Spatial and Temporal Variability of Nonfreezing Drizzle in the United States and Canada

Abstract

A climatology of nonfreezing drizzle is created using surface observations from 584 stations across the United States and Canada over the 15-yr period 1976-90. Drizzle falls 50-200 h a year in most locations in the eastern United States and Canada, whereas drizzle falls less than 50 h a year in the west, except for coastal Alaska and several western basins. The eastern and western halves of North America are separated by a strong gradient in drizzle frequency along roughly 100°W, as large as about an hour a year over 2 km. Forty percent of the stations have a drizzle maximum from November to January, whereas only 13% of stations have a drizzle maximum from June to August. Drizzle occurrence exhibits a seasonal migration from eastern Canada and the central portion of the Northwest Territories in summer, equatorward to most of the eastern United States and southeast Canada in early winter, to southeastern Texas and the eastern United States in late winter, and back north to eastern Canada in the spring. The diurnal hourly frequency of drizzle across the United States and Canada increases sharply from 0900 to 1200 UTC, followed by a steady decline from 1300 to 2300 UTC. Diurnal drizzle frequency is at a maximum in the early morning, in agreement with other studies.Drizzle occurs during a wide range of atmospheric conditions at the surface. Drizzle has occurred at sea level pressures below 960 hPa and above 1040 hPa. Most drizzle, however, occurs at higher than normal sea level pressure, with more than 64% occurring at a sea level pressure of 1015 hPa or higher. A third of all drizzle falls when the winds are from the northeast quadrant (360°-89°), suggesting that continental drizzle events tend to be found poleward of surface warm fronts and equatorward of cold-sector surface anticyclones. Two-thirds of all drizzle occurs with wind speeds of 2.0-6.9 m s Ϫ1 , with 7.6% in calm wind and 5% at wind speeds у 10 m s Ϫ1. Most drizzle (61%) occurs with visibilities between 1.5 and 5.0 km, with only about 20% occurring at visibilities less than 1.5 km.

Process-Oriented Analysis of Environmental Conditions Associated with Precipitation Fog Events in the New York City Region

“…The period from 1977 to 1996 inclusively is considered to ensure homogeneity in present weather-reporting practices (human-made observations, before the widespread introduction of automated weather stations) and maximize the number of reporting stations. A well-known deficiency of the early generation Automated Surface Observing System (ASOS) was its inability to detect very light precipitation as described by Sears-Collins et al (2006). Human-made present weather observations, although subjective in nature, are believed to provide better sampling of conditions in the vicinity of weather stations.…”

Debris flow initiation in postglacial terrain: Insights from shallow landslide initiation models and geomorphic mapping in Southeast Alaska

“…The high landslide density on western Baranof and Kruzof islands, particularly on south‐ and southwest‐facing hillslopes, correlates with ocean‐facing coastal areas particularly exposed to extreme precipitation and winds as southwesterly atmospheric rivers make landfall (Sharma & Déry, 2019, 2020). Regionally, the western coastline of Southeast Alaska experiences demonstrably higher precipitation rates and hours of non‐freezing precipitation than inland areas (Sears‐Collins et al, 2006), resulting in higher frequency of the intense rainstorms that trigger landslides. At a more local scale, observed and modelled storm intensities tend to be higher within Sitka Sound as moisture‐laden atmosphere is funnelled into the sound by the surrounding topography (Jacobs, 2019, personal communication).…”

Giant and Ultragiant Aerosol Particle Variability over the Eastern Great Lakes Region

“…Despite the potential importance of GA in continental clouds, few aircraft observations of GA over continental regions have been published in the formal literature, 1 and none have presented vertical profiles over the same geographical region over a duration sufficient to allow analysis of their temporal variability. Given the relative high tendencies over this region for drizzle to be observed at the surface (Sears-Collins et al 2006) and for the intercept of supercooled drizzle by aircraft (Cober et al 2001), these observations should prove useful to numerical modeling studies that seek to understand these high tendencies, in addition to decreasing the paucity of GA observations over continental areas.…”