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Jianmin Ma and S. M. Daggupaty

Abstract

A variational method is developed to estimate the aerodynamic roughness length and roughness scaling length for temperature based on wind and temperature measurements conducted routinely at an observational network. The problem is formulated to find optimal estimates of roughness lengths for momentum and heat transfer through a minimization of a cost function with respect to these two roughness lengths that measures the errors between observed and predicted wind and temperature profiles. The method has been applied to data collected in two experimental campaigns. Some results are compared with other methods used to compute the aerodynamic roughness length. The variational computations show that the aerodynamic roughness lengths agree well with the estimated z 0m in the experimental campaigns. The roughness scaling lengths for temperature z 0t are in most cases one order of magnitude smaller than z 0m. It was found that the variations of z 0m and z 0t during the course of a day are not likely to follow a simple functional relationship, especially during the daytime, during which both z 0m and z 0t are highly oscillatory. The error test shows that z 0m and z 0t generated from the variational method are not very sensitive to measurement errors.

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Jianmin Ma and S. M. Daggupaty

Abstract

Dry deposition velocities of gases and particles are highly dependent on surface type. In a numerical model, each grid cell may contain multiple surface types, each with a different deposition velocity. Therefore, some kind of averaging technique generally is used to compute the average of the subgrid-scale deposition velocities within a grid cell. In this paper, effective surface parameters are suggested to relate the mean properties of concentration and wind speed to the mean surface fluxes. An effective deposition velocity is computed subject to these effective surface parameters and a weighted-average technique. This effective deposition velocity is compared with an alternate weighted-average deposition velocity that has been used widely in numerical air quality models. For particles, the effective deposition velocity can be significantly different from the weighted-average deposition velocity. For some gases, for which biological factors often control the deposition process, the difference between these two average deposition velocities can still be distinguished for typical gases and surface properties.

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W. M. Washington and S. M. Daggupaty

Abstract

A global circulation model (GCM) developed at the National Center for Atmospheric Research (NCAR) has been used to simulate the large-scale features of the Asian-African summer monsoon. The model has 6 vertical layers of 3-km thickness with a 2½° horizontal latitude-longitude grid. The physical processes incorporated are solar and infrared radiation, with cloudiness explicitly calculated from a model-generated relative humidity distribution. The latent heat released from precipitation is derived from stable lifting and cumulus convection. Also included in the model are subgrid-scale vertical and horizontal transports of momentum, sensible heat, and latent heat.

We compare the computed sea-level pressure, wind, cloudiness, and precipitation patterns with. observed data and, in particular, concentrate on the strong low-level monsoon jet near eastern Kenya and Somalia. The model correctly simulates this jet in position; however, the wind maxima are weaker than observed. Because of the relatively coarse model resolution, we fail to obtain the important monsoon depressions which form in the Bay of Bengal or near Bombay. In nature, these depressions account for a large part of the precipitation in India and its surrounding regions.

This study demonstrates that a global circulation model is capable of simulating many of the features observed in the Asian-African summer monsoon.

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Jian Zhang, S. T. Rao, and S. M. Daggupaty

Abstract

The North American Research Strategy for Tropospheric Ozone-Northeast field measurements taken during 12–16 July 1995 indicate that ground-level ozone concentrations have exceeded the National Ambient Air Quality Standard level of 0.12 ppm for hourly ozone throughout the ozone transport region in the northeastern United States. Analyses of the meteorological conditions conducive to the ozone formation and accumulation reveal that the above ozone exceedances in the Northeast are associated with such meteorological features as the stagnant high pressure system, the Appalachian leeside trough, the frontal trough, the sea breeze, the channeling effects induced by the topography, and the stratified boundary layers. Also, aircraft measurements provide evidence for the buildup of ozone levels in the nighttime residual layer under southwesterly flows from 12 to 15 July. In this paper, it is shown that high ozone trapped aloft mixes downward, elevating the ground-level ozone concentrations as the daytime mixed layer starts to grow in the morning. Simulations using a photochemical box model (the Ozone Isopleth Plotting Package, Research Oriented Version) confirm that ozone trapped aloft in the nocturnal residual layer has a significant impact on the temporal evolution as well as on the peak ozone concentration near the surface. Evidence for the photochemical production and transport of ozone downwind of urban areas is presented using the data from surface as well as aircraft measurements.

