Cloud-scale Simulations of Convection Initiation using Observed Near-cloud Environments from RELAMPAGO-CACTI

Abstract:

The 2018–2019 Remote Sensing of Electrification, Lightning, and Mesoscale/Microscale Processes with Adaptive Ground Observations (RELAMPAGO) and Cloud, Aerosol, and Complex Terrain Interactions (CACTI) projects sought to observe the near-cloud environments of convection initiation by deploying a high spatiotemporal resolution radiosonde network. Previous analysis of these atmospheric soundings (companion presentation in this session) illustrate that Null environments (those producing no precipitating convection despite numerical forecasts from convection-allowing models) contain arguably larger potential to support deep moist convection than environments observed during days when precipitating convection occurred. However, drier and warmer free tropospheres may have limited the ability for CI to occur. In this study, we conduct cloud-scale large eddy simulations using Cloud Model 1 (CM1) in an idealized framework, with 250-m horizontal resolution and homogeneous background state defined by our composite near-cloud soundings to explore physical processes related to interaction of cloud-scale thermal and environmental interactions including vertical wind shear, stability, and moisture.

Preliminary results show that relatively dry free tropospheres, characteristic of the mean Null environments, did not inhibit the growth of convection. Most simulations utilizing the Null-case mean environment, with simplified flat-terrain and a single prescribed warm thermal, produce deeper and more robust convection than the mean environments supporting observed convection. When a prescribed thermal is placed near the top of idealized terrain, however, a strong low-level stable layer in the mean Null environment prevents the developing thermal from rising above the boundary layer, whereas environments from observed CI events produce weak convection but are inhibited by shear. More complex scenarios using slightly more realistic conditions somewhat more realistically produce observed convective outcome: more robust convection forms in the observed CI environments, while delayed initiation of weaker convection occurs using the observed mean Null environment. These results also suggest that successive boundary layer thermals are crucial for obtaining long-lasting and deep convection.