Diabatic heating model of the Indian monsoon

In Part I of this paper, influence functions are derived for the response of a quasi-geostrophic atmosphere to transient heat sources and sinks, assuming that the effects of horizontal advection can be neglected and assuming a fairly reasonable vertical distribution of static stability. The influence is studied for diabatic heating of different horizontal wavelengths and for two different types of the vertical distribution. In Type I, heating is largest at the ground, decreasing to zero at p = 0. In Type II, heating is maximum in the middle atmosphere and decreases parabolically to zero at p = 0 and at the ground. It is shown that, in both types, the horizontal wavelength L of the heating function is very important in determining not only the intensity of pressure fall in the lower levels and of pressure rise aloft in the region of heating, but also the level of maximum pressure effect. It is seen that wavelengths of the order of 15,000 km produce maximum geopotential variations around the 150-mb leve

Meso-scale distribution of summer monsoon rainfall near the Western Ghats (INDIA)

The spatial distribution of southwest monsoon rainfall is studied over Maharashtra State (India), which includes part of the well-known Western Ghats mountain range, near its western boundary, running almost from north to south, perpendicular to the summer monsoon current in the lower troposphere. Meso-scale analysis of daily rainfall is performed for Maharashtra State, including the Western Ghats, for the two mid-monsoon months of July and August, during the 10-year period of 1971-1980. Strong and weak monsoon days were identified for the 5-year period of 1976-1980. The meso-scale pattern of average daily rainfall is obtained separately for strong and for weak monsoon conditions. All these average patterns show the following features: (i) the rainfall increases rapidly from the Arabian Sea coast close to the line of maximum height of the Western Ghats; (ii) there are two rainfall maxima corresponding to the two mountain peaks parallel to the coast line; (iii) between the two mountain peaks, there is a valley which is narrow at the western end (upwind end), broadening towards the east (on the downwind side). Ground contour height of the valley rises eastwards and ends as a part of the Deccan Plateau east of the Ghats. Here the valley opens out like a funnel with higher mountains flanking its two sides. In the valley, the rainfall increases from the coast up to the line of maximum height of the Ghats, and then decreases eastwards towards the plateau. The rainfall isopleths also take a funnel-shaped configuration. An interesting feature is that near the wider section of the valley funnel, there is a rainfall minimum and then the rainfall increases further eastwards on the downwind side. This feature of rainfall minimum is somewhat similar to the rainfall minimum reported by Asnani and Kinuthia (personal communication); Asnani (Asnani GC. 1993. Tropical Meteorology, Vol. I. Prof. G.C. Asnani: Pune, India; 603) attributed the rainfall minimum to the Bernoulli effect. A somewhat similar phenomenon is assumed in the present study area

Rossby wave and pure rotational wave

Let us consider pure inertial motion of particle on a friction less horizontal surface of rotating earth in a B- plane approximatio

Equatorial cell in the general circulation

For some time we have been familiar with a general circulation model in which two tropical Hadley cells have a common limb of upward vertical motion near the equator

Patchy layered structure of tropical troposphere as seen by Indian MST radar

The MST radar observations at Gadanki (13.47° N, 79.18° E) show, almost every day throughout the year, stratified layers of intense reflectivity near the tropopause level (17 km) and also at a couple of levels between 4 km and 10 km. Highest individual reflectivity values occur near 17 km, but they occur for a short while. The region between 11 km and 15 km shows the lowest values of reflectivity alongwith vertical downward motion almost on all days of the year. High values of reflectivity are attributed to the existence of visible or sub-visible clouds; the layered structure of clouds is attributed to inertio-gravity waves with vertical wavelength of 2-3 km. It is suggested that each high reflectivity layer consists mainly of thin sheets and patches of visible and sub-visible cloud material. Hydrometeors inside the cloud material go up and down due to gravity, precipitation-loading, Brunt-Vaisala oscillations, and Kelvin-Helmholtz waves. In these small-scale motions, thin air sheets and patches get formed with sharp temperature and humidity discontinuities through contact cooling, melting, evaporation, condensation and freezing. Also, melting and freezing at low temperatures generate electrical charges in these thin sheets and patches. These thin sheets and patches have vertical dimensions ranging from a few centimetres to several metres and horizontal dimensions of the order of 1km. These thin sheets and patches have corresponding vertical and horizontal discontinuities and sharp gradients in refractive index for the MST radar beam. These show up as regions of high values of reflectivity

New hupothesis on atmospheric tides

LOUD KELVIN1 seems to have been the first to suggest that the equatorial wave of the semi-diurnal oscillation of the atmosphere of Earth may be explained by the resonant tidal effect of the Sun on an atmosphere with a natural period of oscillation close to 12 h. In the thirties, Taylor2 and then Pekeris3 were able to support the concept of a natural period of 12 h by consideration of the then known vertical distribution of temperature in the atmosphere. This theory is no longer satisfactory for the following reasons, (i) Recent measurements of atmospheric temperature do not lead to a period of 12 h. The work of Lindzen4 on the classical theory, however, suggests that the thermal excitation provided by the ozone and water vapour absorptions in the atmosphere may be adequate to yield the proper magnitude for amplitude of the migratory wave at the equator without a critical dependence on the vertical profile of temperature, (ii) The predicted variation of amplitude from equator to pole is substantially different from the observed one. (iii) There is nothing in the theory to account for the polar semi-diurnal oscillation observed at high latitude