Some Clouds Just Really like Mountain Climbing: Relationships between LES-Modeled Deep Convection Initiation Events and Orographic Circulations.

Shallow cumulus cloud fields often organize and deepen within regions of mesoscale ascent associated with orographic flows. Deep convection initiation (DCI) resulting from such thermodynamically and/or mechanically forced orographic convergence along the Sierras de Cordoba (SDC) range in Argentina (and elsewhere globally) often go on to produce intense individual storms and larger mesoscale convective systems with significant hydrological and societal impacts downstream. Numerous DCI events observed along the SDC during the CACTI field project were modeled with large-eddy simulations (LES) as part of the DOE ARM LASSO project. In this presentation, we examine the connection between DCI events during eight LES CACTI cases, and the evolution of ascent associated with the mesoscale orographic circulation and how it modulates the near-cloud meteorological conditions. We are particularly interested in the possible influences of the width, depth, and magnitude of the orographic convergent flow on the width and growth rate of cloudy updrafts.

The overall depth and magnitude of ascent within the swath of orographic convergence along the SDC gradually increases throughout the simulation period leading up to DCI. The width of this swath of ascent typically only slightly increases and is sensitive to the variety of model grid resolutions used (dx = 2.5 km, 500 m, and 100 m). When convective PBL eddies are resolved by the LES, the horizontal spatial scale of their updraft branches seem to scale with the width of the orographic convergent swath, which is perhaps consistent with the thermally forced nature of the circulation.

Preliminary analysis indicates that portions along the SDC where the deepest convective cells form did not necessarily contain the deepest, strongest, or widest orographic ascent in the 1-2 hours leading up to DCI. Thus, a primary function of the orographic low-level convergence in DCI may be to focus transient PBL updrafts into a confined region. Additional important functions of the orographic convergence may be to deepen boundary layer moisture and focus detrainment of cloudy moisture into mesoscale regions. Each of these effects would be expected to minimize negative entrainment effects on growing clouds. To test these hypotheses, we examine correlations between the width and depth of developing cloudy updrafts and: i) nearby properties of the evolving orographic ascent, and ii) convective meteorological ingredients (e.g., CAPE, CIN, moisture, shear, etc.).