Version: 5.0.1

Documentation Examples Download Support Acknowledgments

The Cloud and Surface Parameter Retrieval (CASPR) system is a toolkit for the analysis of data from the Advanced Very High Resolution Radiometer (AVHRR) satellite sensor carried on NOAA polar-orbiting satellites. The toolkit is a collection of algorithms that can be used to retrieve a variety of surface and cloud parameters. These algorithms are integrated into a user-friendly, flexible environment that includes on-line help and a multitude of options for displaying and printing results. CASPR can compute any or all of the following:

Surface and Clouds: Radiation: Cryosphere:
  • Surface temperature, clear or all-sky
  • Broadband albedo, clear or all-sky
  • Cloud particle effective radius
  • Cloud optical depth
  • Cloud particle phase
  • Cloud mask
  • Cloud type
  • Cloud top temperature
  • Cloud top pressure
  • Precipitable water
  • Sunglint mask
  • Downwelling shortwave radiation at the surface
  • Downwelling longwave radiation at the surface
  • Upwelling shortwave radiation at the surface
  • Upwelling longwave radiation at the surface
  • Downwelling shortwave radiation at the top of the atmosphere
  • Upwelling shortwave radiation at the top of the atmosphere
  • Upwelling longwave radiation at the top of the atmosphere
  • Shortwave cloud forcing at the surface
  • Longwave cloud forcing at the surface
  • Ice identification
  • Ice concentration
  • Ice thickness
  • Ice age
  • More to develop
CASPR is written in the Interactive Data Language (IDL) and therefore requires IDL to run. Click here for details on memory requirements and input data files.

WhatCASPR Does Not Do

  1. Atmospheric temperature and humidity profiles are not retrieved, they are input.
  2. Calibration of AVHRR raw data.
  3. Navigation and registration of raw data are not performed.
  4. Retrievals of some parameters for large solar zenith angles.
  5. Retrievals outside of the polar regions, although the primary limitation is the surface temperature retrieval, which could easily be expanded to include lower latitude oceans and land.

A Cautionary Note

CASPR is research code. It does a lot of things well but doesn't do anything perfectly. Many of the algorithms have been validated, some have not. They are all detailed in the Reference Guide. There are three broad problems worth mentioning at the outset. First, everything depends on cloud detection, which sometimes borders on being as much an art as a science when working in the polar regions with the AVHRR. We are, indeed, trying to squeeze water from a stone. We do not claim to have solved the cloud detection problem, but rather provide methods that work reasonably well most of the time. Second, there are fundamental difficulties in estimating some parameters at extreme viewing and illumination geometries common to the high latitudes. We're venturing into territory that is difficult to model so beware. Third, the retrieval of cloudy sky parameters requires temperature and reflectance values underneath the clouds. CASPR interpolates clear sky values to cloudy areas for some parameters. This generally works but can result in large uncertainties in very cloudy areas. Cautionary notes are given throughout the Reference Guide. Please do not ignore them!

What's New

Version 5.0.1 has many significant changes. Polar stratospheric cloud (PSC) detection was added, cloud type classsification algorithm was implemented, sun glint mask parameter was added, NOAA-16 AVHRR post-launch channel degradations were calibrated and corrected for time series, cloud detection procedures were revised, some of cryosphere products were added (ice extent, ice concentration, ice thickness, ice age), and plotting procedures were enhanced. See the revision history for a detailed list of changes.