Sunday, 11 January 2009
Sensitivity of mesoscale wind speed surface analyses to surface observations in the urban-wildlife interface
Phoenix Convention Center
A two-dimensional variational analysis system for wind speed is used over a limited domain (400 km x 400 km) centered over the Los Angeles/San Diego metropolitan area of southern California. Strong Santa Ana winds developed across Southern California during 22 October, 2007 leading to massive wildfires. The variational analysis utilizes a background field from the 1-h forecast of wind speed from the NCEP Rapid Update Cycle downscaled to a 5 km grid, which is generated as part of NCEP's Real-Time Mesoscale Analysis system. Observations of surface wind speed during the period from 08:45-9:12 UTC 22 October from a variety of sources are used to modify the background grid. These observations are comparable to those used in the operational Real Time Mesoscale Analysis. This particular time is examined because: (1) wind speed analyses are particularly difficult to do well during the night as the near-surface winds often become decoupled from the prevailing winds aloft and (2) the strong Santa Ana winds in this case led to very high wind speeds in many localized areas within southern California. The sensitivity of the wind speed analyses is evaluated using data denial experiments as a function of the magnitude of the observational error variance of the surface observations relative to the background error variance as well as the decorrelation length scales of the background error covariance.