Thermally-driven circulation and convection over a mountainous tropical island

Observational data from the 2011 Dominica Experiment (DOMEX) and cloud-resolving numerical simulations are exploited in order to acquire a better understanding of controlling parameters of thermally-driven convection over a mountainous tropical island. Four weak (<4 m/s) background wind days are investigated to obtain a preliminary diagnosis of the conditions favorable for diurnally forced shallow cumulus convection over the island. The observations suggest that the vigor of shallow cumulus convection is largely dependent on the amount of solar radiation received before the first cumulus development rather than the moist stability of the background flow. A “golden” case from DOMEX with a clear diurnal cycle in cumulus convection and quasi-steady large-scale conditions is studied using numerical simulations to better understand the mechanisms and sensitivities of island thermal circulations and cumulus convection. The simulations are quasi-idealized in that they use full model physics and a realistic terrain profile, along with a horizontally homogeneous initial flow based on a single observed sounding. Simulations at different grid resolutions reveal that large-eddy resolution (~100 m) provides the most accurate representation of the in-situ measurements from DOMEX and the radar-derived island precipitation. Sensitivity tests reveal the importance of key factors including island terrain height, land-surface type, cloud-radiative feedbacks, moist stability, and background wind velocity in controlling the thermally-driven convection.