Using Multiple Doppler Retrieval to Investigate Linkages between Boundary Layer Convergence and Convective Initiation

The CACTI/RELAMPAGO campaigns took place in Argentina near the Sierras de Cordoba (SdC), where some of the most intense thunderstorms on the planet occur. During CACTI/RELAMPAGO, a dense network of sensors was deployed near the SdC, providing exceptional observations of severe weather. While deep convection was observed on numerous days during the campaign, the cloud population observed on December 13–14, 2018, are examined more closely as a case study. This case is known for being one of the most intense upscale growth events during the campaign. In addition, CI occurred within range of multiple scanning precipitation radars, providing a baseline for Doppler retrievals.

Radar Doppler velocities at low-elevation scans provide an excellent tool to monitor the wind flow near the surface. The radars can observe non-meteorological echoes prior to convective initiation (CI) that can be used to describe the direction and magnitude of low-level flow, including diagnosing where convergent boundaries were present. Furthermore, the 3D wind field can be obtained with multiple radar observations. However, dual Doppler retrievals are more challenging to obtain near the surface and prior to CI because of beam blockage and the weak returns of non-meteorological echoes. However, with extensive data quality control, the 3D winds can be obtained from these non-meteorological echoes in some locations. In this study, multiple C-band radar observations from the CACTI/RELAMPAGO field campaign were used to quantify the strength of observed near-surface convergent boundaries previously detected in point observations at scattered surface weather stations or vertically pointing instruments. Specifically, non-meteorological echoes at low-elevation scans from the CSAPR-2, CSU-CHIVO, RMA-1, and COW radars were used to retrieve the wind direction near the surface to understand the interaction of the winds with the terrain and whether deepening convection occurred along previously existing shallow convergent boundaries below the level of free convection. The radar observations indicate that a zonally long and shallow convergent boundary (about 1 km deep and several hundred km long) progressed northward through and east of the radar domains. It resembled a large northward moving cold pool that appeared to be linked to CI just east of the SdC range. WRF simulations of this case are leveraged to expand the spatially limited observational dataset and better describe how convection interacted with the low-level convergent boundary as well as the spatially variable thermodynamic field ahead of and behind the boundary.