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Qingyun Zhao and Frederick H. Carr

Abstract

An explicit cloud prediction model has been developed and incorporated into the Eta Model at the National Centers for Environmental Prediction. In this scheme, only one predictive variable, cloud mixing ratio, is added to the model’s prognostic equations to represent both cloud liquid water and cloud ice. Precipitation is diagnostically calculated from cloud mixing ratio. Extensive tests have been performed. The statistical results show a significant improvement in the model precipitation forecasts. Diagnostic studies suggest that the inclusion of cloud ice is important in transferring water vapor to precipitation and in the enhancement of latent heat release; the latter subsequently affects the vertical motion field significantly.

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Lance F. Bosart and Frederick H. Carr

Abstract

The pretropical storm Agnes rainstorm across western New York and Pennsylvania is analyzed using conventional surface and aerological data. Hourly precipitation maps and surface maps showed the north-eastward motion and intensification of a developing rain area over eastern Kentucky at 1200 GMT 20 June 1972. This area remained a separate entity from the main Agnes rainshield. Nonlinear balanced omega as well as kinematic omega computations suggest that a weak short wave in the mid and upper troposphere provided the initial triggering mechanism for the growth of the rain area. Plentiful moisture was available from the Agnes circulation to the south and the western Atlantic. Latent heat release then played a dominating role in modifying the resulting vertical velocity patterns. Finally, some possible general forecast considerations are suggested by these results.

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Mohan K. Ramamurthy and Frederick H. Carr

Abstract

The problem of four-dimensional data assimilation in the tropics has been studied using a limited-area primitive equation model. Of prime concern is the relative importance of different update variables and their impact on data assimilation. Five new experiments complement a set of ten previously reported experiments that investigate the feasibility of four-dimensional data assimilation in the monsoon region using only the wind observations. In addition to assessing the relative importance of update variables, the present study investigates the role of model physics in data assimilation.

The assimilation experiments are carried out for the onset vortex case of the 1979 Indian summer monsoon for which many special FOGE/MONEX datasets are available. The assimilation-forecast system for all of the experiments comprises a 12-h assimilation phase followed by a 24-h forecast period. In all experiments, updating is done via the Newtonian nudging approach which, in our previous study, was found to be more effective than other methods of updating.

It is found that at least for this dataset, the wind data were wore valuable than the temperatures. Although temperature assimilation alone had some unexpected positive results, it did not offer appreciable improvement over wind-only assimilations when the two variables were inserted together. On the other hand, a combination of wind and moisture data produced the most positive results. This confirms the importance of wind and moisture data in the tropics. Finally, it has been found that the incorporation of physical parameterizations during the assimilation period is important for a proper spinup of the model and its smooth transition into the forecast stage.

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Mohan K. Ramamurthy and Frederick H. Carr

Abstract

A limited primitive equation model has been used to study the feasibility of four-dimensional data assimilation in the monsoon region and, further, to study the applicability of several assimilation techniques currently being employed in global models. The two fundamental objectives of this research are

(i) to understand how the model atmosphere responds to the insertion of asynchronous data and its impact on the assimilation-prediction cycle, and

(ii) to determine what assimilation strategies work best for limited-area models in the tropics.

A sequence of ten assimilation experiments are performed using different update procedures; all insertions are carried out with only the wind observations. The model is initialized with the ECMWF FGGE level III-b data for the onset vortex case of 17 June 1979, and assimilations are carried out using the summer MONEX level II-b data during the first 12 hours. From these assimilated states, 24-h forecasts are then made.

The results lend support to the premise that the required initial conditions can be obtained by the process of four-dimensional updating of the prognostic variables. The results also clearly demonstrate the superiority of the continuous assimilation approach via Newtonian nudging over that by indirect insertion. Furthermore, the insertion shocks are significantly minimized by assimilating only the rotational component of the winds. On the other hand, the application of noise control measures only marginally alleviate the insertion shocks accompanying continuous indirect insertion.

