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Snow Cover Frequency Calculations

Video Demonstration of Snow Cover Frequency Calculations

This short video walks you through calculating Snow Cover Frequency using the cloud-based computing tools that we developed. The base images of snow cover are from NASA’s MODIS instrument (MOD10A1).

Snow Cover Frequency Calculations

SCF is computed as the number of days that a remote sensing pixel is mapped as snow-covered (Nsnow) divided by the number of valid observations in each month (Nobs).

SCFmonthly = Nsnow ⁄ Nobs

Thus, each SCF value for a pixel is a frequency between zero and 1. These monthly data are then spatially averaged across the defined watershed. Our pilot study has three watersheds: La Laguna, Chile; Aragon, Spain; and John Day, Oregon, USA.

Because clouds may obstruct the remote sensing instrument, we have implemented a cloud-correction to the SCF calculation using the logic described below.

  • If a pixel is cloud-covered on days that are bounded by snow observations then the cloud-covered days are interpreted as snow-covered.
  • If a pixel is cloud-covered on days that are bounded by snow-free observations, then the cloud-covered days are interpreted as snow-free.
  • If a pixel is cloud-covered on days that are bounded by a prior snow-covered day and a post snow-free day, then the cloud-covered days are interpreted as snow-free.
  • If a pixel is cloud-covered on days that are bounded by a prior snow-free day and a post snow-covered day, then the cloud-covered days are interpreted as snow-covered.
  • A corollary to the four conditions above is that if cloud-covered days are followed by a snow-covered day, then the cloud-covered days are considered as snow-covered. And if cloud-covered days are followed by a snow-free day, then the cloud-covered days are considered snow-free.

For access to the Mountain Hydroclimatology tools, please contact nolina<at>oregonstate.edu.

And finally – A hands-on tutorial of the model with examples from three different watersheds.
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