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- Author or Editor: F. Désalmand x
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Abstract
Four thermal diffusion chambers with parallel plates, having different geometric characteristics, modes for reaching supersaturation (on cooling or heating the plates) and droplet-counting techniques, have been compared in the range of super saturations between 0.15 and 1.25%. The concentrations measured by two chambers which are operated simultaneously may be correlated by a power function fitted with the characteristic law of cloud condensation nuclei N = CSk , where N is the concentration of the nuclei activated at supersaturation S(%), and C and k are parameters depending on the air mass and the whole of the device.
The heating of the upper plate, a small value of the ratio between the diameter of the chamber and its height (aspect ratio), and the inner turbulence are factors which increase the value of the measured concentration.
For values of N at ∼3500 and 4200 cm−3 the responses of the four chambers are very close, and for lower or higher values of N their discrepancies increase.
Abstract
Four thermal diffusion chambers with parallel plates, having different geometric characteristics, modes for reaching supersaturation (on cooling or heating the plates) and droplet-counting techniques, have been compared in the range of super saturations between 0.15 and 1.25%. The concentrations measured by two chambers which are operated simultaneously may be correlated by a power function fitted with the characteristic law of cloud condensation nuclei N = CSk , where N is the concentration of the nuclei activated at supersaturation S(%), and C and k are parameters depending on the air mass and the whole of the device.
The heating of the upper plate, a small value of the ratio between the diameter of the chamber and its height (aspect ratio), and the inner turbulence are factors which increase the value of the measured concentration.
For values of N at ∼3500 and 4200 cm−3 the responses of the four chambers are very close, and for lower or higher values of N their discrepancies increase.
Abstract
Systematic measurements of cloud condensation nuclei (CCN) concentration have been carried out in the Abidjan area from December 1977 to June 1978, at 0700 local time (corresponding to a daily maximum). In that period of the year, from the main dry season to the main rainy season, a general decrease of CCN concentrations has been noted. This decrease is more marked for the more active CCN. In May and June 1978, during the continuous monsoon rains, the concentrations substantially increased, this effect being more important for the fraction of the less active CCN. Nuclei of continental origin accompanying dust haze in December 1977 were progressively eliminated from the atmosphere by dry or wet deposition. They wore replaced by smaller nuclei. originating in vegetation, the production of which was increased by the continuous monsoon rains.
Abstract
Systematic measurements of cloud condensation nuclei (CCN) concentration have been carried out in the Abidjan area from December 1977 to June 1978, at 0700 local time (corresponding to a daily maximum). In that period of the year, from the main dry season to the main rainy season, a general decrease of CCN concentrations has been noted. This decrease is more marked for the more active CCN. In May and June 1978, during the continuous monsoon rains, the concentrations substantially increased, this effect being more important for the fraction of the less active CCN. Nuclei of continental origin accompanying dust haze in December 1977 were progressively eliminated from the atmosphere by dry or wet deposition. They wore replaced by smaller nuclei. originating in vegetation, the production of which was increased by the continuous monsoon rains.
Abstract
The monsoon depressions that form over India during the summer are analyzed using simulations from the Laboratoire de Météorologie Dynamique general circulation model. This type of synoptic system often occurs with a frequency of one to two per month and can produce a strong Indian rainfall. Two kinds of analyses are conducted in this study. The first one is a subjective analysis based on the evolution of the precipitation rate and the pattern of the sea level pressure. The second one is an objective analysis performed using the TRACK program, which identifies and tracks the minima in the sea level pressure anomaly field and computes the statistics for the distribution of systems.
The analysis of a 9-yr control run, which simulates strong precipitation rates over the foothills of the Himalayas and over southern India but weak rates over central India, shows that the number of disturbances is too low and that they almost never occur during August, when break conditions prevail. The generated disturbances more often move north, toward the foothills of the Himalayas. Another analysis is performed to study the effect of the Tibetan Plateau elevation on these disturbances with a 9-yr run carried out with a Tibetan Plateau at 50% of its current height. It is shown that this later integration simulates more frequent monsoon disturbances, which move rather northwestward, in agreement with the current observations. The comparison between the two runs shows that the June–July–August rainfall difference is in large part due to changes in the occurrence of the monsoon disturbances.
Abstract
The monsoon depressions that form over India during the summer are analyzed using simulations from the Laboratoire de Météorologie Dynamique general circulation model. This type of synoptic system often occurs with a frequency of one to two per month and can produce a strong Indian rainfall. Two kinds of analyses are conducted in this study. The first one is a subjective analysis based on the evolution of the precipitation rate and the pattern of the sea level pressure. The second one is an objective analysis performed using the TRACK program, which identifies and tracks the minima in the sea level pressure anomaly field and computes the statistics for the distribution of systems.
The analysis of a 9-yr control run, which simulates strong precipitation rates over the foothills of the Himalayas and over southern India but weak rates over central India, shows that the number of disturbances is too low and that they almost never occur during August, when break conditions prevail. The generated disturbances more often move north, toward the foothills of the Himalayas. Another analysis is performed to study the effect of the Tibetan Plateau elevation on these disturbances with a 9-yr run carried out with a Tibetan Plateau at 50% of its current height. It is shown that this later integration simulates more frequent monsoon disturbances, which move rather northwestward, in agreement with the current observations. The comparison between the two runs shows that the June–July–August rainfall difference is in large part due to changes in the occurrence of the monsoon disturbances.