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Frederick H. Carr and Lance F. Bosart

Abstract

The problem of rainfall prediction for a tropical storm over the United States is examined from a diagnostic point of view. A moisture budget is constructed and the differences between the computed and observed precipitation are discussed. Although the areal averages were comparable, the point-by-point agreement was only fair. Stable and convective precipitation are then computed using methods common to many numerical models. The discrepancies between forecast and observed precipitation is assumed to be due to the incomplete formulation of the amount of moisture available for convection. This leads to an expression for the subgrid-scale moisture supply that provides the missing precipitation. Methods are then suggested in which the deficiencies of a parameterization scheme could be corrected during its use in a prognostic model.

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Frederick H. Carr and James P. Millard

Abstract

Sixty-eight comma-cloud systems over the Great Plains during two spring seasons were examined using satellite imagery and rawinsonde data. Composite soundings were produced for each of ten distinct parts of the comma cloud in order to describe quantitatively the atmospheric structure associated with wave cyclones that produced 585 severe weather events. Composite sectionals and soundings document the different kinematic and thermodynamic environment of each part of the comma-cloud system. Relative-wind, isentropic analyses show air flow relative to the storm and provide additional evidence that the clear region intruding northward east of the cyclone center is an area of strong ascent of dry air with a previous history of subsidence. Stability computations from the mean soundings suggest that the most likely location for severe weather is near the central part of the comma tail, in agreement with the tabulated severe weather reports.

A case study from 21–22 March 1981 is conducted to investigate the generation of a secondary line of convection which formed in the center of a dry intrusion after the main area of convection passed to the east. This “dry-slot convection” is hypothesized to occur if the upper cloud edge of the comma tail moves ahead of the associated surface boundary during the day, leaving a region of moist boundary-layer air exposed to solar heating. With rising motion, adiabatic cooling and drying aloft, and increasing warming and moisture convergence near the surface, a rapid destabilization can occur, resulting in development of secondary lines of convection vigorous enough to produce severe weather.

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Phillip L. Spencer, Frederick H. Carr, and Charles A. Doswell III

Abstract

Kinematic and thermodynamic quantities derived from wind profiler triangles are used to help describe the structure of both an amplifying and decaying baroclinic wave as they traversed portions of the wind profiler demonstration network. The data provide excellent diagnoses of the cyclogenetic processes associated with the amplifying system and the cyclolytic processes associated with the decaying system. The importance of a baroclinic wave's vertical tilt and the associated profiler-derived advective patterns of the systems as they relate to surface evolution are shown to be consistent with conceptual models of baroclinic waves. These structural aspects of the observed baroclinic waves are also shown to vary substantially on short timescales. In addition, a sub-synoptic-scale feature associated with a severe convective event that developed ahead of the decaying wave trough axis was observed quite well by the profiler network. This feature's detection was dependent on the high temporal resolution of the profiler data and was not detectable with data provided by the rawinsonde network.

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Peter A. Stamus, Frederick H. Carr, and David P. Baumhefner

Abstract

A scale-separation technique based on two-dimensional Fourier decomposition is applied to the comparison and verification of analyses and forecasts produced by regional numerical weather prediction systems. A major emphasis of this study is the verification of secondary or derived parameters in addition to the evaluation of primary model variables. Two prediction models are used to illustrate the technique for a variety of forecast fields separated into three separate wavenumber bands. Three different sets of analyses, one from each model system and an independent set, are used for both analysis intercomparison and model verification. The comparison of the analyses is essential to establishing the level of uncertainty for each variable as a function of scale. The synoptic-scale database used to produce the analyses for this study does not allow the verification of scales 800 km or less, no matter how fine the resolution of the model.

