Convective initiation in a convection permitting simulation: Land surface, PBL, and dynamic forcing

A suite of 4km grid spacing convection permitting simulations is performed to investigate convective initiation prior to a well predicted MCS observed during BAMEX on 10 June 2003. Two PBL and two LSM schemes are paired to investigate the models convective initiation processes under moderate large scale forcing. This study contrasts other CI cases where the focus has been exclusively on dryline processes.

Using the NOAH LSM and MYJ PBL schemes as a baseline, changing the PBL scheme to the YSU scheme delays CI by 2 hours while switching to the RUC LSM delays CI by 1 hour. We analyzed the CI regions among the models finding that, while conditions were favorable for convection IF it initiated, positive vertical velocity was needed for a prolonged period to initiate and sustain convection. Preliminary results indicate that dry soil patches enhanced sensible heat fluxes in an area where the PBL top was saturated. From this moist, absolutely unstable environment convection formed in regularly spaced intervals drifting NE with the mean wind. Vertical velocity bands expanded SE during the next 2 hours when deep convection produced 2 unique areas of cool, dry downdrafts forcing a gust front. The gust front helped convection organize into the MCS.

The CI processes modeled here do not differ markedly from other studies, but the environment in which CI occurred did. As convection permitting models become more widely used for assessing convective potential and evolution, understanding the model CI processes will help forecasters utilize such data in the forecast process. The implications of the physics impact and land surface state on CI will be discussed in further detail at the conference.