(The following is an excerpt from the introduction of the Level 3 Products Description and Interface Control Document. The entire document can be viewed from the CloudSat Standard Data Product Interface Control Document links page.)
This document will provide an overview of the CloudSat data sampling strategy and the CloudSat Level 3 Products that are generated from CloudSat Level 2 data. The initial CloudSat Level 3 data products are derived from either the diagnostic output of the applications that produce Level 2 products or the Level 2 output itself. CloudSat Level 3 products will be generated in two parts. The first will be summary statistics which are gathered for each granule (referred to as "Level 3A"). These statistics will then be averaged over a period of time (ie. monthly) and mapped onto a common space-time grid (referred to as "Level 3B").
CloudSat data products will be generated and archived as "granules". A CloudSat granule represents one full orbit of data. CloudSat will complete an orbit in approximately 110 minutes at a mean altitude of 705 km. CloudSat is a nadir-pointing instrument which provides a vertical "profile" every 160 milliseconds. The CPR footprint is approximately 1.4 km in diameter and is averaged along track over the 160 millisecond interval to produce a footprint for the profile of 2.5 km along track and 1.4 km across track, with the center point of each profile being approximately 1.1 km apart along the sub-track. This spacing provides approximately 36,383 profiles per granule.
Each application that generates CloudSat Level 2 Standard Data Products will produce an output text file that contains quality assessment statistics. These statistics will be used by the operators to identify potential problems in the data processing algorithms or the science data itself. An example, of the type of QA statistics will be produced, is the number of CPR profiles, within a granule, that contain cloud. If this value is higher or lower than a reasonable threshold, determined by the science team investigator, the operator will perform a set of checks to see if they can determine the cause of the discrepancy, if any. It may turn out to be an anomaly in which the granule was simply unusually cloudy or cloud-free, or it could be caused by noisy or missing data. Because statistics such as this one can also be quite useful for looking at long term trends or for comparison with other cloud retrieval techniques, the value will also be stored for eventual input to a Level 3 product application.
The majority of the initial CloudSat Level 3 products will be sampled on a 2.5 degree lat/lon grid. A select group level 3 products will be summarized over latitudinal bands as follows:
Band Name Description Location
Tropic Tropics (23.5 S to 23.5 N)
N_Sub_Tropic north subtropics (23.5 N - 35 N)
S_Sub_Tropic south subtropics (23.5 S - 35 S)
N_Mid_Lat north middle latitude (35 N - 55 N)
S_Mid_Lat south middle latitude (35 S - 55 S)
N_High_Lat north high latitude (55 N - 90 N)
S_High_Lat south high latitude (55 S - 90 S)
The latter will serve as the primary Quality Assessment statistics that will be used by the operator to determine if there is a potential problem with the cloud analysis application or the input data, while the 2.5 degree gridded data will form the basis for comparison with other global geophysical data sets.
Also see the CloudSat Data I/O Interface Specifications