Myles Allan, Constraints for Probabilistic Forecasts Myles Allan ...

Bruce Hewitson, Cohesive spatial changes of precipitation and seasonality over South
Africa
Bruce Hewitson
CSAG, University of Cape Town, South Africa
While much historical climate change analysis has been undertaken for many regions of the
world, this is most often focused on point source data – typically surface station observations.
This raises the question over whether the station is spatially representative, and following from
this, whether appropriate inferences may be made about change on the regional scale.
Compounding this is the question of whether the change is occurring in the base values of the
measured variable, or in the statistics or characteristics of occurrence of events.
This talk examines these issues over South Africa using a new gridded data set of daily
precipitation, interpolated from a high station density, and with explicit recognition of the
variable spatial representivity of each station. This latter attribute is further refined by
recognizing that the spatial representivity of any given station is itself a function of the
synoptic weather state. The 50-year period from 1950-1999 is examined in terms of the daily
precipitation values as expressed in the monthly statistics of wet and dry spell duration, wetwet
and dry-dry day probabilities, median rainfall, number of raindays exceeding a threshold,
and the 90 th percentile values. The spatial cohesiveness of historical change is assessed in
terms of trend, variability, and seasonal changes; collectively showing regional trends within
South Africa.