For a number of gridcells, all between the 500mm and 800mm rainfall isohyets in sub-Saharan Africa, I've got dekadal rainfall (from FEWS). They are pretty spread out over the continent and there are a lot of them, so they'll have different seasonality, though they should all be roughly unimodal (particularly given the latitudes that I've got).
I want to define an algorithm to define the average start and end of the rainy season. So far, I'm thinking of the following:
For each gridcell:
- Define the month of average maximum rainfall
- Find the month prior to the average maximum rainfall month that minimizes the sum of 3 lags of average monthly rainfall, subject to that month containing at least 20% of annual rainfall. Define it as the average start.
- Find the month after the average maximum rainfall month that does the same thing, only with 3 forward lags of average monthly rainfall. Define that month as the end of the rainy season.
The 20% figure seems arbitrary, though I'm not sure what else to do. I'm not really a climate scientist, though maybe I could play one on television. Is there an accepted convention here? If not, any suggestions on improving my algorithm?