Dynamical Processes of Orographic Cumuli II

University Of Wyoming, Laramie WY

Investigators

Abstract

In the summer of 2006 a field campaign focused on orographic cumulus convection and boundary-layer circulations was conducted. The field campaign was called CuPIDO (Cumulus Photogrammetric, In-situ and Doppler Observations). This campaign deployed a unique combination of instruments, including an airborne cloud radar, atmospheric soundings, a surface mesonet, and stereo-photogrammetry. The present project involves further analysis of the CuPIDO dataset and builds on research completed in the initial two years following the field campaign. The main objective of the new research is to describe the observed fine-scale vertical and horizontal structure of the radar-observed cloud circulations and reflectivity fields within orographic cumuli congesti, and to use this in combination with flight-level measurements and other CuPIDO data to test the following key hypotheses: 1) Toroidal circulations surround the buoyant cores of growing cumuli. Significant entrainment occurs due to fine-scale instabilities at the updraft interface, and the mixed air is efficiently transferred to the cumulus core by the toroidal circulation, and 2) In places where a cumulus grows in the detritus of older clouds, its growth is enhanced, in particular at levels where the environment is dry (moisture-convection feedback hypothesis). The second objective is to use a high-resolution cloud-resolving model to statistically compare observed vs. modeled cumulus properties and patterns of entrainment and detrainment, and to use the continuous and dynamically consistent model output to assist in the interpretation of the observations, in particular regarding cumulus-environment interactions. The model will be used also in a more idealized way to assess the impact of ambient conditions (wind, wind shear, stable or dry layers) on the evolution of orographic (locked-source) convection. Intellectual Merit. In the past half-century several field studies of cumulus dynamics have been conducted by means of ground-based precipitation radars and aircraft making in situ measurements. The CuPIDO experiment places flight-level observations, collected while penetrating a cumulus, in the context of the radar-derived echo and velocity field, at a resolution of 40 m or better, in both vertical and horizontal planes. This combination constitutes a powerful tool for the study of fundamental cumulus dynamics. In particular, cloud radar data reveal entrainment events at various scales, and close-proximity aircraft data allow assessment of the thermodynamic and cloud microphysical characteristics of these events. The combination of the rich CuPIDO dataset with numerical modeling of orographic convection at resolutions matching that of the cloud radar will improve our understanding of characteristic cumulus properties and evolutions, the patterns and scales of entrainment, and mechanisms of cumulus-environment interaction. Broader impacts. Most warm-season precipitation results from deep convection. Cumulus convection operates over a range of horizontal and vertical scales, both over mountains and elsewhere. Not all scales can be resolved by operational numerical weather prediction models, now or in the foreseeable future. Parameterization of the effect of these unresolved cloud circulations on the larger-scale resolved circulations remains one of the greatest uncertainties in these models and also in climate models. Our analyses and numerical simulations of the fine-scale structure and evolution of cumuli, and of the interaction of cumuli with their environment, should lead to more accurate parameterization of the effects of cumulus convection on the resolved scales of these models.