Measurements of Drop Size Distribution in a Megacity

The Metropolitan Area of São Paulo (MASP), Brazil, is one of the largest urban environments of the world where diabatic heating and sea breeze circulation are ingredients for very deep thunderstorms in summer time. About 70% of all major flash floods in the MASP between 1999 and 2013 were related to this type of convective system. This work is about weather radar polarimetric and disdrometer measurements of convective systems in the MASP region aimed at characterizing the drop size distribution of thunderstorms and their time evolution from the early phase to the decaying phase between September 2009 and January 2010, a total of 76 precipitating events. The morphology and vertical structure of these very deep convective systems is also analyzed with polarimetric measurements (e.g., ZDR and KDP) made with a mobile XPOL weather radar (MXPOL) as well as cloud dynamics, thermodynamics, and microphysics under favorable synoptic conditions that trigger them in the early afternoon hours, generally. The overall drop spectra and ZR relationship (a=165 and b=1.5) are presented. It is shown an instance of such very deep convective systems occurred in 2 January 2010 under heat island effect and early morning wet surface conditions that produced thunderstorm domes above 18 km altitude, normally observed in the Amazon region. The results indicate a bimodal drop size distribution at about 2 mm and 1 mm under heavy rainfall and transitions to monomodal distribution centered at about 1 mm in the decaying phase, similar to the ones observed in the Amazon.