Thermodynamic Characteristics of Intense Convective Events Over the Mexico City Valley

Abstract:

There has been ample discussion in the community regarding the nature and origin of extreme precipitation trends in the Mexico City Valley where, on the one hand, emissions of gases and particulate matter can potentially alter the pathways of mycrophysical processes,that lead to intense precipitation and, on the other, extensive land use land cover change (LULC) in the Valley over the last 50 years has radically changed thermodynamic conditions under which local convection takes place. In this study we extend the work by Ochoa et al 2015 to include an in-depth analysis of intense convective events in the light of observational and simulated data. We use three independent observational data sets: 1) 24­hour accumulated precipitation from a very dense hydrological network in Mexico City, 2) cloud top temperature (CTT) from GOES satellite data and, 3) CAPE data from daily morning soundings. These 3 data sets help to define a measure of intense convective events during the month of September from 2002 to 2011. A subset of intense convection and precipitation events is obtained and its spatial statistics are analyzed. Preliminary results indicate intense precipitation events to be strongly dependent on orography where the upper decile of precipitation and CTT are found at the piedmont on the western side of the Valley. Analyses of daily CTT, for these events, reveal two patterns of convection: 1) initiating over the mountain top and descending over the Valley and, 2), convective systems entering the Valley from its north eastern end, forced by the easterlies. The observed spatial statistics of CTT is roughly matched in a cloud­resolving simulation with the Weather Research and Forecasting (WRF) for ten Septembers from 2002 to 2011, that uses the 1993 USGS Land Use/Land Cover (LULC). Additional simulations are performed with a reduced urban version of the 1993 USGS LULC. Preliminary results show important changes in the initiation of convection and on the spatial patterns of precipitation.