Examining the spectra of difference fields with time allows one to study the evolution of model error (or differences between two models) as a function of wavenumber. In some instances where traditional statistical measures of skill indicated good agreement between two forecasts, spectral scale selection of the difference fields shows that the spatial distribution of the errors was quite different, pointing to different error-growth characteristics of the models. The technique allows one to partially separate phase and amplitude errors and, hence, barotropic-versus baroclinic-type error structure. It was found, as expected, that forecast skill decreases more rapidly with time for smaller scales, but this is not true for all parameters examined. The presence of lateral boundary conditions strongly influences the evaluation of skill in a regional model for the primary variables, but not as much for some secondary variables. Verification of secondary variables nearly always indicates significant errors in the forecast before serious problems in the primary variables are detected.

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Frederick H. Carr, Richard L. Wobus, and Ralph A. Petersen

Abstract

The Regional Analysis and Forecast System at the National Meteorological Center consists of an optimum interpolation objective analysis scheme, an adiabatic nonlinear normal model initialization (NNMI) and a hemispheric Nested Grid Model (NGM) to provide 48 h forecasts. We investigate here the effect NNMI has on the analyses and forecasts produced by this system. An eight vertical mode NNMI procedure led to significant reductions of the divergent component of the analyzed wind field in regions of heavy precipitation. This is shown to contribute to a systematic spinup error in NGM 0–12 h precipitation forecasts, especially from the 0000 UTC runs. Forecasts starting with no initialization had unacceptable noise levels. NNMI using two vertical modes yielded the best combination of noise-free forecasts and unsuppressed initial precipitation rates. A physical interpretation of this result is provided using the vertical structure functions of the normal modes. Tests of the two-mode NNMI in an operational setting confirmed that the 0–12 h NGM precipitation amounts increased along with a reduction of an excessive precipitation bias in the 12–36 h period. The two-mode procedure was implemented operationally in August 1987. It is suggested that if one has a reliable data assimilation system, especially if the 6 h forecasts used as background fields for the analysis have realistic precipitation rates and attendant divergent circulations, then a two-vertical-mode adiabatic NNMI provides many of the benefits of a traditional diabatic normal mode initialization procedure.

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Mark A. Shafer, Donald R. MacGorman, and Frederick H. Carr

Abstract

Cloud-to-ground (CG) lightning data are examined relative to digitized radar data for a storm system that occurred in Oklahoma on 26 May 1985. This system evolved through three stages: 1) two lines of cells, one near the dryline and the other 60 km ahead of it; 2) a supercell storm; and 3) a mesoscale convective system (MCS). The behavior of lightning in each stage was different. Initially no ground flashes were observed in either line until reflectivity increased to ≥46 dBZ and vertically integrated liquid (VIL) increased to ≥10 kg m−2; then ground flash rates remained <1.2 min−1 for >1 h. Most ground flashes in the line of storms near the dryline were negative (18 −CG, 3 +CG), while most in the leading line were positive (11 +CG, 3 −CG), a pattern of polarity opposite to what usually has been observed. Approximately 3 h after radar detected the first storm, ground flash rates increased to >5 min−1 and remained so for 6 h. A mesocyclone formed approximately 30 min after flash rates exceeded 5 min−1, and a few positive ground flashes occurred near it. Ground flash rates increased briefly to >20 min−1 as the mesocyclone dissipated and then remained >10 min−1 as a squall line formed along the outflow boundary from the dissipating supercell and produced a stratiform region. Most ground flashes in this MCS occurred in the convective line and had negative polarity. The few ground flashes in the stratiform region tended to be positive (42 +CG, 32 −CG during 3 h). During 1 h of the MCS, ground flash rates decreased and then increased again simultaneously in both the convective and stratiform regions, a previously undocumented behavior. It is possible that this was caused by updrafts in both the convective line and stratiform region changing at roughly the same time. It is also possible that most ground flashes in the stratiform region originated near the convective line, and so were influenced by the line. Overall trends in ground flash density, flash relative frequency, reflectivity, VIL, and severe hail reports appeared similar as the storm system evolved.

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