Relative Roles of Mesoscale Forcing and Surrounding Convective Ingredients in Controlling Cumulus Updraft Width and Depth

Shallow cumulus cloud fields often cluster and deepen within regions of mesoscale flow convergence and ascent. Several studies employing idealized simulations and theory indicate that the probability of a cloud deepening to the point of precipitating is greater for those whose updrafts are relatively wide near cloud base, which has the effect of mitigating damping effects of entrainment-driven dilution. However, there is significant uncertainty in the relative roles of the mesoscale sub-cloud dynamic forcing, convective thermodynamic ingredients (e.g., CAPE, moisture, LFC), and cloud-scale factors ultimately controlling the size of cloudy updrafts in realistic conditions. In this study, we use LES (100-m horizontal grid spacing) with realistic background conditions to objectively identify and measure properties of mesoscale orographic ascent occurring along the Sierras de Córdoba range in central Argentina for 4 days in which deep convection was observed during the Cloud, Aerosol, and Complex Terrain Interactions (CACTI) field project. We tag these ascent metrics and nearby convective sounding parameters to individual cloudy updrafts and examine statistical relationships between updraft properties (e.g., width, strength, and depth) and the surrounding mesoscale meteorological conditions (e.g, instability, moisture, orographic forcing, etc.).

Deep convection initiation does not always occur in regions of the strongest or widest orographic ascent along the mountain range. The depth and near-cloud-base width of convective updrafts are weakly positively correlated with the depth of their adjacent mesoscale sub-cloud ascent. Although cloud width had a comparatively strong positive correlation with the nearby magnitude of orographic ascent in one case, it bore statistically insignificant correlations for all other cases. In fact, the local width of the band of orographic ascent was negatively correlated with most updraft metrics in all cases, especially updraft depth and lower-cloud updraft speed. Thus, most measured properties of sub-cloud mesoscale ascent do not seem to pose a direct or consistent control on updraft width. Conversely, stronger and statistically significant positive correlations are found in most simulations between updraft metrics (width, strength, and depth) and thermodynamic convective ingredients (e.g., CAPE, moisture, and 1/CIN). Relative humidity in the lower-to-middle free troposphere contained the strongest overall relationships with updraft properties, suggesting that updraft-scale entrainment-driven dilution processes are a significant governor of the width and depth achieved by cloudy updrafts. This may indicate that the primary role of the mesoscale convergence is to passively locally destabilize, moisten, and focus detrainment of preceding cloud moisture that reduces negative entrainment effects for subsequent growing updrafts.