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E. G. Astling, S. M. Daggupaty, and K. R. Leslie

Abstract

This paper discusses the results obtained from a study of a non-deepening disturbance which developed and dissipated in the Caribbean Sea. Careful analyses Of the temperature moisture, and wind fields Were performed at six levels for a two-day, period while the disturbance was dissipating. The data were used to compute vertical motions from a nonlinear balance model for the purpose of diagnosing the reasons for decay of the disturbance.

The vertical motions that were derived from the diagnostic computations compared favorably with the synoptic-scale cloud distribution. The results suggested that the presence of clouds within the disturbance on the first day was attributed to 1) frictionally induced ascending motions in the lower troposphere where there were large values of specific humidity, 2) the Laplacian of thermal advection which produced rising motions in the lower and upper troposphere where there were weak, though significant, temperature gradients, and 3) ascending motions in the upper troposphere which were contributed by convective latent heat release.

When the disturbance decayed on the second day, the diagnostic computations produced weak subsidence in the lower troposphere by the synoptic-scale frictional and thermal effects. As a consequence, the low-level moisture convergence was suppressed and the latent heat release was negligible. The results point out the importance of synoptic-scale circulation and convection as being cooperative mechanisms for extensive cloud formation.

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C. L. Smith, E. J. Zipser, S. M. Daggupaty, and L. Sapp

Abstract

In Part 1 of the two-part paper, we present an analysis of a portion of a tropical wave in the BOMEX Phase IV ship network on a much smaller sale than normally attempted using conventional observations and data from a number of research aircraft. The results indicate the existence of a strong mesoscale cyclone with a lifetime of less than 12 h within the synoptic-scale wave. At the time of maximum data density, the analysis time, the system was near its maximum intensity.

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T. N. Krishnamurti, S. M. Daggupaty, Jay Fein, Masao Kanamitsu, and John D. Lee

The zonally asymmetric climatology of the tropical large-scale motion field is an interesting GARP topic. Understanding of the maintenance of various quasi-stationary features will be a challenging problem during the FGGE (the First GARP Global Experiment) and Monex (the Monsoon Experiment). In this paper we present some current thoughts that are relevant to the climatology of the tropical upper troposphere during the northern summer.

A review of some of the results from various numerical general circulation models and theoretical studies is presented for northern summer conditions. The relative success or failure of simulations of 200-mb climatology is discussed. It is pointed out that a proper simulation of the belt of anticyclones over the Asian highlands is somewhat crucial for a proper simulation of the summer climatology over the rest of the tropics.

Observations of the semipermanence of the Tibetan high pressure cell during northern summer at 200 mb suggests that it acts somewhat like a barrier. In order to illustrate this we consider a problem related to the evolution of barotropic non-divergent flows past a barrier. The flows are initially zonal, with speeds varying in the north-south direction according to northern summer observations. The barrier, whose shape is based on observations of a blocking thermal high, is impulsively introduced at initial time. The flows are kept zonal at frictionless walls at 25S and 45N. The initial north-south distribution of the zonal flows is shown to have no inflection point in its profile, thus it does not satisfy the necessary condition for barotropic instability. The presence of an impulsively introduced barrier, however, results in the evolution of transient as well as steady wave motions in long term numerical integrations. It is shown that a 30-day mean motion field contains many of the well known climatological features such as the African high, the mid-Atlantic trough, the mid-Pacific trough, the Mexican high and a weak easterly jet south of the Tibetan high. Calculations of kinetic energy exchanges between waves and zonal flow in this simple experiment is compared with corresponding calculations for tropical observations and recent general circulation experiments carried out by Abbott. The impulsively introduced barrier simulates an energy source for zonal wavenumber 1, quite similar to observations in a tropical belt. Although this experiment is fairly crude, it is found to be very illustrative in many respects. Many diverse experiments along these lines can be carried out to reveal various aspects of atmospheric circulations.

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