A Study of the Early Winter Effects of the Great Lakes.I: Comparison of Very Fine Scale Numerical Simulations with Observed Data

Douglas B. Boudra Rosenstiel School of Marine & Atmospheric Science, University of Miami. Miami, FL 33149

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Abstract

Development of a framework for study of the Great Lakes' effects on late fall-early winter cyclones andArctic air masses has been initiated. The central theoretical component is a three-dimensional numericalprimitive equations model. The 40-45 km horizontal grid point spacing provides for resolution of the shapeand dimension of each lake, and 15 levels in the vertical resolve interaction of boundary layer and midtropospheric processes. Over land, locally computed vertical diffusion coefficients depending on static stabilityand Richardson number are used to parameterize subgrid-scale turbulent processes throughout the depthof the atmosphere. At over-lake grid points, the Businger et al. (1971) formulation of the Monin-Obukhov(1954) similarity equations is used in the constant flux layer, and the combined O'Brien (1971) K profileand Deardorff (1974) boundary layer depth prediction equation are used for turbulence parameterizationin the rest of the boundary layer. The prognostic variables, pressure, u, v, and water vapor mixing ratio,are unstaggered on the Cartesian finite difference grid. Horizontal differencing operators are fourth-orderaccurate and the vertical operators are of second-order accuracy formulated for an uneven grid. The equationsare cast over a limited area 50 x 40 x 15 grid points, and the Perkey-Kreitzberg (1976) method isincorporated for the variable tendencies near the lateral boundaries. A real data case exhibiting vigorouscyclone development and subsequent strong cold air advection over the lake region is chosen for simulation.The dynamic variables are analyzed with the optimum iaterpolation scheme on isentropic surfaces developedby Bleck (1975) and are interpolated to the model grid matrix. Likewise, the moisture field is analyzedusing a scheme described by Perkey (1976). The model is initialized with the first of four analyses atconsecutive radiosonde observation times. The remaining three analyses are used in the lateral boundaryregions for the eternally specified tendencies during a 36 h simulation.

The remaining components of the study consist of several forms of observation taken during the periodof the simulation: 1) the objective analyses of radiosonde data at 12 h intervals, 2) surface analysis on theNorth American synoptic chart, 3) the hourly precipitation data network in the United States, 4) radarsummaries and 5) satellite imagery. The operational forecast generated on the LFM2 model by the NationalMeteorological Center is used as an additional point of reference. It is found that the LFM2 forecast, whichignores the Great Lakes, predicts the cyclone motion realistically, but makes considerable error otherwise.Agreement between the very fine scale simulations and observation is generally much better and encouragesdevelopment of diagnostic methods for a rigorous comparison of the observed and simulated lake effects.

Abstract

Development of a framework for study of the Great Lakes' effects on late fall-early winter cyclones andArctic air masses has been initiated. The central theoretical component is a three-dimensional numericalprimitive equations model. The 40-45 km horizontal grid point spacing provides for resolution of the shapeand dimension of each lake, and 15 levels in the vertical resolve interaction of boundary layer and midtropospheric processes. Over land, locally computed vertical diffusion coefficients depending on static stabilityand Richardson number are used to parameterize subgrid-scale turbulent processes throughout the depthof the atmosphere. At over-lake grid points, the Businger et al. (1971) formulation of the Monin-Obukhov(1954) similarity equations is used in the constant flux layer, and the combined O'Brien (1971) K profileand Deardorff (1974) boundary layer depth prediction equation are used for turbulence parameterizationin the rest of the boundary layer. The prognostic variables, pressure, u, v, and water vapor mixing ratio,are unstaggered on the Cartesian finite difference grid. Horizontal differencing operators are fourth-orderaccurate and the vertical operators are of second-order accuracy formulated for an uneven grid. The equationsare cast over a limited area 50 x 40 x 15 grid points, and the Perkey-Kreitzberg (1976) method isincorporated for the variable tendencies near the lateral boundaries. A real data case exhibiting vigorouscyclone development and subsequent strong cold air advection over the lake region is chosen for simulation.The dynamic variables are analyzed with the optimum iaterpolation scheme on isentropic surfaces developedby Bleck (1975) and are interpolated to the model grid matrix. Likewise, the moisture field is analyzedusing a scheme described by Perkey (1976). The model is initialized with the first of four analyses atconsecutive radiosonde observation times. The remaining three analyses are used in the lateral boundaryregions for the eternally specified tendencies during a 36 h simulation.

The remaining components of the study consist of several forms of observation taken during the periodof the simulation: 1) the objective analyses of radiosonde data at 12 h intervals, 2) surface analysis on theNorth American synoptic chart, 3) the hourly precipitation data network in the United States, 4) radarsummaries and 5) satellite imagery. The operational forecast generated on the LFM2 model by the NationalMeteorological Center is used as an additional point of reference. It is found that the LFM2 forecast, whichignores the Great Lakes, predicts the cyclone motion realistically, but makes considerable error otherwise.Agreement between the very fine scale simulations and observation is generally much better and encouragesdevelopment of diagnostic methods for a rigorous comparison of the observed and simulated lake effects.

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