Friday, February 10, 2012

PRECIPITATION PROJECTIONS

This is the time of year when climate data sets are updated to include annual totals for the preceding year (in this case 2011). Most sites concentrate on temperature - though sometimes include not just observed atmospheric temperature but also variables such as modelled projections and temperature in the oceans. One variable which is often forgotten is precipitation. After all, the positive feedback from water vapour assumes that it remains in the atmosphere rather than becoming precipitation.

On the chart below we use two data sets. The first is the NCDC 5° gridded precipitation anomaly at http://www1.ncdc.noaa.gov/pub/data/ghcn/v2/grid/grid_prcp_1900-current.dat.gz. To get a monthly global figure we averaged the data, cosine weighted on latitude to compensate for reducing grid sizes. The values are in millimetres. The second data set was of precipitation hind-cast/projected downloaded from the Climate Explorer web site at http://climexp.knmi.nl. The data set used was described as “all models, 20c3m/sresa1b” and included 23 models. These data were in mm/day so to convert then to equivalent units they were multiplied by the number of days in the month. They were adjusted to give values relative to the period 1980 to 2010. As trends were masked by month-to-month variations the five year centred moving averages are also plotted.




This shows, as stated in the IPCC TAR4 report, that the variance of the simulated precipitations is less than that of the observed values (TAR4 section 9.5.4.2.1). The difference, based on the 5-year moving average, is as high at 2 mm/month which is equivalent to 1.88 W/m2. (1 mm evaporation over 1 m2 weighs 1 kg. The latent heat of evaporation of 1 kg of water is 2.45 Mj. 1 kWh is 3.6 Mj.)

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