Lake-Breeze-Front-Like Features During Large-Scale Onshore Flow and Their Influence on Convection Initiation

Understanding the influence of the Great Lakes on convection initiation (CI) and the environment in which convection evolves remains a substantial forecasting challenge, due to the influence of the lake breeze and/or the inland-advected marine atmospheric boundary layer (MABL). It has been long known that pronounced lake-breeze fronts (LBF) develop under conditions of weak or offshore flow, with the LBF serving as a focus for CI. However, there have been fewer investigations of CI during conditions of large-scale onshore flow, where inland advected MABL may destabilize sufficiently for CI to occur in the absence of a pronounced LBF. Moreover, the classic modeling studies investigating this utilized relatively coarse grid spacing, which may result in poor representation of surface fluxes regardless of the direction of large-scale flow, and may “wash out” any areas of weak convergence and/or thermodynamic gradients embedded within large-scale onshore flow. These potential deficiencies warrant an updated study of CI during both onshore and offshore large-scale flow, through use of large-eddy simulations (LES).

This talk will present results from 24 LES that were run using the PSU/NCAR Cloud Model 1, with simulations that vary the onshore base-state wind (2.5 and 5 m s^-1), radiative forcing (15 June, 15 July, 15 August), and water surface temperature (25th, 50th, 75th, and 90th percentiles for Lake Michigan corresponding to the radiative forcing dates). In all simulations, the initial shore-parallel wind component was equal to 0 m s^-1. The base-state thermodynamic profile was constructed to allow for initiation of deep convection and was defined by a capping inversion at approximately 875 hPa (1240 m AGL) with a weakly stable layer below and a well-mixed layer above. The model domain was 200 × 20 × 18 km in the W-E, N-S and vertical dimensions, with a surface type of water in the western 50 km and land in the eastern 150 km. Inclusion of radiative parameterization, friction, and surface heat fluxes enabled development of realistic boundary layer structure and CI. Analysis of simulations will be presented in the context of offshore base-state wind simulations (to be presented separately). In all simulations an area of diffuse convergence develops, with weak return flow aloft. This feature is collocated with weak thermodynamic gradients, progresses inland with time, and separates portions of the domain where CI does or does not occur. All together, these results suggest that LBF can develop, maintain a somewhat coherent structure, and influence CI, embedded within large-scale onshore flow of at least 5 m s^-1.