CLOUDMAP2 ATSR cloud products

The data contained in this directory is the result of a contract to Eumetsat for application to SEVIRI and partly under CLOUMAP2 a European framework 5 project.

The data contained here are the cloud parameters, cloud top pressure , height, phase, fraction, effective radius and optical depth derived using a variational analysis method applied to ATSR-2 data for the European area, A full global data set will follow soon.

A full description of the physical principles behind the algorithm is given in [1]. Here we summarise. The (A)ATSR-2 instrumentation measures TOA radiances in seven wavelengths from the visible (0.55 micron) to the infrared (12 micron). All channels have window characteristics, i.e. they are chosen to be as little sensitive to the cloud-free atmosphere as possible within a given wavelength range. Clouds, however, impact all the measurements and do so, because of the window characteristics, to a strong degree. Visible channels are not absorbed by cloud water, only scattered, and this leads to their utility is sensing the total column of cloud water present (represented by the optical depth in the system). Near infrared channels (at 1.6 and 3.7 micron) are slightly absorbed by cloud water and the balance between scattering and absorption thus produced, makes them sensitive to the particle size and phase (ice or liquid) present. Infrared channels are strongly absorbed so that particle size has little effect and, for anything other than very thin clouds, these channels are responsive to the cloud temperature (and therefore by proxy, height or pressure).

That all channels respond to all cloud parameters and additional information is required to constrain the system leads to the use of Optimal Estimation (OE) for the parameter extraction method. OE combines the measurements with prior information (e.g. NWP estimates of skin temperature, detection estimates of cloud fraction) using proper statistical weighting. Measurements are mapped to and from parameter space using a fully specified (but fast approximation) radiative transfer model (RTM) and additional fixed constraints (e.g. temperature and humidity of the atmosphere, surface reflectance) are provided through inputs to the RTM. Profiles originate from ECMWF analysis. The OE system minimises a cost or distance function by adjusting the cloud parameters so that the solution best fits the measurements and the prior information in a statistical sense.

This methodology is very powerful as it allows inclusion of diverse sources of information and, crucially, allows a high degree of quality control.

The first quality control method checks the solution cost is sufficiently low. If it is not, there is some indication that the scene being analysed does not conform in a crucial way to the model assumed. In this case, such a non-conformity is likely to be multi-layer or highly structured cloud, shadows etc, and the entire retrieval is rejected.

Given a low solution cost, the second quality control method is used. This is based on the quantitative evaluation of the statistical likelihood of the solution and is a result of the sensitivity of the measurements to each parameter and the level of measurement and prior error assumed. Quality control here is applicable to each parameter separately; optically thin cloud may have high errors for particle size estimation but still retain low errors in the pressure estimation for example.

[1] Study on Cloud Properties derived from Meteosat Second Generation Observations. Watts, Mutlow, Baran, Zavody. Available from Eumetsat.

This data is currently restricted to members of the CLOUDMAP2 consortium and should not be passed on to members outside the consortium.

To gain access to the data you must apply for access to cloudmap data.

CLOUMAP data can be accessed through the web (using a login name) or via FTP.

NOTE:

The data is in HDF5 format. Documentation describing the CLOUDMAP File Format is available.

• For further information please see the CLOUDMAP2 WEB SITE
• Global Retrieval of ATSR Cloud Parameters and Evaluation (GRAPE) data is a more complete and uses a better algorithm than CLOUPMAP. GRAPE is publicly available (after registering with the BADC) and can be accessed here

EUMETSAT, [Poulsen, C.A.] . CLOUDMAP2 ATSR cloud products, [Internet]. NCAS British Atmospheric Data Centre, 2003-, Date of citation. Available from http://badc.nerc.ac.uk/view/badc.nerc.ac.uk__ATOM__dataent_cloudmap

If you have any questions concerning data availability or data access then you should contact the BADC Support line. If you have any questions concerning scientific issues such as the limitations of the data then you should contact Caroline Poulsen.