Precipitating Clouds: Description and examples


  1. Goal of the Precipitating Cloud product
  2. Precipitating Clouds algorithm description
  3. List of inputs for Precipitating Cloud
  4. Coverage and resolution
  5. Description of Precipitating Cloud outputs
  6. Example of Precipitating Cloud visualisation

Goal of the Precipitating Cloud product

The online-processing for the Precipitating Clouds product derives the precipitation likelihood from avhrr and microwave data.

The objective of the pc product is to support detailed precipitation analysis for nowcasting purposes, especially in areas for which no radar data is available. The focus is on the delineation of non-precipitating and precipitating clouds and on giving a rough indication on intensity to be expected. The output is the likelihood of precipitation in three intensity intervals of light precipitation (0.1-0.5mm/h), moderate precipitation (0.5-5.0mm/h) and heavy precipitation (>5mm/h).

Precipitating Clouds algorithm description

The algorithm to retrieve information on the presence (including rough intensity estimations) of precipitation is based on the Cloud Type output, and it uses the spectral information in the vis/ir of the avhrr (or viirs ) and, when available, it also uses the mw information of the amsu-A/B. For the vis/ir, the day-time algorithm relies on the microphysical information available in the 1.6 micron and 3.7 micron channels, and the night-time algorithm relies on the 11 and 12 micron channels. When amsu-data is present the algorithm takes advantage of as well the avhrr as the amsu-data, where the amsu-algorithm utilises the scattering signal of precipitation size ice particles in the cloud.

The Precipitating Clouds product is aimed for Nowcasting applications. It shall give the forecaster a quick overview over the probability of precipitation in a few coarse intensity intervals. Important features are that the scheme shall work seamlessly over land and sea, and shall work for both day and night, and for both stratiform and convective cases. The product can be used by the forecaster as a complement to radar data, especially in areas without radar coverage.

List of inputs for Precipitating Clouds

pps products as input: Cloud Type

Satellite data:

Auxillary data:

Coverage and resolution

avhrr or viirs data alone: Full avhrr or viirs resolution.

Using avhrr and amsu data together: Full avhrr (1 km) resolution, but the image is still influenced by the amsu resolution; where the smallest amsu footprint is 15 km.

Description of Precipitating Clouds outputs

The pc product consists of a numerical value for the likelihood of precipitation within the following four precipitation intensity classes:

For each intensity the following probability classes are used:

No separate quality flag is included in the pc product. Provision of the probability of precipitation in each of the intensity classes gives an indication of the uncertainty of the estimate. The output also includes some processing flags, giving information about how processing was performed.

Example of Precipitating Clouds visualisation

noaa19_20120514_1125_16826.mesanX.precipclouds.jpg (96000 bytes) Colour coding:

The image of the precipitating clouds product is created as an rgb colour composite where:

  • layer 1 (red) is assigned to the probability for precipitation intensity class 3, intensive precipitation
  • layer 2 (green) is assigned to the probability for precipitation intensity class 2, light/moderate precipitation
  • layer 3 (blue) is assigned to the probability for class 1, risk for precipitation.

Bright colours means well defined precipitation intensities (sharp probability functions) and greyish colours mean flat probability distributions. For instance bright red indicates high likelihood for heavy precipitation.

noaa19_20120514_1125_16826.mesanX.avhrr.ch_06_09_11.thumbnail.jpg (476000 bytes) noaa19_20120514_1125_16826.mesanX.precipclouds.jpg (96000 bytes)

Here an example image of the pps Precipitating Clouds product over northern Europe is displayed. To the right is the Precipitating Cloud product with data from a noaa19 overpass (orbit 16826) May 14, 2012, 11:25 UTC, as received and processed at smhi, Norrköping, Sweden. On the left the corresponding rgb images using avhrr channel 0.6µm,0.9µm,11µm and . Below is the corresponding radar composite based on the Baltic radar network. Please observe that this radar composite does not cover eg. off the Norweigan coast or southern Finland

The Precipitation Cloud image shown here has been generated with the nwcsaf/pps software. It displays only part of the information available in the product. The image is an rgb image where red is assigned to the probability of heavy precipitation, green to the probability of moderate precipitation and blue to the probability of light precipitation.

The pps Cloud products are most appropriately visualised using a dedicated Graphical User Interface (gui) tool allowing the display of all relevant information available in the hdf5 file, including flags, geographical location, etc. smhi has developed such a tool running under Unix and freely available upon request. Otherwise the hdf-viewer (hdfview) available from ncsa is recommended.