Airborne dual-Doppler syntheses are used in combination with ight-level in situ data to explore the velocity fields in vertical and horizontal cross-sections of cumulus turrets. The multidimensional representations of the rising thermals led to an improved conceptual model of cumulus cloud growth. The Wyoming Cloud Radar (WCR) on-board the University of Wyoming KingAir research aircraft is a 95 GHz Doppler radar capable of quasi-simultaneous multi-fixed-beam scanning. A technique was developed to analyze and merge data collected by pairs of WCR beams yielding two-dimensional wind field syntheses in horizontal or vertical planes. The gridding methodology can be customized to follow the scanned surface, with grid cell sizes between 30 and 45 m. The algorithm to solve the velocity inverse decomposition problem takes advantage of available a-priori and external information about the target and its environment. An estimate of the ambient winds is employed to evaluate the velocity component normal to the solution plane. The accuracy of the two-dimensional (2-D) velocity is on the order of 1-2 m s-1, and it is dependent on: the radar system design, the data collection process, and the weather target properties. A methodology to calculate each error component and to infer the maximum uncertainty in the retrieved 2-D velocity is discussed. The clouds sampled in this study were cumulus congestus forming in cold, dry and nearly neutrally stable environments over land. Observations focused on newly-developed turrets and followed their evolution over periods of up to 15 min. The turrets ranged from 1 to 2 km in horizontal size. Measurable radar reflectivities covered vertical depths of up to 2 km. Cloud bases were at temperatures of 0 to -5 ºC, and cloud top temperatures were -20 to -30 ºC. The cumuli formed and evolved through sequences of updraft pulses, or thermals, consisting of well defined vortex-ring structures at the top, and trailing turbulent wake flows behind them. The rate of rise of the thermal is about half the maximum updraft speed. The reflectivity and velocity fields, in both horizontal and vertical transects, show that the kinematic patterns contribute to a spatial organization of the hydrometeors within the cloud volumes. Gradients and characteristic length scales are similar for the horizontal and vertical planes. Convergence at the bases of the toroids has significant implications for hydrometeor recycling and for entrainment. Dry air intrusions are visible from the velocity fields and are characterized by weaker radar echoes associated with the inflow quadrants of vortices seen in both the horizontal and vertical transects. Ambient air intrusions were also found to accompany the vortex-enhanced downdrafts in the proximity of the cloud upper boundaries. Small scale (100 m) instabilities, in the form of overturning protrusions at the cloud/environment interface, were also detected. Vertical vorticity was observed to persist for 5 min or more. Pairs of counter-rotating horizontal circulations enclose areas of stronger reflectivity and bound the regions where the updrafts were likely located. The model of toroidal thermal is extended to include the effect of ambient wind cross-flow and shear, identifying the tilting of the azimuthal vorticity within the thermals as a possible source for the observed vertical vorticity. The results of this work point to the fact that the three-dimensional vortical kinematics is fundamental for the assessment of cloud microphysical properties as well as for the prediction of its overall dynamical development. The evidence presented herein is the clearest demonstration to date of vortical circulations in growing cumulus turrets, and can provide a reliable benchmark for numerical simulations and model parameterizations.
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