Diurnal cycle of extreme convective elements in the Andes Mountains of the tropical South America

Radar reflectivity fields from a Doppler dual-polarimetric C-band radar that sits on top of the northern edge of Los Andes range in the city of Medellín, Colombia are used to evaluate the characteristics and distribution of the forms of extreme convection that occur near these inter-Andean valleys in South America. The radar that is operated by the Early Warning System of El Valle de Aburrá (SIATA) has two operational scanning modes, one that survey the volume in 4 different elevations angles (SUR), and the other that uses 4 fixed azimuths for scanning in a series of incremental elevations (RHI) to obtain fine vertical resolutions. The SUR and RHI scans are sequential recording data about every 5 minutes since January 2013 up to date, forming a unprecedented database to study the tropical composition of the population of convective cloud elements occurring in this mountainous region highly affected by extreme types of convection.

A method that combines the advantages of using both scanning strategies is used, identifying extreme convective features based on the existence of cloud systems in the form of deep convective cores (DCC, contiguous 40 dBZ echoes in RHI scans that reach at least 8 km in height), wide convective cores (WCC, contiguous 40 dBZ echoes in SUR scans with areas > 100 km²), and broad stratiform regions (BSR, stratiform echo areas in SUR scans that are greater than 5,000 km²). The rainfall observed by the radar shows a marked diurnal cycle with maximum accumulations at mid-night, a second maximum in late afternoon, and a minimum value at midday. The diurnal composite of the occurrence of those extreme echo features shows similarly a marked diurnal cycle; with DCC elements occurring at late afternoon, WCC occurring at midnight, and BSR occurring in the first hours after mid-night. The sequence of echo elements peaking at successive times during the day is closely associated with the various stages in the life cycle of the population of convective storms with mesoscale characteristics. However, the cycle seems much more rapid than the one observed in oceanic regions. In addition, composites of observational (rain gauges and microwave profiler) and reanalysis data for the times of occurrence of the identified echo features shows how the mean atmospheric state varies with the change in the population of convective elements.

The results presented in this work are critical not only for assessing the diurnal distribution of storms exhibiting different convective character and that occur in the mountainous regions of the tropics, but also helpful in providing metrics for the evaluation of the output of both mesoscale and GCM modeling frameworks that simulate convective processes and precipitation in tropical latitudes.