On the Sub-tropical Jet Stream and its Role in the Development of Large-scale Convection

Abstract

This paper contains mainly the results of a synoptic and climatological study of the largescale convection in northern India and Pakistan during the three months preceding the onset of the southwest monsoon. It has been shown that the sea-level and lower tropospheric charts give little clue to the development of the large-scale convection and that the latter is overwhelmingly determined by the divergence in the waves in the sub-tropical jet-stream. It has further been shown from detailed synoptic evidence that nor'westers, andhis and the majority of the thunderstorms without squalls in northern India and Pakistan in the pre-monsoon period are fundamentally the same phenomenon. The role of cold-air advection in the middle and upper troposphere in the development of large-scale convection has also been discussed. This study has further revealed that the regions of upper-divergence and convergence can be qualitatively located by identifying certain typical patterns on the high-level maps more than 12 hours before the usual time of commencement of convection and that, consequently, these maps can be used as effective tools in the issue of area-warnings against thunder in general and nor'westers and andhis in particular.A general study has also been made of the large-scale convection in southeast Australia, southeast Brazil and southeast United States and the similarities between the large-scale convection in these countries and in Indo-Pakistan have been brought out. O n the basis of these studies, it has been suggested that the jet stream plays the very important role of producing large-scale convection in the subtropics all over the world wherever it over-runs on its equatorward side, moist air possessing a high degree of latent instability.

Winter westerly disturbance dynamics and precipitation in the western Himalaya and Karakoram: a wave-tracking approach

“…The general dissociation between premonsoon storms in this region and lower-tropospheric circulation was first observed by Ramaswamy (1956), in which the usefulness of upper-tropospheric charts for forecasting these events was also discussed. Furthermore, any methodology employing data below the 500-hPa level would be significantly affected by high topography over much of central Asia and yield dubious results.…”

“…In December, January, and February, the average geopotential height anomaly is up to 70 m deeper than in March, April, or November. From December through February, the meridional temperature gradient across the northern hemisphere is at a maximum, and thus, mid-latitude storm intensity peaks during this time (Ramaswamy 1956). WWD occurring during these months are dynamically more intense, with deeper troughs and stronger wind speeds than those occurring in March and April.…”

“…March is the most variable month as WWD transition from primarily orographic to orographic and convective forcing. After April and before the onset of the monsoon, large-scale convection is unlikely as the subtropical jet has migrated to the north of the Tibetan Plateau and upper-level divergence associated with waves in the jet is absent (Ramaswamy 1956). …”

Out‐of‐phase relationships between convection over northwest India and warm pool region during the winter season

“…On the other hand, the baroclinic instability, which is associated with vertical shear, grows by converting available potential energy associated with the mean horizontal temperature gradient, which provides thermal wind balance for the vertical shear in the jet stream. The upper-level horizontal shear increases the upper-level divergence towards its equator-ward side of the jet stream core (Ramaswamy, 1956). The large shifts in storm-track structure occur in response to changes in the subtropical jet due to the two-way interaction between storm tracks and the mid-latitude planetary-scale flow (Cai and Mak, 1990).…”

Seasonal Weather Variations in Khumbu Himal

“…Clear days without any clouds were frequent in the first half of November. It was pointed out that some of the precipitation in the later part of monsoon season falls under the influence of the upper westerly current (Ramaswamy, 1956;Reiter and Heuberger, 1960). Therefore, if we define one series of cloudiness Since that day, the STJ at 500 mb was also fixed south of the Himalayas (Fig.…”