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The fields of atmospheric modelling and numerical weather prediction are ever changing and evolving. Theoretical and observational studies confirm that orographically - forced gravity waves lead to a significant drag in the troposphere rather than the stratosphere. The present study describes the parameterization of orographic gravity-wave drag as used in LMD-GCM. The complete wave-breakdown is assumed to occur in the model layer, where the convective instability condition is first satisfied. The beneficial impact of the parameterization of gravity wave drag helps in removing the excess of westerlies in the northern hemisphere. There is very little impact on southern hemispheric circulation.

In two balloon flights carried out in the Southern Hemisphere, a region of the sky near the galactic center has been explored with a Spark Chamber telescope with the aim of investigating the gamma-ray emission at energies above 20 MeV from possible celestial sources.

A gamma-ray telescope with a 850 cm2 multiplate spark chamber as a detector, has been used in an experiment of six balloon flights to investigate the Crab Nebula Pulsar 0532. The triggering signal is provided by a plastic scintillator-directional Čerenkov counter system. The spark chamber events are photographed by a stereo camera. The time of arrival of each event is recorded in UTC time with one millisecond accuracy, to search for a possible pulsed gamma emission from NP0532 above 50 MeV.

WE have investigated the gamma-ray emission from the Crab Nebula in a series of balloon flights of a multiplate spark chamber triggered by a Cerenkov detector-scintillator coincidence system1. The previously quoted gamma-ray energy threshold for pulsed emission from the Nebula and NP 0532 (refs. 1, 2) is ∼50 MeV; a detailed investigation of the telescope efficiency as a function of energy has now shown that the efficiency extends down to 20 MeV.

The gamma-ray emission from the Crab Nebula and NP-0532 has been investigated in a series of balloon flights of a multiplate Spark Chamber triggered by a Cerenkov detector-scintillator coincidence system (Parlier et al., 1973; Leray et al., 1972; Vasseur et al., 1971), having a gamma-ray energy threshold for detection at 20 MeV. Figure 1 shows a schematic view of the gamma-ray telescope; Figure 2 the view of the telescope without its thermal protection.

A SEARCH for pulsed gamma ray emission from NP 0532 has been carried out with a balloon-borne experiment using a spark chamber as the central detector. This pulsar, situated in the Crab Nebula, is already known to pulsate in the radio, optical and X-ray range1–5.

Results from 90-day simulations with the LMD GCM are described, where sea surface temperatures of 1987 or 1988 years are respectively prescribed. The initial states correspond to 1 June 1987 and 1 June 1988. The simulated precipitation rates over India show a strong contrast between the two years, with drought occurring during summer 1987 and abundant rainfall during summer 1988. The dry regime simulated during 1987 corresponds to an eastward displacement of the outflow at 200 mb and a weaker westerly flow at the surface as compared with 1988, both features being in agreement with reality. Because it is more difficult for models to simulate rainfall differences than to simulate wind variations between the two years, the changes in simulated rainfall over India are studied in more detail. In particular, more integrations are carried out to test the sensitivity of rainfall variations to initial conditions, and the result is that the decrease of rainfall in 1987 compared to 1988 is a robust feature ...