Cold‐pool‐driven convective initiation: using causal graph analysis to determine what convection‐permitting models are missing. (5th May 2020)

• Record Type: Journal Article
• Authors: Hirt, Mirjam; Craig, George C.; Schäfer, Sophia A. K.; Savre, Julien; Heinze, Rieke
• Is Part Of: Quarterly journal of the Royal Meteorological Society. Volume 146:Number 730(2020)
• Page Start: 2205
• Page End: 2227
• Publication Date: 2020-05-05
• Subjects: causal effect estimation -- numerical weather prediction -- convection organization; Meteorology -- Periodicals; 551.5
• DOI: 10.1002/qj.3788
• Languages: English
• ISSNs: 0035-9009

Abstract

Abstract: Cold‐pool‐driven convective initiation is investigated in high‐resolution, convection‐permitting simulations with a focus on the diurnal cycle and organization of convection and the sensitivity to grid size. Simulations of four different days over Germany were performed using the ICON‐LEM model with grid sizes from 156 to 625 m. In these simulations, we identify cold pools, cold‐pool boundaries and initiated convection. Convection is triggered much more efficiently in the vicinity of cold pools than in other regions and can provide as much as 50% of total convective initiation, in particular in the late afternoon. By comparing different model resolutions, we find that cold pools are more frequent, smaller and less intense in lower‐resolution simulations. Furthermore, their gust fronts are weaker and less likely to trigger new convection. To identify how model resolution affects this triggering probability, we use a linear causal graph analysis. In doing so, we postulate a graph structure with potential causal pathways and then apply multi‐linear regression accordingly. We find a dominant, systematic effect: reducing grid sizes directly reduces upward mass flux at the gust front, which causes weaker triggering probabilities. These findings are expected to be even more relevant for km‐scale, numerical weather prediction models. We thus expect that a better representation of cold‐pool‐driven convective initiation will improve forecasts of convective precipitation. Abstract : By comparing cold‐pool‐driven convective initiation in different, high‐resolution model simulations, we find that cold pools are more frequent, smaller, less intense and trigger less new convection in lower‐resolution simulations. We further use a linear causal graph analysis to understand how model resolution influences triggering probability. We find that reducing grid sizes directly reduces upward mass flux at the gust front, which causes weaker triggering probabilities.