Temporal evolution of raindrop size distributions from mixed clouds in Mexico City

Numerical modeling studies for warm-rain predict fixed shapes for equilibrium raindrop size distributions (DSD). Generally, equilibrium DSD show a multimodal behavoir (i.e., with several peaks) and are only dependent on rainfall rate. According to these models, the shape of a DSD remains the same independently of the initial drop spectra, given that a period of time long enough is allowed for the microphysical processes (collision - coalescence – breakup) to reach steady-state conditions. On the other hand, clustered rain can be referred to as zones with different raindrop concentrations falling through an area. This is normally observed by noticing that the rain intensity changes in time over a single precipitación event. However, when rain measurements are made (for example with a rain gauge) it is possible to detect short periods of time over which rain intensity remains almost constant.

Mexico City is located in the Southwest part of the Mexico Basin at an altitude of 2,280 masl, and its rainy season occurs during the summer and autumn. This geographical location, along with the meteorological characteristics during the rainy season, allow for the development of convective clouds with warm bases (950 m above the ground), although their tops show the presence of ice particles formed and developed during the early stages of precipitation. The heigth of the 0°C isoterm is, on average, located at 1,500 meters above cloud-base, thus allowing for small ice particles to melt during their fall before reaching the ground.

In the present investigation, microphysical data obtained with two 2-D optical array spectrometer probes are analyzed in order to observe the evolution of clustered rain originating from mixed clouds in Mexico City. The observations were made at ground level and stratified in terms of time periods with similar values of rain intensity. The results show that, for periods of heavy rain, there is a trend for similar drop counts and interarrival time values when the observational time period lasts for more than 3 minutes. This may be interpreted as an indication of spatial homogeneity of rain inside a cluster and give information regarding the drop interaction processes occurring during different precipitation stages.