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lmd_all1996_abstracts.html

1996 .

(7 publications)

K. Layal, R. Raghava, J. Polcher, R. Sadourny, and M. Forichon. Simulations of the 1987 and 1988 Indian Monsoons Using the LMD GCM. Journal of Climate, 9:3357--3372, December 1996. [ bib | DOI | ADS link ]

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 of the model.Very early, the importance of evapotranspiration in simulating land rainfall was emphasized. Additional integrations are performed in order to study the impact of the new vegetation scheme introduced in the LMD GCM. It is shown that the contrast in rainfall between the two years is better simulated when the evapotranspiration rate of vegetation cover is represented. When vegetation is not represented in the model, the model does not simulate accurately the interannual variation of the precipitation rates.

A. Harzallah, J. O. Rocha de Aragão, and R. Sadourny. Interannual Rainfall Variability in North-East Brazil: Observation and Model Simulation. International Journal of Climatology, 16:861--878, August 1996. [ bib | DOI | ADS link ]

The relationship between interannual variability of rainfall in north-east Brazil and tropical sea-surface temperature is studied using observations and model simulations. The simulated precipitation is the average of seven independent realizations performed using the Laboratoire de Météorologie Dynamique atmospheric general model forced by the 1970-1988 observed sea-surface temperature. The model reproduces very well the rainfall anomalies (correlation of 091 between observed and modelled anomalies). The study confirms that precipitation in north-east Brazil is highly correlated to the sea-surface temperature in the tropical Atlantic and Pacific oceans. Using the singular value decomposition method, we find that Nordeste rainfall is modulated by two independent oscillations, both governed by the Atlantic dipole, but one involving only the Pacific, the other one having a period of about 10 years. Correlations between precipitation in north-east Brazil during February-May and the sea-surface temperature 6 months earlier indicate that both modes are essential to estimate the quality of the rainy season.

W. Yu, M. Doutriaux, G. Sèze, H. Le Treut, and M. Desbois. A methodology study of the validation of clouds in GCMs using ISCCP satellite observations. Climate Dynamics, 12:389--401, May 1996. [ bib | DOI | ADS link ]

The cloudiness fields simulated by a general circulation model and a validation using the International Satellite Cloud Climatology Project (ISCCP) satellite observations are presented. An adapted methodology is developed, in which the issue of the sub-grid scale variability of the cloud fields, and how it may affect the comparison exercise, is considered carefully. In particular different assumptions about the vertical overlap of cloud layers are made, allowing us to reconstruct the cloud distribution inside a model grid column. Carrying out an analysis directly comparable to that of ISCCP then becomes possible. The relevance of this method is demonstrated by its application to the evaluation of the cloud schemes used in Laboratoire de Météoroligie Dynamique (LMD) general circulation model. We compare cloud properties, such as cloud-top height and cloud optical thickness, analysed by ISCCP and simulated by the LMD GCM. The results show that a direct comparison of simulated low cloudiness and that shown from satellites is not possible. They also reveal some model deficiencies concerning the cloud vertical distribution. Some of these features depend little on the cloud overlap assumption and may reveal inadequate parameterisation of the boundary layer mixing or the cloud water precipitation rate. High convective clouds also appear to be too thick.

Z. X. Li. Correlation of the astrometric latitude residuals at Mizusawa and Tokyo with the southern oscillation index on an interannual time scale. Astronomy Astrophysics, 309:313--316, May 1996. [ bib | ADS link ]

The El Nino/southern oscillation (ENSO) is the most prominent interannual fluctuation in the atmosphere-oceanic system. A single index SOI (Southern Oscillation Index), based on the sea level pressure difference between Tahiti and Darvin, is conventionally used to describe the ENSO phenomenon. Its linkage to other geophysical phenomena is being studied now. The paper studies the correlation of SOI with the latitude residuals by means of cross correlation in using the latitude observational data of the six astrometric instruments at Mizusawa and Tokyo: the Zenith Telescope (1900-1978), the Photographic Zenith Tube No. 1 and No. 2 (1962-1975; 1975-1992), the Floating Zenith Telescope(1967-1984) and the astrolabe (1966-1984) at Mizusawa; the Photographic Zenith Tube at Tokyo (1966-1988). It appears that the latitude residuals at Mizusawa and Tokyo have a significant correlation at interannual time scale with the SOI, the SOI leading latitude residual of about 2-3years.

