Climatologies of Precipitation Bands over the USA and UK

In the 1970s and 1980s, Bob Houze’s research led to a classification of rainbands in extratropical cyclones that is still used today.  Today, with nationwide networks of Doppler radar, freely available datasets, and computational power, we can construct a climatology of banded precipitation across the United Kingdom and the 48 contiguous states of the United States; the results of such an analysis are presented here.  Over the United Kingdom, radar data from 2006–2015 is used, whereas over the United States, radar data from 2003–2014 is used.  A band is defined as a contiguous feature with an intensity exceeding 0 mm per hour rain rates (UK) or radar reflectivity equal to or exceeding 20 dBZ (USA) that is at least 100 km long with an aspect ratio (length of major axis to length of minor axis) of at least 3:1.  By applying an automated feature-based detection algorithm to composite radar imagery, over 1.8 million bands were identified in the UK and over 7.2 million bands were identified in the USA.  

The spatial maps of these two climatologies illustrate the most frequent locations of banded precipitation in the UK and USA.   Specifically, in the UK, areas on the west coast of England, Wales and Scotland experience about 50% more banded precipitation than equivalent areas in the east.  When convective bands are considered (precipitation rates exceeding 10 mm per hour over at least 100 square km), southwestern England is highlighted, likely due to the converging sea breezes along the South West Peninsula.  In the USA, areas east of the Rockies experience banded precipitation nearly three times more frequently than areas west of the Rockies.  Areas downwind of the Great Lakes show prominent cool-season maxima associated with lake-effect snowstorms.  Local maxima in the Dakotas and east of the Colorado Rockies are consistent with previous case studies of banded precipitation.

In the UK, banded precipitation features comprise 18% of all precipitation features by occurrence, but contribute 26% of the total precipitation.  In contrast, in the United States, banded-precipitation features comprise only 14.8% of all precipitation features in the United States, and contribute 21.9% of the annual precipitation occurrence.