NWC REU 2002
May 12 - July 20



A Climatology of Drizzle for North America

Addison Sears-Collins, Bob Johns, and Dave Schultz



Drizzle is a significant forecast challenge. Although most recent efforts to understand drizzle have been associated with marine stratocumulus and stratus clouds, few studies have examined the spatial and temporal distribution of drizzle. This paper investigates those subjects through a climatology created using surface observations from stations across North America between 1976 and 1990.


A monthly distribution of maximum drizzle occurrence reveals that 40% of the stations in North America have a drizzle maximum from November to January. These stations cover much of the United States. In Canada, the temporal variability is greater, and maximum drizzle occurs from September to December.


An analysis of the hour of most frequent occurrence of drizzle for North America reveals that east coast stations of the United States tend to report maximum drizzle earlier in the day (~10-11 UTC) than West coast stations (~14-15 UTC). This result suggests that drizzle is affected by the diurnal cycle because the increase in mixing of the boundary layer at sunrise produces instability, resulting in turbulence that increases opportunities for collision-coalescence to produce drizzle. Freezing drizzle also may be related to the diurnal cycle. The result of a recent study on freezing drizzle will be used for comparison.


An examination of the average number of hours of drizzle at each hour per year reveals that the temporal distribution at these high drizzle frequency locations in North America is varied. In addition, due to either local effects or a low number of drizzle observations over the 15-year period, a small percentage (13%) of stations dispersed around the continent report drizzle maxima from June to August.

Paper available upon request.