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Ralph A. Petersen

Abstract

Recent developments in meteorological objective analysis have been directed towards improving numerical model initialization, often at the expense of preserving detail in data rich regions. The objective scheme presented here is designed to give detailed, yet computationally efficient analyses over data rich regions and to be independent of any numerical model influences. The results, which accurately preserve observed data and gradient information, are particularly suited to diagnostic studies. The analysis scheme uses both observed station data and interstation gradients to interpolate pressure, wind, and moisture information from vertical cross sections onto isentropic surfaces using cubic polynomials. Three-dimensional grid point mass and moisture analyses at specified isentropic intervals are obtained by first constructing a series of nearly parallel, north–south vertical cross sections over an area of interest and then filling in the space between adjacent vertical analyses using a second set of nearly orthogonal cross sections. The choice of isentropic coordinates allows the objective analysis to use detailed information included within the vertical structure of sets of individual, quality-controlled soundings to capture significant horizontal thermal gradients. Gradients obtained from the mass analyses are then combined with observed wind data to obtain observed wind analyses which have spatial variations consistent with the scales of variations noted in the detailed mass fields. Examples are given which illustrate the detail and continuity inherent in the technique when applied to radiosonde data obtained at a variety of scales.

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Ralph A. Petersen

Abstract

Several different forms of lifted index (LI) calculations are available from the Limited-Area Fine Mesh Model (LFM) and Nested-Grid Model (NGM) run operationally at the National Meteorological Center (NMC). The differences between the index calculations are presented, various applications of the LI are discussed, and a case demonstrating some of the strengths and weaknesses of each is presented.

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Ralph A. Petersen
and
Louis W. Uccellini

Abstract

An explicit technique for computing atmospheric trajectories, based on Greenspan's discrete model formulation, is presented as an alternative to the commonly used implicit scheme. The method provides an economical means of objectively obtaining computer-generated trajectories and accounts for the variable accelerations and local ψtendencies along the entire trajectory path. The initial results presented show that the explicit computations are stable and very nearly energy-conservative. An application of the discrete model approach to a real data base and comparisons with trajectories determined by the implicit method yield favorable results, illustrating the utility of the explicit technique as a diagnostic tool.

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Thomas M. Whittaker
and
Ralph A. Petersen

Abstract

Overlapping second-order Lagrangian polynomials are used to construct cross-sectional isentropic analyses from upper air sounding observations. Estimates of geostrophic winds are obtained through the thermal wind relationship. Thermal information is also combined with normal components of the observed wind to obtain a thermally enhanced, observed wind analysis. Results using these techniques are presented and compared with a previous objective analysis technique developed by Shapiro and Hastings (1973). Examination of the results indicates the applicability of the techniques both for operational and research purposes.

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Ralph A. Petersen
and
Jeffrey H. Homan

Abstract

The recent advancement of mini- and microcomputers into the local-work environment can provide local forecast offices with the capability to run simple numerical models for specific nowcasting and short-term forecast needs. While the capabilities of sophisticated, centralized modeling systems continue to progress, short-term forecasts are hampered by a number of inadequacies in this guidance, ranging from numerical deficiencies of the models to the inability to access the gridded model output in a timely manner and view it at frequent enough time intervals or to enhance particular local aspects of the initial analyses. In many instances, observations and extrapolations are still considered to be more important than numerical guidance in the very short range.

In this paper, an isentropic forecast technique is used to provide temporally and spatially detailed short-term advective forecasts of temperature, moisture, and wind changes. While geostrophic potential vorticity conservation is used to simplify the model, the advective computations can use both geostrophic and ageostrophic wind data. The inclusion of pronounced baroclinic structures in the detailed initial isentropic analyses allows the model to preserve large observed vertical and horizontal wind shears, to monitor local stability changes, and to detect areas of strong subsidence and overrunning.

