High-Resolution 3D Wind Analysis 2008

CASA has been producing high-frequency (~every 3 to 5 minutes) realtime wind analyses at high spatial resolutions since the spring of 2008. Different from typical dual-Doppler wind synthesis method, the analysis uses a three-dimensional variational (3DVAR) method and utilizes all available data to produce complete 3D wind fields without holes typically associated with radar data voids. The radar data coverage can be single, dual, multiple Doppler or even without any Doppler wind coverage. The system optimally combines a prior estimate from an operational coarse-resolution numerical weather forecast with radar data, and other conventional observations such as those from a surface observation network, for the purpose of producing the best possible forecasting and decision tools and optimizing end user response. Below is an example of low-level wind analysis for the February 10, 2009 case produced at 200 m resolution - the analysis captures the closed circulation (right panel, pointed to by the arrow) in the weak echo region at the tip of a hook echo (left panel). The circulation is associated with a weak vortex that is also shown as having large values of azimuthal shear (middle panel). Such spatially and temporally high-resolution wind analyses have been presented to and evaluated by the National Weather Service forecasters in realtime in an experimental setting, and have been found by the forecasters to be much more useful than the raw radial velocity observations, and give them much more confidence in determining the character and strength of low-level wind features such as tornadic vortices and strong damaging winds. Such products are not possible without the dense adaptively scanning radars of CASA type. Strong wind and tornado detection algorithms based on such wind analyses will be developed and linked with the radar scan control system of CASA, to help improve the adaptive scanning of the radars.

(left panel) Reflectivity observed by the CASA radars at 4:10 pm local daylight time, February 10, 2009, showing a thin 'finger' of radar echo having a weak 'hook' at its tip (pointed to by the arrow), (middle panel) velocity shear calculated from the radial velocity data from one of the CASA radars showing a heightened shear region (bright color shading), and (right panel) the low-level 3DVAR wind analysis at 200-m resolution showing a well-defined smaller-scale circulation centered on the strong-shear region. The reflectivity field is overlaid with the winds.