Distribution of hydrometeors in monsoonal clouds over the South American continent during the austral summer monsoon: GPM observations

Abstract

The Global precipitation measurement (GPM) was launched in February 2014 and provides the three-dimensional attenuated corrected radar reflectivity factor (Zₑ) along with the raindrop size distribution (DSD) parameters. The DSD parameters consist of the mass-weighted hydrometeors size (Dm in mm) and normalized hydrometeors concentration (Nw in mm⁻¹ m⁻³). The present study investigates the vertical and spatial distribution of hydrometeors in intense convective clouds that form over South America (SA) during Austral summer monsoon seasons. We defined Cumulonimbus towers (CbTs) and intense convective cells based on 8 km (ICC8s) and 3 km (ICC3s), using vertical profile of radar reflectivity algorithm and then their properties are explored over eight selected areas over SA. CbT is defined by using the Zₑ≥20 dBZ at 12 km with base height less than 3 km altitude. The ICCs are defined by using the Zₑ thresholds at 8 and 3 km altitude, and Zₑ threshold value belongs to the top 5% of the Zₑ value at the reference height. Subtropical areas including Sierra de Cordoba (SDC) and La Plata basin (LPB) consist of a higher frequency of CbTs and ICC8s, whereas ICC3 is nearly uniformly distributed over the SA continent and the Atlantic Ocean (AO). Eastern foothills of the Andes mountain also consist of a higher frequency of CbTs and ICC8s. Irrespective of the height and Zₑ thresholds used in the present study, the SDC and LPB consist of the most intense convective clouds with higher echo top altitude, and similar to that observed over Western Himalaya foothills over South Asia. Land and ocean differences are visible in the cloud cells based on the 3 km reference height, which is well below the freezing level. CbTs and ICC8s do not show the land and ocean differences as in both the cases the AO has comparable Zₑ in average vertical profiles compared to the land areas, but in ICC3, the AO has the weakest cloud cells with the least cloud top height. The absolute slope of Zₑ in mixed-phase altitude is highest in ICC3 and reflect the local precipitation fallout in mixed-phase regions and suggested that fewer hydrometeors of larger sized are lifted in the upper atmosphere. The least slope over SDC and Central Foothills indicates that higher sized of hydrometeors are lifted in the upper atmosphere in the monsoonal clouds compared to other parts of SA continent. The DSD parameters indicate that in general, intense cloud cells consist of large-sized hydrometeors although their concentration is low. The spatial averages of the DSD parameters also indicate larger sized of hydrometeors exist on average for all types of clouds at 3 and 8 km in southeastern areas of SA including SDC, LPB, and Brazilian highlands. The single meteorological feature is not responsible for the intense and deep convection, but the combined meteorological variables are responsible for producing the intense and deeper convection.