Influence of urban features on the vertical development of cumulus clouds during TRACER
David M. Romps, University of California, Berkeley (Principal Investigator)

Rusen Öktem University of California, Berkeley (Co-Investigator)  

With their coupling of buoyancy and phase changes, convective clouds are a particularly challenging and still unsolved problem among societally relevant turbulent flows. In particular, there is still not a satisfactory theory for the maximum height attained by such clouds, even under relatively simple conditions. The maximum height of these clouds is of primary importance for air quality during a wildfire (as the height of the smoke dictates where the winds carry it), for the climate impact of large urban firestorms (as in a "nuclear winter" scenario, where soot lofted sufficiently high could send temperatures plummeting), and for the energy budget of our past, current, and future climate (since the heights of clouds strongly influence their lifetimes and their potential to reflect, absorb, and emit radiation). The overall aim of this project is to improve our theoretical understanding and modeling capability with regards to cloud heights using data from the TRacking Aerosol Convection interactions ExpeRiment (TRACER). The TRACER campaign was conducted in the Houston area using the first Atmospheric Radiation Measurement (ARM) Mobile Facility (AMF1), which included a pair of stereo cameras. Those stereo cameras took synchronized photographs at 20-s intervals, which ARM then processed to generate stereo reconstructions of the clouds. A preliminary investigation of those data has revealed the presence of pinned clouds over the Houston area, i.e., convecting clouds that, despite their dynamic fluid flow, remained pinned, however tenuously, to a feature in the urban landscape. These clouds present a unique opportunity to observe the heights of clouds in a quasi steady state, which is best suited to developing and testing both theories and models for clouds' vertical development. The approach of this project is to combine two lines of evidence -- 1. observations of clouds and their environments during TRACER, and 2. computational fluid dynamics simulations of pinned clouds in the environments measured during TRACER -- to test theories for the convective-cloud heights. Particular attention will be paid to the representation of turbulence in the clouds, the radial structure of the clouds, the treatment of water vapor and condensational heating, and the influence of horizontal wind shear. The project aims to deliver an improved theoretical understanding and modeling capability for predicting the heights of pinned clouds, with applications to predicting the vertical development of clouds in an urban environment, as well as to forecasting the vertical distribution of soot from wildfires or large urban firestorms.