Characterizing the raining systems observed in the metropolitan area of São Paulo – Brazil

The metropolitan area of São Paulo (RMSP) is considered one of the megacities of the world where more than 22 million people live. RMSP is located in the state of São Paulo, Brazil, and it encompasses 39 cities in an area of 851 km2 that mixes urbanized, residential and industrialized areas, with parks, forests and deforested areas in addition of been 40 km from the coastline. RMSP has more than 20 thousand industries and more than 8 millions of automobiles travelling through the cities. Due to its tropical location, topography and proximity to the sea, RMSP has an annual precipitation above 1,600 mm and more than 50% of the total volume is observed in summertime. Over the last 80 years, RMSP experienced a precipitation increase that follows the expansion of the RMSP, industries, urbanization, deforestation as well as the population. As a result, the drainage areas decreased and flash floods have been more frequent. Therefore, to understand how the raining systems affect the RMSP it is important to depict the main precipitation features (size, duration, time of initiation, propagation direction, single or multi-cellular cells) and evaluate possible urban and anthropic effects as well as the impact of large, mesoscale and sea breeze circulation in the development of the raining systems.

In this matter, this study investigates the main features observed in the raining systems that propagated or developed over the RMSP during the year of 2019. For this analysis we have used the rain estimates from the SELEX S band dual polarization Doppler weather radar from DAEE/CTH deployed at Salesópolis city that it is 70 km distant from RMSP. The rain estimates were based Ryzhkov et al. (2005) methodology and were retrieved every 5 minutes with a resolution of 0.5 x 0.5 km2. To depict the temporal and spatial evolution of the rainstorms, we have employed TITAN algorithm to retrieve the life cycle of the individual storms that propagated or formed over the RMSP. TITAN was adjusted to use rain estimates fields instead of volumetric weather radar. To cluster the raining systems, a threshold of 1 mm/h has been defined in TITAN. Based on 2019 radar data, it was possible to identify and track 26,851 rainstorms over a radar domain of 120 km radius. 36% of those rainstorms were observed during summer, while autumn had 30%, winter showed 8% and spring presented 26%. Moreover, 40.5% of those systems have been characterized as single storms, while 59.5 % were complex, i.e., it suffered split or merge during their lifecycle. Of the total number of tracked rainstorms, 19% were observed in the RMSP area and 2.5% were classified as single storms and 16.5 % were complex type of systems. The majority of single storms had duration of one hour while complex systems vary from 2 to 4 hours. In terms of time of initiation, single storms formed between 16-18 UTC while complex storms at 18 UTC. In terms of precipitation diurnal cycle, the maximum precipitation occurred between 16-19 UTC for both type of storms. Considering only the raining systems that formed inside the RMSP area, we found that most of the single storms initiated at 17 UTC while the complex systems were at 20 UTC. For those that formed outside the RMSP and move in, we found that single storms formed around 16 UTC while complex systems do not present a well defined time of initiation. Differences were also observed in rain volume. For example, singles storms that propagate over the RMSP produced 65% of the rain volume of the complex systems. Complex systems were responsible for more than 90% of the total rain volume observed in the RMSP as they are more numerous and present larger size.

For the conference we will expand the analysis to other two more years and explore how the rain features change with the different circulations.