Observations of the transition from shallow to deep convection during CuPIDO 2006

The elevated terrain in the southwest United States provides an ideal location to observe the onset and development of deep convection. Thunderstorms associated with the North American Monsoon occur on a fairly regular diurnal cycle, with shallow convection developing under typically clear skies over the highest peaks a few hours after sunrise. The shallow convection builds as a series of successively taller turrets that rise through the remnants of the preceding ones, transitioning into cumulus congestus and eventually cumulonimbus.

The development of the deep convection is often slow, occurring over a period of hours despite the presence of sufficient CAPE. One possibility is that as the shallow turrets rise into the relatively dry air aloft, they evaporate by entraining the environmental air. Given suitable conditions, such as weak winds aloft, the shallow convection can moisten the column and the successive turrets grow through a moistened environment. Another possibility for conditioning the vertical profile for deep convection is an adiabatic adjustment via the action of gravity waves forced by nearby convection.

Observations of convection over the Santa Catalina Mountains in southern Arizona have been ongoing for several years. Using stereo pairs of digital cameras and automatic image processing techniques, a method for determining the three dimensional structure of convection has been developed. This preliminary work motivated a series of coordinated observations using a variety of platforms that took place in July and August of 2006.

The CuPIDO (Cumulus Photogrammetric In-situ and Doppler Observations) field program employed a network of 10 surface meteorological stations, 4 of which were equipped to measure surface fluxes, two mobile GPS based radiosonde systems and two arrays of digital cameras that were used to generate time lapse movies and 3-dimensional cloud structures. In addition, the University of Wyoming King Air (WKA) provided a variety of in-situ wind, thermodynamic, cloud microphysical and doppler radar data.

The CuPIDO program consisted of a series of 15 intensive observing periods. The experiment design called for coordinated soundings and aircraft observations beginning just prior to the onset of the shallow convection and through the transition to cumulonimbus. This provided the opportunity to sample the pre-convective environment and determine changes that occur as the convection develops. Soundings were located upwind of the orography and taken at hourly and in some cases 30 minute intervals. Digital imagery and surface data were collected continuously through the period. The CuPIDO dataset will provide a valuable resource for understanding the mutual interaction of cumulus clouds with the environment and for validating cloud resolving models and convective parameterizations.

The presentation will discuss the CuPIDO experiment design, provide a description of the algorithm that was developed for generating the 3-dimensional cloud structures and present a case study of the onset and development of the deep convection.