Combined CloudSat spaceborne radar and CALIPSO spaceborne lidar cloud fraction dataset

The following was contributed by Jennifer Kay (NCAR), February, 2012:

#CloudSat (a 94 GHz radar) and CALIPSO (a 532/1064 nm depolarization lidar) provide a unique global view of cloud vertical structure and spatial distributions.  When combined, they provide the best available global estimate of cloud vertical structure.  I have used the radar cloud mask (CloudSat dataset name 2B-GEOPROF R04) and the co-located lidar cloud mask (CloudSat dataset name 2B-GEOPROF-LIDAR P2 R04) datasets to produce cloud fraction climatologies.  An example of the synergy of CloudSat and CALIPSO to detect clouds along a single orbit is shown in Figure 1.  The lidar detects thin cloud at cloud top and low cloud that the radar misses (green), while the radar detects most mid cloud that the lidar misses due to attenuation (blue).  There are some interesting low clouds that have radar-only tops (a resolution issue?) and lidar-only bottoms (radar cannot detect cloud close to surface due to ground clutter).  I combined data from multiple orbits to produce the gridded cloud fraction climatologies in netcdf format.  For example - Figure 2 shows the zonal annual mean vertical cloud fraction from the combined CloudSat and CALIPSO product.  CloudSat and CALIPSO provide the first ability to look at the vertical structure of clouds in this way.  Figure 3 shows the annual mean total cloud fraction map from CloudSat and CALIPSO.

CloudSat and CALIPSO provide new insight into the vertical distribution of clouds at all latitudes where they are available (82 S to 82 N).  Because of their limited temporal sampling, I have only used CloudSat and CALIPSO to look at seasonal or annual mean cloud amounts.  For comparison of CloudSat and CALIPSO observations to models, it is important to use satellite simulators (see COSP page).  

Though my climatologies are global, I have used CloudSat and CALIPSO primarily in the polar regions (e.g., Kay et al. 2008, Kay and Gettelman 2009, Bromwich et al. 2012).  Unlike many satellite-based estimates of cloud amount, CloudSat and CALIPSO are "active" instruments that send radiation out and measure the return signal.  As such, CloudSat and CALIPSO do not rely on thermal or albedo contrast to detect clouds and thus unlike many satellite retrieval algorithms, their cloud fraction masks are not confused by the atmospheric inversions or bright surfaces that are common in polar regions.  In summary, CloudSat and CALISPO are particularly useful in the polar regions where there are few cloud observations, and the observations that do exist are not as reliable as they are at lower latitudes.

Marchand et al. 2009 describes the methods for hydrometeor detection using CloudSat.  Mace et al. 2009 is another useful reference.  They analyze combined CloudSat and CALIPSO cloud masks globally.  The CloudSat science team also produced a number of Level 3 products, including one similar to mine that combines the radar and lidar cloud masks (http://www.cloudsat.cira.colostate.edu/).##