A 25-yr climatology of large-amplitude hourly surface pressure changes over the conterminous United States

Hourly surface pressure observations for a 25-yr period (1949-63; 1984-93) for the conterminous United States have been used to map the distribution o f large hourly pressure changes defined as falls or rises in excess of 4.25 hPa. Initially, 8431 reports satisfying this pressure change threshold wer e obtained. After error checking, this number was reduced to 5380 occurrenc es. Large hourly surface pressure changes are most common over the Great Pl ains and New England and least common over the Southeast and Southwest. Lar ge pressure falls are confined almost exclusively to the Plains, upper Midw est, and New England, and are virtually absent over the Intermountain West and parts of the Appalachians. A manual (subjective) analysis identified 1038 inertia-gravity wave (IGW) o ccurrences from the 5380 large hourly surface pressure change occurrences. IGW occurrences are most common across the Plains and from the Great Lakes toward western New England, and are virtually absent across the Intermounta in West. A partitioning of IGW occurrences on the basis of weather type rev eals that IGWs associated with cyclones occur preferentially from Nebraska eastward across the Great Lakes to New England, while IGWs associated with convection are mostly confined to the Plains. IGW occurrences are most likely seasonally in winter and spring and diurnal ly near 0300 and 1200-1300 LST. Considerable interannual variability is see n in the distribution of 579 IGW events (an event may include IGW occurrenc es at several stations). Twenty-three IGW events (roughly 2 events per mont h) occur each pear over the conterminous United States and 8 of the 23 even ts involve multiple stations. IGW events are most prevalent during the Marc h-June period. Representative composites of mean sea level pressure, 1000-500-hPa thicknes s, 500-hPa geopotential height, and 500-hPa geostrophic wind are shown for cool season (November-April) IGW events associated with cyclones and early warm season (May-June) IGW events associated with convection. The composite s show that IGWs occur preferentially in a warm air advection region to the northeast of the surface cyclone center poleward of a frontal zone and ben eath a band of relatively strong southwesterly flow at 500 hPa. The composi te results are in broad agreement with the signatures identified by Uccelli ni and Koch as generally common to occurrences of large-amplitude IGW event s identified in the literature.