Thursday, 8 August 2013
Holladay-Halsey (DoubleTree by Hilton Portland)
A thorough understanding of the dynamics that modulate the development, maintenance, precipitation structure, and kinematics of cloud-top precipitation generating cells is crucial to understanding heavy snowfall in winter cyclones. These cells are hypothesized to be responsible for the initial generation of precipitation particles, which then undergo aggregation, accretion, and diffusional growth as they fall through stable frontal layers and approach the surface. High-resolution remote sensing and in-situ observations of precipitation generating cells were made during the 2009-2010 Profiling of Winter Storms (PLOWS) field campaign. Observations from the Wyoming Cloud Radar (WCR, data resolution ~15 m) aboard the NSF/NCAR C-130, paired with thermodynamic analyses, indicate that generating cells have bases above stable frontal layers and tops at the tropopause. Such cells were observed in all 13 PLOWS IOPs including NSF/NCAR C-130 flight legs, and were ubiquitous atop the warm frontal and comma-head regions of these cyclones. Observed generating cell dimensions were ~0.75 1.5 km wide and ~1.5 2.0 km deep, with maximum vertical velocities of approximately +/- 3 m s-1
In this presentation we will discuss the physical structure and kinematics of generating cells realized in ultra-high-resolution (50 m horizontal grid spacing) idealized Weather Research and Forecasting (WRF) simulations in relation to the WCR observations. These idealized simulations were designed to represent: 1) a strong cyclone with classic comma-head structure (8-9 December 2009), and 2) a cyclone in which generating cells were observed despite having a poorly defined comma-head structure (14-15 February 2010). Output from these simulations will be used to test hypotheses for generating cell development, maintenance, precipitation structure, and kinematics. While the WCR provides an excellent two-dimensional dataset of the precipitation structure and radial velocity of generating cells (time-height cross sections), it cannot be used to infer their three-dimensional structure. Therefore, other data from PLOWS will be used to compare simulated three-dimensional structure of generating cells to structures suggested by theories for organization of convective precipitation.
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