Qualifying swaths with cloud cover estimation

Each swath can be qualified with cloud cover estimation, as a result of intersection analysis.

Two parameters are calculated:

Cloud Statistic

It provides the cloud probabilitypercentage for the month when the swath is scheduled, measured for the intersection between AOI and Swath.

It is calculated against a grid of 1 x 1 degree geographical.

Cloud Forecast

Cloud probability percentage forecast for the date and hour of the swath, measured for the intersection between AOI and Swath.

It can be obtained when the swath date is in the boundary [now - 30 days, now + 16 days], with "now" meaning current time. Out of those boundaries, or in the absence of connection to the weather server, the cloud forecast will be -1%, meaning "estimation not available".

It is calculated against a grid of 0.25 x 0.25 degree geographical.

Enabling cloud cover calculation

Cloud cover estimations will require additional processing, with a some impact in calculation performance.

In some cases, e.g. SAR satellites, you may not be interested in cloud cover estimations, so you can gain some performance improvement by disabling them.

If internet is not available, it is advisable to disable cloud forecast calculation.

To enable or disable cloud cover estimation go to Edit / Properties ... /Swaths/Benchmarks and select between STATISTIC & FORECAST, FORECAST, STATTISTIC or NONE.

With cloud cover calculation disabled, the cloud estimation parameters (either Statistic or Forecast) in each swath will be set to -1 %, meaning "estimation not available"

Cloud estimation performance

Performance impact is noticeable mostly when activating cloud forecast, due to the need to download weather forecast data.

The measured impact is in the order of 3 to 4 seconds for every 3-hour period of the simulation window which contains swaths. For example, if your simulation window is 48 hours, it leads to 48 / 3 = 16 "3-hour periods". Assuming that only 11 of these periods contain valid swaths, the performance impact would be 33 to 44 second approximately, in addition to the regular calculation without cloud forecast.

Weather data downloaded remains in the local cache and is used in the next calculation. Therefore the performance impact should be observed only the first time when running a simulation for a given simulation window.

We recommend that you test SaVoir in your own environment with and without cloud forecast estimation to obtain a feeling of the performance impact.

Cloud data sources

Cloud Statistic

The cloud statistical source is a predefined dataset distributed with SaVoir , resident in the local disk in the SaVoir installation path.

The dataset contains monthly average, minimum, maximum and standard deviation estimates of cloud probability obtained from NASA satellites TERRA (morning) and AQUA (afternoon). Statistics are available for each month of the year at two different hours of the day, 10:30 UTC, averaged between years 2000 to 2016, and 13:35 UTC, averaged between years 2002 to 2016.

Cloud Forecast

Cloud forecasts are obtained by accessing a GRIB weather service via internet during swath calculation. By default SaVoir is configured to use NOAA's national weather service. It is possible to configure a different server address in Edit / Properties .../Swaths / Clouds/ Forecast server.

SaVoir uses the "Entire Atmosphere" cloud dataset of NOAA.

Calculating cloud probability

The following steps are followed to calculate the cloud probability:

  1. Calculate the swath intersecting the AOI
  2. Isolate the intersection area between AOI and swath.
  3. Calculate the bounding box of the intersection area
  4. Obtain a cloud probability texture (statistic or forecast) for the swath date and time and crop it to the bounding box
  5. Calculate the average cloud fraction, as the average of all points of the cropped texture which fall inside the intersection area.



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