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.
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. This grant used new technology (esp. a cloud radar) and more advanced numerical modeling to examine the development of the convective boundary layer (CBL) around a mountain, the interaction of thermally-forced orographic circulations with cumulus convection, and Cu dynamics. The research builds on the CuPIDO (Cumulus Photogrammetric, In-situ and Doppler Observations) campaign conducted in the summer of 2006 around the Santa Catalina Mountains in Arizona (Damiani et al. 2008). CuPIDO involved the UW King Air aircraft with Wyoming Cloud Radar, a network of 10 flux stations around the mountain, and soundings. Geerts et al. (2008) developed a technique to compute horizontal perturbation pressure gradients from data collected at different elevations and demonstrate the diurnal cycle of solenoidal forcing that drives anabatic surface flow. Demko et al. (2009) examined the evolution of mountain-scale convergence, using data from both stations positioned around the mountain and UWKA data. They found some evidence for a toroidal heat-island circulation. The daytime evolution of the CBL and orographic circulation were studied further using WRF simulations that assimilate CuPIDO surface and sounding data in two papers: the evolution without deep convection appears in Demko and Geerts (2010a), and the interaction with deep convection is described in Demko and Geerts (2010b). Demko received his PhD in 2009. A 2nd PhD student, Yonggang Wang, authored 7 papers on Cu dynamics and defended in December 2011. Based on composite data in the exit region of cumulus clouds, Wang and Geerts (2009) proposed a correction for measured temperature (using a commonly used immersion sensor) in and near clouds. Wang and Geerts (2010) then improved the description of humidity variations near Cu by means of a new fast-response lyman-apha humidity probe. These first two steps were needed because of the uncertainty in buoyancy estimation of buoyancy near clouds, due to uncertain humidity and temperature measurements in and near clouds. Wang et al. (2009) used the improved measurements to document buoyancy variations in and near Cu. Yonggang Wang then incorporated WCR with the in situ data, first to document Cu detrainment patterns (Wang and Geerts 2011), and then to describe the typical Cu cloud top vortex circulation using single-Doppler (Wang and Geerts 2013) and dual-Doppler data (Wang and Geerts 2014, in review for Mon. Wea. Rev.). Buoyantly- and dynamically-induced pressure perturbations within and near orographic Cu are analyzed and composited in Wang and Geerts (2014b, in preparation). A 3rd graduate student, Xin Zhou (MS Dec 2012), examined the relationship between soil moisture and the development of the CBL, the thermally-forced orographic circulation, convection, and precipitation, through two months of CuPIDO observations (Zhou and Geerts 2013) and WRF simulations (Zhou and Geerts 2014, in review for Mon. Wea. Rev.). Broader impacts. Most of the warm-season precipitation results from deep convection. Cumulus convection operates at a range of horizontal and vertical scales, both over mountains and elsewhere. Not all scales can be resolved by operational numerical weather prediction (NWP) models, now or in the foreseeable future. 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 penetrating convection will remain in the realm of parameterizations. Our observations and numerical simulations of the fine-scale structure and evolution of cumuli, and of the interaction of cumuli with their environment, have lead to new insights about the following: the diurnal cycle of upslope and downslope flow, pressure perurbations, and CBL temperature around the Catalina mountains during the summer monsoon, the importance of elevated heating (rather than mountain-scale convergence) on orographic convective initiation, improved temperature measurement in cumulus cloud, a better characterization of the buoyancy field near the edge of cumuli, a description of the cumulus detrainment process, a description of the charecteristic cloud top solenoidal circulation, and the importance of soil moisture variations on the development of a CBL, upslope flow, CBL depth, Cu convection, and convective precipitation.
