Simple Rainfall Assimilation
Problem:
- in test assimilations, noticed dry zone in South America
- dry zone were associated with very dry soil moisture
- positive feedback, especially in data-poor regions
- wanted a way to constrain the soil moisture (not nudging)
- must be global
A simple scheme:
- combined the observed and model precip. to correct the soil moisture
- Xie-Arkin's pentad precipitation (satellite and gauge measurements)
- takes observed precip, model precip and model runoff for the
previous pentad to calculate a correction for the next pentad
- if model runoff is zero then
- use (ObsPrecip - ModelPrecip) to correct top soil layer
- if ObsPrecip < (ModelPrecip - Runoff) then
- correct soil moisture by ObsPrecip - (ModelPrecip - Runoff)
- otherwise
- assume error in the model precip affected the runoff
- misc. details
- errors in the 5-day precipitation are "corrected" over the next
5 days
so that "right" amount of water enters the soil.
- correction is lagged in time
- daily observed precipitation would be better (smaller lag)
- nice feature: no error in the model precip. => no correction term
- no correction is done with the soil is frozen
- if the correction drys out top layer, soil moisture is
removed from the middle layer
- if the correction saturates the top layer, soil moisture
is
added to the middle layer
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