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T. N. Krishnamurti
and
H. S. Bedi

Abstract

The eastern and the western tropical oceans usually show a considerable zonal asymmetry in the extent and depth of deep cumulus convection. Earlier versions of a simple cumulus parameterization based on GATE observations have revealed some limitations in differentiating this type of zonal asymmetry. The aim of the proposed scheme is to provide global statistical corrections to a Kuo-type cumulus parameterization scheme and thus to optimize the moistening, heating and rainfall rates over different regions. The base data for this study are the recently analyzed global FGGE IIIb datasets. Three months of daily datasets during the global experiment were utilized in order to evaluate the coefficients of a multiple regression analysis. These multiple regression coefficients vary in space and provide different measures of a moistening parameter b and a mesoscale convergence parameter η. A clear distinction in the strength of convection is found, based on the regression parameters, between the western and the eastern oceans. This generalization of a modified Kuo-type scheme is derived for a spectral resolution of 42 waves. The impact of the aforementioned scheme is investigated in several medium range prediction experiments. Forecast comparison with a simpler version of the Kuo scheme is also carried out. Our interest in these experiments is an evaluation of precipitation forecasts, for which the proposed global cumulus parameterization is compared with other experiments that were based on GATE coefficients and with the observed measures of precipitation. The results of the global forecasts show a very marked improvement in the short range (1 to 2 day) prediction from the use of the globally varying parameterization coefficients. On the other hand, the precipitation amounts predicted from an application of the local GATE coefficients underestimate the rainfall rates over most regions.

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T. N. Krishnamurti
,
H. S. Bedi
,
William Heckley
, and
Kevin Ingles

Abstract

A dynamic relaxation technique is examined to update a spectral model. The technique consists of constraining selected time dependent model variables towards their predetermined space–time estimates, while the remaining variables evolve unconstrained. The scheme involves gradual assimilation of data and thus is essentially free from data insertion shocks generally associated with data assimilation schemes. The scheme can also be used to update the model variables consistent with the observed estimates of diabatic forcings. The spectral formulation is particularly suited to relax the current estimates of model variables towards their observed estimates scale-by-scale.

The scheme has been applied to initialize model variables by relaxing vorticity, divergence and total mass (surface pressure) fields through one to three observation periods using an 11-layer model with T-42 spectral resolution. In addition, the moisture field and diabatic heating rates have been relaxed consistent with the observed estimates of precipitation rates. The explicit two-day Newtonian relaxation of the streamfunction, velocity potential (consistent with rainfall estimates) and the surface pressure and an implicit treatment of the humidity (again consistent with rainfall estimates) results in a realistic initialization. Tropical rainfall, humidity analysis and the divergence field show considerable consistency and improvement. The study addresses the model initialization by this scheme and its impact on medium range forecasts using FGGE IIIb data. The reduction of the spinup time is accomplished by this procedure at the initial time. Globally averaged evaporation and precipitation exhibit an equilibration by this procedure.

A major result of this study is the ability to initialize an observed rainfall field from the use of a reverse Kuo algorithm, the Newtonian relaxation and the overall physical initialization within this model.

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T. N. Krishnamurti
,
Wei Han
,
Bhaskar Jha
, and
H. S. Bedi

Abstract

The main theme of this paper is on the intensity forecast of a hurricane (Opal) and interpretation of factors contributing toward it. The paper illustrates the results of assimilation and prediction for Hurricane Opal of 1995 from a very high-resolution global model. The assimilation makes use of a detailed physical initialization that vastly improves the nowcasting skill of rainfall and the model-based outgoing longwave radiation. Some of the interesting aspects of Hurricane Opal’s history occurred between 1200 UTC 1 October 1995 and 1200 UTC 5 October 1995. During this period the storm made landfall over the Florida panhandle. The storm reached maximum wind speed of over 130 kt on 4 October 1995. The intensity issue of Opal has drawn much attention. Issues such as the potential vorticity impact from a middle-latitude trough, the angular momentum of the lower-tropospheric inflow layer, the warm ocean temperature anomalies of the northern Gulf of Mexico, and the possible role of mesoconvective concentric eyewall are discussed in this paper.

The main finding of this study is that a reduction of the gradient of angular momentum occurs above the regions of maximum convective heating. This contributes toward stronger cyclonic spinup of parcels that enter the storm environment from the middle latitudes. Another major contributor is the import of angular momentum along the lower-tropospheric inflow channels of the storm. These channels were found to be open, that is, uncontaminated with a plethora of deep convection and heavy rain. This permitted the high angular momentum to advance toward the storm’s interior thus contributing to its intensification.

