The Lidar Simulation Model simulates space-based DWL shots on a volume
utility grid system that is referenced from the DWL’s swath path defined by the satellite/scanner geometry and the
satellite ground track. For example, the bracketing OSSE’s "
Best Case" experiment grid volume is 200 km by 200 km by 1 km above the PBL and 200 km by 200 km by 250 m below the
PBL. For the proposed NPOESS/OP platform, there are approximately 40 DWL shots per azimuth perspective (forward and aft) in
each grid volume above the PBL. The LSM computes the single shot DWL LOS winds from backscattering sources such as aerosols,
molecules and clouds. Each backscattering source and perspective is considered to provide a unique measurement in each grid
volume. A single representation of the wind in each grid volume for each source and perspective is computed by accumulating
the single shot DWL LOS estimates. The LSM also computes the standard deviation of the accumulated single shot LOS winds.
This standard deviation reflects not only system accuracy, but also
representativeness as long as the single shots are distributed throughout the grid volume. Consider the following as an example of the disposition of 100 shots as they pass through the lowest 20 km of the earth’s atmosphere. Assume 50 shots are taken with a forward perspective and 50 from an aft perspective. The target data product for assimilation by a global analysis is a single LOS for each of the two perspectives within a volume of 200 x 200 x 1 km3. In this example the global model (eg ECMWF) calls for 25% cloud coverage at 12 km, 50% coverage at 4 km and 20% coverage at 1km. At 12 km the clouds are taken to be cirrus, but the 25% coverage is not interpreted literally. Instead, we assume 100% cirrus coverage and use the value of 25% to scale the optical depth between 0 and 1.4 km. Thus all 100 shots would yield returns, 50 "forward" and 50 "aft", from cloud material at 12 km and pass, attenuated, through to lower levels. The two sets of 50 shots would be processed to produce one LOS wind speed for each perspective and be assigned to the height of the top of the cirrus layer. For the non-cirrus clouds at 4 km, the 50% coverage is taken literally. Hence, 25 forward/25 aft shots are intercepted by cloud and 25 forward/ 25 aft shots provide aerosol returns. In this case, four LOS wind speeds are computed; forward and aft LOSs from the top of the cloud, and forward and aft LOSs assigned to the middle of the 1 km layer of aerosols. Only the shots that missed the clouds continue on to lower levels. At 1 km, 5/5 shots are processed to provide two LOSs assigned to the top of the cloud, and 20/20 shots are combined to provide two LOSs for the aerosol layer and then continue on to yield surface returns.
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| This page managed by Sidney A. Wood | Last modified: 9 Sept 1999 |