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Mukut B. Mathur

Abstract

Three new schemes to assimilate the rainfall observations for improving the initial conditions are developed and tested. A reverse cumulus parameterization procedure is used in which the model convective precipitation is nudged in scheme 1, and the model total precipitation is nudged in schemes 2 and 3, toward the observations. At the nonconvective grid points, the reverse procedure is not invoked. A method to enhance the rainfall at such points, in case the predicted total rainfall is weaker than the observed, is included in scheme 3. Numerical results are presented to show that the thermodynamic and wind fields and the distribution of condensational heating are improved with the use of any of the above rainfall assimilation schemes. The incorporation of the method to enhance the rainfall at the nonconvective grid points and the use of the total rainfall in place of only the convective rainfall in the reverse procedure ameliorate the initial conditions.

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MUKUT B. MATHUR

Abstract

Numerical results of time integration of divergent barotropic primitive equations by a quasi-Lagrangian advective scheme, without smoothing, show that the proposed model has the desired property of conserving total energy and potential vorticity following a particle. A scheme to forecast boundary conditions in a manner similar to that in the interior is incorporated which significantly improves the results. The tests are made utilizing a large amplitude sinusoidal perturbation on a basic current as the initial state. Tests are carried out by performing the integration for 8 days.

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Mukut B. Mathur

Abstract

A four-level primitive equation model is integrated to 96 hr. A multiple-grid system is used to increase the resolution near the center of the hurricane. The convective and nonconvective release of latent heat, surface friction, transfer of sensible and latent heat from the sea surface to air, and the variation of Coriolis parameter with latitude are incorporated in the model.

The initial balanced state is derived from conventional and aircraft reconnaissance data over the Gulf of Mexico on 10 October 1964, defining a weak tropical depression. Results of the integration show that the model simulates fairly well the movement, the rate of intensification, the asymmetries in the wind field in the lower and the middle troposphere, and the banded structure in the vertical motion field which were observed in Isbell 1964. The temperature lapse curve at the center of the simulated hurricane lies close to the lapse curve that was observed in the eye of Isbell.

Numerical results suggest three stages in the life cycle of the simulated hurricane-formative, storm and hurricane. In the formative stage (00–48 hr), the low-level circulation becomes well marked and extends to the middle troposphere. Appreciable warming in the middle troposphere occurs. During the period 48–72 hr, the depression intensifies into a storm. Well-marked zones of convergence form in the boundary layer. Scattered bands in the vertical motion field appear in the middle and the upper troposphere and are located at considerable distance (150 km) from the center. Intense warming in the middle and the upper troposphere takes place in the hurricane stage (72–96 hr). Realistic magnitudes of the maximum surface pressure gradient (20 mb in 37 km) and the rate of intensification (21 mb during the period 84–96 hr) are simulated. Other features of hurricanes which are realistically simulated include organized bands in the vertical motion field close to and surrounding the eye, downward motion in the eye, and the cyclonic out-flow in the upper troposphere.

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Mukut B. Mathur

Abstract

The developments of the propagating and the stationary bands in a three-dimensional model of a hurricane (Isbell, 1964) are investigated. Propagating bands in the vertical motion fields in the middle and the upper troposphere form in the regions of strong heating in the upper troposphere and weak cooling in the middle troposphere. The structures of the wind, temperature and pressure fields in these bands are similar to those observed in the outer radar bands of hurricanes. Strong, nearly stationary bands form close to the center in the intense storm stage.

Results of two experiments, one (M1) in which the so-called nonconvective release of latent heat in the upper troposphere is included and the other (M2) in which this heating is not incorporated, are compared. Convective release of latent heat is included in both experiments. The stationary bands in which form in M1, also develop in M2. The propagating bands which form in M1, however, do not develop in M2. The rate of intensification of the simulated storm in M1 is nearly the same as observed in Isbell; it is, however, significantly weaker in M2. It is shown that the inclusion of nonconvective release of latent heat in M1 enhances the upper tropospheric outflow which induces strong zones of convergence in the boundary layer. The resulting increase in the upward motion at the top of the boundary layer augments the convective release of latent heat and leads to a rapid intensification of the disturbance.

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Mukut B. Mathur

Abstract

A regional numerical weather prediction model is designed using the quasi-Lagrangian method for operational forecasting of synoptic and mesoscale disturbances. The nonlinear advective terms and the total forcing experienced by a fluid parcel are evaluated with high order accuracy. Many physical processes such as convective and nonconvective release of latent heat are incorporated.

The model can be integrated over any geographical area, with any horizontal and vertical resolution, and with either a uniform grid or two nested grids.

Results are presented for 48 h forecasts produced by the model utilizing operational limited area analyses. Lateral boundary values were obtained from a previous run of the operational large-scale model. The model's predictions are compared to those obtained using the National Meterological Center's currently operational regional model.

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Mukut B. Mathur

Abstract

A description is presented of the National Meteorological Center's Quasi-Lagrangian Model (QLM), which is used for operational hurricane prediction. The model uses the primitive equations with high horizontal and vertical resolution, and includes parameterizations of principal physical and dynamical processes that affect the motion and development of a hurricane.

Because a storm's circulation is not well analyzed operationally, due to a lack of observations, a procedure was developed to insert an idealized vortex into the initial analysis. The geopotential height and wind fields in the vortex satisfy the gradient wind relation with variable Coriolis parameter, and its structure depends on the size and intensity of the observed storm.

