The Role of Soil Moisture Gradients in Summertime Convection Initiation over the Colorado Front Range: An Idealized Numerical Modeling Study

Abstract

This study investigates how horizontal soil moisture gradients influence the initiation of convection (CI) along the Colorado Front Range using idealized convection-permitting simulations with the Weather Research and Forecasting (WRF) model. Using the tornado outbreak event of 21–22 June 2023 as a case study, eight perturbation experiments were conducted where the atmospheric initial and boundary conditions were held constant and only land surface soil moisture was modified. Soil moisture gradients were imposed across northeastern Colorado in both zonal and meridional orientations, with perturbation magnitudes derived from a 46-year NLDAS-2 June climatology. Across all experiments, a consistent response emerges: CI preferentially occurs over the dry portion of the soil moisture gradient. This is not due to enhanced moisture availability, but rather the dynamical response to differential surface heating. Dry soils suppress the evapotranspiration, increasing the Bowen ratio and enhancing sensible heat flux, which then deepens the daytime mixed layer and creates horizontal temperature gradients between adjacent wet and dry patches. Through hydrostatic balance, these temperature differences produce lower surface pressure over the warmer dry regions, driving low-level convergence along the soil moisture boundary. This convergence locally reduces convective inhibition and helps lift parcels to their level of free convection. This mechanism is robust across gradient orientations and spatial scales, with sharper gradients producing stronger responses. Overall, the results support a convergence-driven land–atmosphere coupling regime where surface thermal contrasts matter more than near-surface moisture availability in setting where deep convection is triggered. This highlights the importance of representing land surface heterogeneity in high-resolution numerical models used for convective prediction in semi-arid, complex terrain environments like eastern Colorado.