain rates and also for the different precipitation types (convective, transition and stratiform). The TC DSD also varies with distance from the TC centre, as rainfall near the TC centre comprises of relatively smaller drops which are strongly evident at small to moderate rainfall rates (< 30 mm hr-1). These iii variations in the DSD have implications for the parameters used in the algorithm that converts radar reflectivity to rainfall rate, as well as for the analytical expressions used in describing the observed DSD employed in cloud modelling parameterizations. Finally, the feasibility of estimating the DSD parameters using (i) the TRMM precipitation radar (PR) and (ii) a combined PR and TRMM Microwave Imager (TMI) algorithm (COM) is investigated using the Darwin C-band dual polarised (C-POL) ground radar (GR) as the reference. The correspondence of the TRMM instruments with the GR is generally dependent on the precipitation type: the PR and the COM usually overestimate (underestimate) the reflectivity and the rainfall rate from events that are highly stratiform (convective) whereas they mostly overestimate the median volume diameter (Do) of all rainfall types whereby the overestimation increases with an increase in the percentage of convective fraction. Also, the COM reflectivity estimates are similar to the PR estimates but it has a smaller bias in the Do for most of the greater stratiform events. This suggests that combining the TMI with the PR adjusts the Do towards the “correct” direction if the GR is taken as the reference. Moreover, for the TC events considered in this study, the association of the TRMM estimates with the GR is similar to the highly stratiform non-TC events (there is no significant difference) but it differs largely from the majority of the highly convective non-TC events. iv Declaration This is to certify that: (i) this thesis comprises only my original work towards the Doctor of Philosophy except where indicated, (ii) due acknowledgement has been made in the text to all other material used, (iii) the thesis is less than 100,000 in words in length, exclusive of tables, maps, bibliographies and appendices. _______________________________________________________ v Acknowledgements This research would not have been successful without the kind support of various people and institutions. A sincere appreciation is extended to the Australian Government Department of Education for funding my PhD at the University of Melbourne through the Endeavour Postgraduate Award. A hearty thanks to my supervisor Prof. Kevin Walsh for his expert guidance and support during my candidature. Thanks, also to the two PhD panel members, Dr. Robyn Schofield and Dr. Richard Dare, for their guidance and advice. I would also like to extend my gratitude to Dr. Stephen Munchak of the Goddard Space Flight Centre (GSFC), NASA and Dr. Alexander Peltier of Meteo-France, New Caledonia for their kind contribution to certain aspects of this thesis. Thanks to Dr. Ravin Deo of Monash University, Dr. Savin Chand of Federation University Australia, Dr. Visagaperuman Ramachandran of Fiji National University and Prof. Sushil Kumar of the University of the South Pacific for their support and guidance during the PhD application process. I would also like to thank Drs. Alain Protat, Michael Whimpey, Valentin Louf, Yen Jun Chen, and Surendra Rainuyar from the Bureau of Meteorology and Dr. Robert Warren of Monash University for their support on certain aspects of this project. Thanks to the Fiji Meteorological Services and Swastika Sharan from the same institution and the Australian Bureau of Meteorology for providing the necessary data. I would also like to acknowledge the Melbourne International Fee Remission Scholarship for covering the tuition gap. Thanks to the ARC Centre of Excellence for Climate System Science and its respective institutions for funding my attendance to workshops, winter school and a visit to the GSFC NASA laboratory. Special thanks to Dr. Munchak and Jasmine Smith from GSFC for providing the necessary support during my visit. Thanks to the members of the School of Earth Sciences especially Toni Fraser, Charlotte Mcloughlin and Katrina Swell for their administrative support. My appreciation vi also to Cheryl Goult, Laveena Lobo and Ashleigh Cook from Scope Global Pty Limited, responsible for managing the Endeavour Awards, for their caring and support. Finally, a sincere graduate to my parents, Mrs. Gyan Wati and Late Mr. Prem Deo, who passed away during my final year of candidature, my wife Madhu Devi, my daughter Adria Deo, my brother Ashneel Deo and my in-laws Mr. and Mrs. Anand Kumar for their continued support. vii Publications The thesis comprises the following manuscripts which are either published or submitted for publication in international referred journals: Chapter 4: Deo, A., Walsh, K.J.E., Peltier, A., 2016. Evaluation of TMPA 3B42 precipitation estimates during the passage of tropical cyclones over New Caledonia. Theor. Appl. Climatol.1-17. Chapter 5: Deo, A., Walsh, J.E., 2016. Evaluation of TRMM multi-satellite precipitation analysis during the passage of tropical cyclones over Fiji. Journal of Southern Hemisphere Earth Systems Science, 66, 442-456. Chapter 6: Deo, A., Walsh, K.J.E., 2016. Contrasting tropical cyclone and non-tropical cyclone related rainfall drop size distribution at Darwin, Australia. Atmos. Res., 181, 81-94. Chapter 7: Deo, A., Munchak, S.J., Walsh, K.J.E., 2018. Cross-validation of rainfall characteristics estimated from the TRMM PR, a combined PR-TMI algorithm and a C-POL ground-radar during the passage of tropical cyclone and non- tropical cyclone events over Darwin, Australia. J. Atmos. Oceanic Technol (Accepted for Publication, October 2018) viii Contents 1 Introduction and Objectives ..................................................................................... 1 1.1 Introduction .................................................................................................................. 1 1.2 Aims and Objectives .................................................................................................... 4 1.3 Thesis Outline .............................................................................................................. 5 2 Literature Review ...................................................................................................... 6 2.1 TC Genesis ................................................................................................................... 7 2.2 Quantitative Precipitation Estimation (QPE) during the passage of TCs .................... 7 2.2.1 Evaluation of TRMM-based precipitation products during the passage of TCs .. 9 2.3 Cloud Dynamics and Microphysics ........................................................................... 11 2.3.1 Warm Clouds (liquid phase) ............................................................................... 11 2.3.2 Cold clouds (mixed phase) ................................................................................. 13 2.3.3 Dynamical and Microphysical Processes in Stratiform and Convective Precipitation ........................................................................................................ 16 2.4 Empirical Formulation of the DSD ...........................................................................