Observing and Understanding the Variability of Water in Weather and Climate
17TH Conference on Hydrology


The temporal and spatial variability of drizzle in North America

Addison L. Sears-Collins, University of Virginia, Charlottesville, VA; and D. M. Schultz and R. H. Johns

Drizzle is a significant forecast challenge. Although most recent efforts to understand drizzle have been associated with marine stratocumulus clouds, few studies have examined the temporal and spatial distribution of drizzle and the associated surface and upper-air conditions. This paper investigates those subjects through a climatology created using surface observations from 609 stations across North America between 1976 and 1990 and the North American Radiosonde Database.

A frequency distribution for the hour of most frequent occurrence of drizzle for North America reveals a sharp increase from 9 UTC to 12 UTC followed by a steady decline from 13 UTC to 23 UTC. East coast stations of the United States tend to report 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.

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 North America, except for central Canada where drizzle occurs most frequently in September. 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.

The combination of low visibility and supercooled drizzle droplets presents a weather hazard, especially for aviation. An analysis of the surface conditions associated with drizzle indicates that 68% of the drizzle observations occurred at visibilities less than or equal to 6400m (4 mi). Many stations in Alaska, Canada, and the northern United States have reports of drizzle at these low visibilities with subfreezing temperatures. An understanding of the hourly and monthly distributions, surface conditions, and upper-air conditions associated with drizzle in this region will help forecasters better anticipate these potentially hazardous events.

extended abstract  Extended Abstract (196K)

Joint Session 6, Spatial and Temporal Variability of Water in All its Phases: Part 3 (Joint with the Symposium on Observing and Understanding the Variability of Water in Weather and Climate and the 17th Conference on Hydrology)
Wednesday, 12 February 2003, 8:30 AM-9:30 AM

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