Tuesday, 7 August 2007
Halls C & D (Cairns Convention Center)
In an effort to provide improved ground validation information for the upcoming NASA Global Precipitation Measurement (GPM) mission, a technique to retrieve snowflake size distributions (SSDs) from a 915 MHz profiler is being developed. The SSD and uncertainty information will be valuable to aid in the improvement of satellite estimation of snowfall rate. While retrievals of drop size distributions from VHF and UHF profilers are now understood fairly well, there has been little work in retrieving SSDs using these profilers. The retrieval problem is more difficult in snowfall due to increased variability in fall speeds and density, which are both dependent mainly on crystal type. There is also the problem of decreased fall speeds, and a narrow returned spectrum resulting from a fairly weak size-fall speed relationship (in general). The issue of an unconstrained retrieval problem is dealt with in two parts. The first is to use surface SSD and crystal type estimates from a video disdrometer to provide estimates of terminal velocity and density through the use of published size-density and size-fall speed relationships. The second is to estimate mean vertical air motion and spectral broadening due to turbulence by following the constraints outlined in Williams (2002). Increasing the resolution of the profiler spectra is explored as a way to alleviate the narrow spectrum problem. Simulated spectra are used to begin to quantify the uncertainty of the retrieval method. Also, the surface SSDs will be compared to the lowest range gate of the profiler to further estimate retrieval uncertainty. Vertical SSD profiles will also be produced providing a more complete characterization of snowfall. Different types of snowfall events will be examined to gauge the robustness of the retrieval technique. The uncertainty analysis and retrieval performance for several cases will be presented.
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