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T. N. Krishnamurthi
,
H. S. Bedi
,
Darlene Oosterhof
, and
Vivek Hardiker

Abstract

A high-resolution global model forecast of the formation of Hurricane Frederic of 1979 is analyzed by means of several diagnostic computations on the model's output history. The formation is addressed from an analysis of limited-area energetics where the growth of eddy kinetic energy is examined. The question on internal versus external forcing during the formative stage of the hurricane is explored by means of the Kuo-Eliassen framework for the radial-vertical circulation of the hurricane. The intensity of the predicted hurricane is diagnosed from a detailed angular momentum budget following the three-dimensional motion of parcels arriving at the maximum wind belt. Overall, the successful simulation of the hurricane has enabled us to make such a detailed diagnosis of the predicted hurricane at a high resolution. The principal findings of this study are that a north-south-oriented beating function maintained a zonal easterly flow that supplied energy barotropically during the growth of an African wave. The growth of eddy kinetic energy is somewhat monotonic and slow throughout the history of the computations. The initial development of the easterly wave appears to be related to the widespread weak convective heating that contributes to a covariance of heating and temperature and of temperature and vertical velocity. The hurricane development period is seen as one where both the barotropic and convective processes contribute to the growth of eddy kinetic energy. During this developing stage, the growth of radial-vertical circulation is largely attributed to convective, radiative, and frictional forcings. The role of eddy convergence of momentum flux appears to be insignificant. The intensity issue of the storm (maximum wind of the order of 37 m s−1) was addressed by means of a detailed angular momentum budget following parcel motion. The pressure torque in the model simulation had a primary role in explaining the intensity of the predicted storm. It is only in the storm's inner rain area where the frictional stress becomes quite large. But at these small radii the frictional torque is still smaller compared to the contribution from the (small but significant) azimuthal asymmetries of the pressure field and the resulting pressure torques.

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Mukut B. Mathur
,
H. S. Bedi
,
T. N. Krishnamurti
,
Masao Kanamitsu
, and
Jack S. Woollen

Abstract

Sparsity of conventional data over tropical oceans makes it difficult to analyze well the moisture and divergence fields, and therefore the diabatic forcing of the tropical atmosphere is not well predicted in numerical models. A nudging procedure to improve the precipitation forecast in the National Meteorological Center (NMC) Medium Range Forecast Model (MRF) is developed. The convective parameterization scheme is modified to adjust the predicted rainfall amounts toward the observations in this method. In the absence of conventional data, the rainfall estimates from the satellite measures of the outward-going longwave radiation are utilized as the observed precipitation.

Several forecasts from the MRF are presented to show the improvements in intensity and location of the intertropical convergence zone and tropical disturbances with the application of the nudging procedure. Additionally, spurious cyclone and excessive rainfall that were predicted without this procedure either failed to form or their intensifies were considerably reduced.

Results from incorporation of the modified convective scheme in the global data-assimilation system within the NMC forecast model are also discussed. The analysis, the subsequent 72-h forecast circulation, and the rainfall amounts are improved with the use of this scheme.

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T. N. Krishnamurti
,
H. S. Bedi
,
G. D. Rohaly
, and
D. Oosterhof

Abstract

The emphasis of this paper is on residue-free budgets of seasonal climate forecasts. It is possible to ask the following question: given a seasonal mean geopotential height simulation from a climate model, what is a breakdown of that contribution from different areas of the model physics and dynamics? In that context, the authors have examined the maintenance of a monsoonal 500-mb ridge, the eastward shift of the Tibetan anticyclone during an El Nin˜o year, and the Pacific-North American pattern. The salient results of this study include a substantial contribution from the advective nonlinear dynamics toward the maintenance (positive or negative) of the seasonal climate.

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Saad Mohalfi
,
H. S. Bedi
,
T. N. Krishnamurti
, and
Steven D. Cocke

Abstract

A two-stream scattering scheme based on the delta-Eddington approximation is incorporated into the Florida State University Limited Area Model for computing the shortwave radiative fluxes due to dust aerosols over the Saudi Arabian region and to study their impact on synoptic-scale systems and the diurnal cycle over the region. The radiative properties of dust corresponding to different categories of dustiness are determined from the results of field experiments. Satellite imagery and visibility are used to determine the intensity and extent of the dust layer.

Two parallel simulations, one including the radiative effects of dust aerosols and the other without them, were made over a 6-day period starting with 1200 UTC 25 June 1979 using First GARP (Global Atmospheric Research Program) Global Experiment IIIb data analyses from ECMWF. A comparison of the two experiments shows that the dust aerosol radiative heating strengthens the heat low over Saudi Arabia. Furthermore, the radiative heating of the heavy dust concentrated at low levels during the dust outbreak episode protects the heat low from its possible destruction due to strong cold winds from the northwest.

A significant improvement in the diurnal cycle of temperature at middle levels occurs with the introduction of dust aerosols. The extension of the dust layer over the Arabian Sea also warms the middle levels in the vicinity of the dust layer and cools the layer below it, thus intensifying the inversion above the monsoon flow. The presence of dust aerosols over the Arabian Sea is also found to affect the intensity of the low-level Somali jet and the diurnal cycle of the sea breeze. These model results are found to be consistent with observations.

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