The primary purpose of the QLM is to provide numerical guidance for forecasting hurricane track. Considerable improvement in the track and the structure of a storm is obtained through the use of the idealized vortex. Further improvement in forecasts is attained with the use of a secondary circulation (a dipole). Based on the current storm motion, the dipole imposes a steering current on the vortex.

Several 72-h track forecasts from the QLM and the most accurate statistical-dynamical track prediction model (NHC83) in use over the Atlantic Ocean area are compared. Results show that the QLM forecasts of landfall compare well with the NHC83, but over the open oceans, where observations are sparse, the NHC83 performs better than the QLM.

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Mukut B. Mathur and Janene Ruess

Abstract

A comparison is presented of the forecast track errors in the 1990 North Atlantic hurricane season cases from three operational models. The Quasi-Lagrangian Model (QLM) is a baroclinic model. The National Hurricane Center Model 1990 (NHC90) is an advanced statistical-dynamical model, and CLIPER (climatology and persistence) is a simple statistical model that uses only climatology and persistence. Mean errors over the season were smaller in the QLM than in the NHC90 or CLIPER. The forecasts were also divided into several 5°-10°latitude zones based on the initial storm latitude. The mean errors in the QLM were the smallest in many zones after the first 12 h.

Mean forecast track errors in some storms are presented to show the variation in the performance of the models from one storm to another. Individual cases in these terms are also discussed to gain further insight into the relative accuracy of the models. The QLM generally performed better than the NHC90 and CLIPER for storms that moved northward or had unusual tracks.

The analysis and forecasts of an operational global model are used to derive the QLM initial state and for prediction over QLM lateral boundary points, respectively. Both the global analysis and forecast programs were modified in 1991. It is shown that the performance of the QLM relative to the CLIPER and NHC90 was not altered despite the changes in the global model.

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Mukut B. Mathur, Keith F. Brill, and Charles J. Seman

Abstract

Numerical forecasts from the National Centers for Environmental Prediction’s mesoscale version of the η coordinate–based model, hereafter referred to as MESO, have been analyzed to study the roles of conditional symmetric instability (CSI) and frontogenesis in copious precipitation events. A grid spacing of 29 km and 50 layers are used in the MESO model. Parameterized convective and resolvable-scale condensation, radiation physics, and many other physical processes are included. Results focus on a 24-h forecast from 1500 UTC 1 February 1996 in the region of a low-level front and associated deep baroclinic zone over the southeastern United States. Predicted precipitation amounts were close to the observed, and the rainfall in the model was mainly associated with the resolvable-scale condensation.

During the forecast deep upward motion amplifies in a band oriented west-southwest to east-northeast, nearly parallel to the mean tropospheric thermal wind. This band develops from a sloping updraft in the low-level nearly saturated frontal zone, which is absolutely stable to upright convection, but susceptible to CSI. The updraft is then nearly vertical in the middle troposphere where there is very weak conditional instability. We regard this occurrence as an example of model-produced deep slantwise convection (SWC). Negative values of moist potential vorticity (MPV) occur over the entire low-level SWC area initially. The vertical extent of SWC increases with the lifting upward of the negative MPV area. Characteristic features of CSI and SWC simulated in some high-resolution nonhydrostatic cloud models also develop within the MESO. As in the nonhydrostatic SWC, the vertical momentum transport in the MESO updraft generates a subgeostrophic momentum anomaly aloft, with negative absolute vorticity on the baroclinically cool side of the momentum anomaly where outflow winds are accelerated to the north.

Contribution of various processes to frontogenesis in the SWC area is investigated. The development of indirect circulation leads to low-level frontogenesis through the tilting term. The axis of frontogenesis nearly coincides with the axis of maximum vertical motion when the SWC is fully developed. Results suggest that strong vertical motions in the case investigated develop due to release of symmetric instability in a moist atmosphere (CSI), and resultant circulations lead to weak frontogenesis in the SWC area.

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Aleksandr Falkovich, Eugenia Kalnay, Stephen Lord, and Mukut B. Mathur

Abstract

A method to assimilate observed rain rates in the Tropics for improving initial fields in forecast models is proposed. It consists of a 6-h integration of a numerical forecast model; the specific humidity at every time step at each grid point is modified (nudged) in such a way that the total model precipitation accumulated during this integration becomes very close to that observed. An increase in the model precipitation is achieved by moistening the lower troposphere above a grid point with prescribed supersaturation; a decrease in the model rainfall is brought about by decreasing the specific humidity in the lower troposphere in proportion to the difference between the model and reference specific humidity profiles. The modified values depend on the difference between the model and target precipitation. The depth of the atmospheric column in which the humidity is changed is proportional to the target rain rate. Quality criteria of a rain assimilation procedure are proposed. The quality of the assimilation method was verified using a test in which precipitation generated by a forecast model without nudging (“control” experiment) was considered to be “quasi-target” data and the nudging procedure was used for assimilation of the rain produced in the control experiment. The following experiments were performed: control (C)—without nudging, “simulated nudge” (S)—nudging to the 6-h accumulated rainfall from the C experiment, and “satellite nudge”—nudging to the 6-h accumulated satellite-retrieved (observed) rainfall. Each experiment consisted of a 6-h forecast (first guess), analysis, next 6-h forecast (first guess), next analysis, and 24-h forecast. Nudging was applied during the two successive 6-h calculations of the first guess over the tropical belt. Parameters of the nudging procedure were determined in such a way that the assimilation procedure converged quickly and simulated the observed precipitation very closely. The difference in forecast fields between the S and C experiments after a 24-h forecast turned out to be small, indicating high quality of the assimilation procedure. The high sensitivity of forecast fields to the quality of rain retrieval is demonstrated.

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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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