Global Rainfall

From Clive Best

Has rainfall increased or decreased with rising temperatures? NCDC Daily contains the raw precipitation measurements from about 100,000 weather stations back to 1780. I put my computer  (iMAC-i7) to calculate this , but it took a week of CPU time!  Here are the results as compared to temperature anomalies.

Since 1975 global average rainfall on land has increased by about 1mm per day while simultaneously land average temperatures have risen by 1C. However, it appears that rainfall was equally high during the 19th century.

The calculation is similar to that used for GHCN-DAILY temperatures. Where possible I calculate for each station a normalised monthly rainfall between 1961-1990. I then use a 5 level icosahedral grid for averaging and also calculate the average (normal) monthly rainfall within each bin between 1961-1990.

It is very interesting to view how this ‘normal’ daily seasonal rainfall looks  on a 10242 node icosahedral grid. It shows beautifully how rainfall follows the sun. It may seem counter-intuitive but on average there is more rainfall during the summer months, than in those damp cold winter months.

Ocean surfaces warm during summer months, thereby increasing evaporation which then drives rainfall.

The NCDC monthly rainfall ‘anomalies’ are plotted below.

Could the observed increased rainfall between 1810 and 1870 perhaps be due to two extremely large volcanic eruptions – Tambora (1815) and Consiguina (1835)?  There is also a smaller rainfall peak immediately following Krakatoa (1883).

In general though a warmer world is a wetter world.

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