A series of short-range forecasts of the preconvective environment associated with severe convection is discussed. High time frequency fields of midtropospheric static stability, low-level moisture and temperature, tropopause height, and conventional stability indices are presented and combined with observed surface data to provide updated stability and buoyancy guidance. In addition, forecasted winds are used to advect VAS mid- and low-level moisture imagery, which can be used to predict areas of changing convective potential and extend the lead time required to issue forecasts for severe convective storms.

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Ralph A. Petersen
and
James E. Hoke

Abstract

The response of the Regional Analysis and Forecast System (RAFS) low-level forecast fields to the geographical distribution of snow cover is discussed. The errors produced by an improper specification of this field in the forecast model can have a wide variety of local forecasting implications, ranging from poor forecasts of lee-side Great Lakes snowfalls, to errors in forecasts of the earth's surface temperature in areas where the snow cover is changing rapidly. The use of the snow-cover data in the forecast model and its effect on forecast guidance are described.

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Ralph A. Petersen
and
John D. Stackpole

Abstract

An overview of the regularly scheduled sequence of computer analyses and forecasts produced at the National Meteorological Center (NMC) is presented. The available computer resources and time schedule constraints are discussed, the sources and treatment of incoming data are described, the purposes and configurations of the operational analysis/forecasting systems are outlined, and the mechanisms for product distribution are presented.

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Timothy J. Wagner
and
Ralph A. Petersen

Abstract

Routine in situ observations of the atmosphere taken in flight by commercial aircraft provide atmospheric profiles with greater temporal density and, in many parts of the country, at more locations than the operational radiosonde network. Thousands of daily temperature and wind observations are provided by largely complementary systems, the Airborne Meteorological Data Relay (AMDAR) and the Tropospheric Airborne Meteorological Data Reporting (TAMDAR). All TAMDAR aircraft also measure relative humidity while a subset of AMDAR aircraft are equipped with the Water Vapor Sensing System (WVSS) measure specific humidity. One year of AMDAR/WVSS and TAMDAR observations are evaluated against operational National Weather Service (NWS) radiosondes to characterize the performance of these systems in similar environments. For all observed variables, AMDAR reports showed both smaller average differences and less random differences with respect to radiosondes than the corresponding TAMDAR observations. Observed differences were not necessarily consistent with known radiosonde biases. Since the systems measure different humidity variables, moisture is evaluated in both specific and relative humidity using both aircraft and radiosonde temperatures to derive corresponding moisture variables. Derived moisture performance is improved when aircraft-based temperatures are corrected prior to conversion. AMDAR observations also show greater consistency between different aircraft than TAMDAR observations do. The small variability in coincident WVSS humidity observations indicates that they may prove more reliable than humidity observations from NWS radiosondes.

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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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Anthony Mostek
,
Louis W. Uccellini
,
Ralph A. Petersen
, and
Dennis Chesters

Abstract

Retrievals from the VISSR Atmospheric Sounder (VAS) an combined with conventional data to assess the impact of geosynchronous satellite soundings upon the analysis of a pre-convective environment over the central United States on 13 July 1981. VAS retrievals of temperature, dewpoint, equivalent potential temperature, precipitable water, and lifted index are derived with 30 km resolution at 3 hour intervals. When VAS fields are combined with analyses from conventional data sources regions with convective instability are more clearly delineated prior to the rapid development of the thunderstorms. The retrievals differentiate isolated areas in which most air extends throughout the lower troposphere (and are therefore more conducive for the development of deep convective storms) from those regions where moisture is confined to a thin layer near the earth's surface (where convection is less likely to occur). The analyses of the VAS retrievals identify significant spatial gradients and temporal changes in the thermal and moisture fields, especially in the regions between radiosonde observations. The detailed analyses also point to limitations in using VAS data. Even with nearly optimal conditions for passive remote sounding (generally clew skies, minimal orographic effects, and a rapidly changing moisture field), the VAS retrievals were still degraded in some regions by small clouds which are unresolved in the infrared imagery. These analyses, however, demonstrate that the geosynchronous VAS can be used in a case study mode to produce high-resolution spatial and temporal measurements that are useful for the quantitative analysis of a cloud-free pre-convective environment.

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