M. Collins, S. R. Lewis, P. L. Read, and F. Hourdin. Baroclinic Wave Transitions in the Martian Atmosphere. Icarus, 120:344--357, April 1996. [ bib | DOI | ADS link ]

Surface pressure data from the Viking Lander mission and from GCM simulations of the martian atmosphere have been analyzed using singular systems analysis. Very regular oscillations are found with frequencies that are distributed bimodally with peaks corresponding to periods of approximately 2-4 days and 5-7 days, respectively. Reconstructions of the amplitudes of the two oscillations are often negatively correlated; i.e., when the amplitude of one oscillation is large, that of the other is small. The GCM simulations show that the negative correlation in the amplitudes of the two oscillations can be explained as a flipping between two different wavenumber modes. In the absence of diurnal forcing in the model, transition from an unrealistically regular high frequency mode to a similarly unrealistic regular low frequency mode occurs at most once during the northern winter season. The diurnal cycle in the model, however, acts in a non-linear sense to stimulate the transitions between the two wavenumbers and thus increases the frequency of mode flipping events. The corresponding simulations bear a closer resemblance to the observations.

C. J. Stubenrauch, G. Seze, N. A. Scott, A. Chedin, M. Desbois, and R. S. Kandel. Cloud Field Identification for Earth Radiation Budget Studies. Part II: Cloud Field Classification for the ScaRaB Radiometer. Journal of Applied Meteorology, 35:428--443, March 1996. [ bib | DOI | ADS link ]

Gaining a better understanding of the influence of clouds on the earth's energy budget requires a cloud classification that takes into account cloud height, thickness, and cloud cover. The radiometer ScaRaB (scanner for radiation balance), which was launched in January 1994, has two narrowband channels (0.5 0.7 and 10.5 12.5 m) in addition to the two broadband channels (0.2 4 and 0.2 50 m) necessary for earth radiation budget (ERB) measurements in order to improve cloud detection. Most automatic cloud classifications were developed with measurements of very good spatial resolution (200 m to 5 km). Earth radiation budget experiments (ERBE), on the hand, work at a spatial resolution of about 50 km (at nadir), and therefore a cloud field classification adapted to this scale must be investigated. For this study, ScaRaB measurements are simulated by collocated Advanced Very High Resolution Radiometer (AVHRR) ERBE data. The best-suited variables for a global cloud classification are chosen using as a reference cloud types determined by an operationally working threshold algorithm applied to AVHRR measurements at a reduced spatial resolution of 4 km over the North Atlantic. Cloud field types are then classified by an algorithm based on the dynamic clustering method. More recently, the authors have carried out a global cloud field identification using cloud parameters extracted by the 3I (improved initialization inversion) algorithm, from High-Resolution Infrared Sounder (HIRS)-Microwave Sounding Unit (MSU) data. This enables the authors first to determine mean values of the variables best suited for cloud field classification and then to use a maximum-likelihood method for the classification. The authors find that a classification of cloud fields is still possible at a spatial resolution of ERB measurements. Roughly, one can distinguish three cloud heights and two effective cloud amounts (combination of cloud emissivity and cloud cover). However, only by combining flux measurements (ERBE) with cloud field classifications from sounding instruments (HIRS/MSU) can differences in radiative behavior of specific cloud fields be evaluated accurately.

W. T. Hutzell, C. P. McKay, O. B. Toon, and F. Hourdin. Simulations of Titan's Brightness by a Two-Dimensional Haze Model. Icarus, 119:112--129, January 1996. [ bib | DOI | ADS link ]

We have used a 2-D microphysics model to study the effects of atmospheric motions on the albedo of Titan's thick haze layer. We compare our results to the observed variations of Titan's brightness with season and latitude. We use two wind fields; the first is a simple pole-to-pole Hadley cell that reverses twice a year. The second is based on the results of a preliminary Titan GCM. Seasonally varying wind fields, with horizontal velocities of about 1 cm sec -1at optical depth unity, are capable of producing the observed change in geometric albedo of about 10% over the Titan year. Neither of the two wind fields can adequately reproduce the latitudinal distribution of reflectivity seen by Voyager. At visible wavelengths, where only haze opacity is important, upwelling produces darkening by increasing the particle size at optical depth unity. This is due to the suspension of larger particles as well as the lateral removal of smaller particles from the top of the atmosphere. At UV wavelengths and at 0.89 μm the albedo is determined by the competing effects of the gas and the haze material. Gas is bright in the UV and dark at 0.89 μm. Haze transport at high altitudes controls the UV albedo and transport at low altitude controls the 0.89-μm albedo. Comparisons between the hemispheric contrast at UV, visible, and IR wavelengths can be diagnostic of the vertical structure of the wind field on Titan.

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