Dynamical Processes of Orographic Cumuli

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This research is part of the CuPIDO (Cumulus Photographic Investigation and Doppler Observations) project. This campaign, to be conducted in the summer of 2006, intends to describe early cumulus development over the Santa Catalina Mountains in Arizona at the meso-gamma and cloud scales, with an emphasis on boundary-layer evolution and the effects of entrainment, detrainment, and glaciation on individual convective towers that may develop in succession and culminate in the cumulonimbus stage. This research will utilize an advanced airborne cloud radar (Wyoming Cloud Radar; WCR) and in situ aircraft thermodynamic measurements. The project will be collaborative with a University of Arizona scientist who will apply newly developed techniques in digital stereo-photogrammetry. The geo-located cloud-edge maps, inferred from the digital stereo-images, will complement the WCR observations since the cloud base and the earliest stages of cumulus development cannot be detected by the radar. The primary objective is to describe the radar echo and kinematic structure and evolution of cumuli at an entrainment-resolving scale, and to merge this description with in situ cloud and thermodynamic data, in order to gain new insights into the fundamental dynamical processes controlling the growth and decay of cumulus congestus. In this context, the mountains provide a natural laboratory, given the regular, repetitive, and geographically fixed development of cumuli. The secondary objective specifically deals with the orographic forcing of cumulus clouds. The spatial and temporal changes in the environment in which orographic cumuli develop and decay will be studied in order to gain insights into the synergy between deep cumulus convection and orographically-induced mesoscale circulations. In the past half-century several field studies of cumulus dynamics have been conducted by means of aircraft making in situ measurements, and by means of ground-based precipitation radars. These studies form the basis of current conceptual representations of entrainment; however, entrainment remains the least understood factor in cumulus development. Newly developed technology provides the ability to place flight level observations, collected while penetrating a cumulus, in the context of radar-derived echo and velocity field at a resolution of 40 m or better. This combination constitutes a powerful tool for the study of fundamental cumulus dynamics. In particular, the WCR reveals entrainment events at various scales in the airflow and echo fields. Close proximity aircraft data allow assessment of the thermodynamic and cloud microphysical characteristics of these events. Cumulus convection operates at a range of horizontal and vertical scales and these scales are not resolved by operational numerical weather prediction (NWP) models. Thus the simulation of its impact is challenging, and cumulus parameterization remains one of the greatest uncertainties in NWP models and general circulation models (GCMs). New generation operational NWP models may resolve mesoscale orographic circulations, but the effect of cumulus convection will remain in need of parameterization. Observations of the fine-scale structure and evolution of cumuli, and the resulting new insights into the fundamental dynamical processes of cumuli interacting with their environment, should lead to a more accurate parameterization of the overall effect of cumulus convection on the resolved scales of NWP models and GCMs.