Geographical Comparison of Potential Temperature and Specific Humidity Fields around Extra-tropical Cyclones in the Northern Hemisphere

Abstract

Temperature and humidity fields in the low-level atmosphere around cyclones, which are assumed to differ for each individual cyclone, were objectively analyzed through a year on the hemispheric scale to detect their representative spatial patterns. Geographical and seasonal characteristics for the appearance of cyclones as related to these patterns are comparatively shown over the middle latitudes of the Northern Hemisphere.
Cyclone positions at 1000 mb and surrounding potential temperature (θ) and specific humidity (q) fields at 850 mb are objectively computed using NMC data from August, 1985 to July, 1986. The distribution of the air-mass fields and the cyclone positions in the analysis year were firstly examined in relation to the former climatological maps. Then, representative spatial patterns of θ and q around cyclones with cloud areas are obtained as a function of the three rotated principal components (RPCs). The first two spatial fields of the RPCs-θ show a relatively symmetric correlative structure with southwest-northeast and southeast-northwest dipole patterns, respectively. However, the first spatial field of RPC-q shows a concentric structure with meridional asymmetry. Case studies indicate that the RPCs-θ pattern represents the actual distribution of temperature and its evolutional variations along the border between air masses, and RPCs-q represents the distribution of absolute humidity gradient, which is recognized as the margin of the sub-tropical air mass in the middle to lower latitudes in summer.
The frequency distributions of cyclone appearance with representative plus and minus RPCs are composed for θ and q, and shown with seasonal mean θ and q fields over the Northern Hemisphere. The appearance of cyclones with the first two RPCs-θ patterns was determined by the relative location along the edge of the cold air mass trough, which is formed by the intrusion of a cold air mass from the Arctic region. Large-scale topography, such as straits, large plains and chained mountains, determines the path of the cold air mass from the north, and reflects the strong regional characteristics of the RPCs-θ values affecting the appearance of cyclones. Prevailing regions for critical RPCs-θ values shift longitudinally according to the season and according to the major routes of air mass intrusion. Meanwhile, cyclones with strong RPC-q patterns appear from the southwestern Tibetan Plateau to the southeastern side of Eurasia, from the northeastern Tibetan Plateau to the northwest Pacific and southern North America to Canada along the east side of Rocky Mountains, in summer. These regions overlap with the western margins of maritime Polar Frontal Zones and the Siberian-Canadian Arctic Frontal Zone.