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Kenneth A. Campana

Abstract

Real data experiments are conducted using a semi-implicit version of NMC's six-layer primitive equation model to investigate reduction in spatial truncation error by higher order finite-differencing. The higher order approximations are applied to advection terms in the equations. Detailed results art presented for two developing winter storm cases over North America.

Comparisons between second- and fourth-order versions of the coarse-mesh (381 km) model show that the higher order scheme produces improved predicted translational speeds of meteorological features. Forty-eight hour forecasts made by the fourth-order model are more accurate in both mass field and accumulated precipitation predictions.

Fine-mesh (one-half the grid distance) forecasts have been made for one of the cases with both second- and fourth-order differencing. Second-order fine-mesh results are remarkably similar to those from the coarse-mesh fourth-order model, although the finer mesh does produce a more accurate forecast of precipitation. Fourth-order differencing in the fine mesh produces further improvements. The computational efficiency of the higher order coarse-mesh alternative to the fine mesh is an attractive feature to be considered in operational forecast environments.

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John A. Brown Jr. and Kenneth A. Campana

Abstract

A simple method for integrating the primitive equations is presented which allows for a timestep increment up to twice that of the conventional leapfrog scheme. It consists of a time-averaging operator, which incorporates three consecutive time levels, on the pressure gradient terms in the equations of motion. An attractive feature of the method is its case in programming, since the resulting finite-difference equations can he solved explicitly.

Presented here are linear analyses of the method applied to the barotropic and two-layer baroclinic gravity waves. Also presented is an analysis of the method with a time-damping device incorporated, which is an alternative in controlling linearly amplifying computational modes.

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Yu-Tai Hou, Kenneth A. Campana, Kenneth E. Mitchell, Shi-Keng Yang, and Larry L. Stowe

Abstract

CLAVR [cloud from AVHRR (Advanced Very High Resolution Radiometer)] is a global cloud dataset under development at NOAA/NESDIS (National Environmental Satellite, Data, and Information Service). Total cloud amount from two experimental cases, 9 July 1986 and 9 February 1990, are intercompared with two independent products, the Air Force Real-Time Nephanalysis (RTNEPH), and the International Satellite Cloud Climatology Project (ISCCP). The ISCCP cloud database is a climate product processed retrospectively some years after the data are collected. Thus, only CLAVR and RTNEPH can satisfy the real-time requirements for numerical weather prediction (NWP) models. Compared with RTNEPH and ISCCP, which only use two channels in daytime retrievals and one at night, CLAVR utilizes all five channels in daytime and three at night from AVHRR data. That gives CLAVR a greater ability to detect certain cloud types, such as thin cirrus and low stratus. Designed to be an operational product, CLAVR is also compared with total cloud forecasts from the National Meteorological Center (NMC) Medium Range Forecast (MRF) Model. The datasets are mapped to the orbits of NOAA polar satellites, such that errors from temporal sampling are minimized. A set of statistical scores, histograms, and maps are used to display the characteristics of the datasets. The results show that the CLAVR data can realistically resolve global cloud distributions. The spatial variation is, however, less than that of RTNEPH and ISCCP, due to current constraints in the CLAVR treatment of partial cloudiness. Results suggest that if the satellite cloud data is available in real time, it can be used to improve the cloud parameterization in numerical forecast models and data assimilation systems.

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Shi-Keng Yang, Yu-Tai Hou, Alvin J. Miller, and Kenneth A. Campana

Abstract

This study presents an evaluation of the NCEP–NCAR Reanalysis (the reanalysis) by comparing its components of the earth radiation budget to satellite data. Monthly mean clear sky (CS) and total sky of outgoing longwave radiation (OLR), as well as reflected solar radiation (RSW) for 1985 and 1986, are compared to the top-of-the-atmosphere (TOA) measurements from the Earth Radiation Budget Experiment (ERBE). The ERBE-derived data of Staylor and Wilbur are also utilized to validate surface albedo. There are two objectives to this study: (i) to document the general quality of the reanalysis radiation budget, and (ii) to identify some of the general problem areas in the reanalysis global data assimilation system (GDAS).

The OLR comparisons show that the global annual mean from the reanalysis is approximately 1.5% higher than that of ERBE. The zonal-average differences are strongly seasonal, which is particularly evident at high latitudes for the CS OLR, and at most latitudes for total-sky OLR. For the geographical distribution, the synoptic patterns from the reanalysis are in good agreement with the observations. Yet many regions in the Tropics and subtropics pose significant systematic biases. Possible causes are from shortcomings in the the cloud/moisture parameterizations of the reanalysis GDAS. The complex topography unresolvable by the T62 model could also be the cause for the biases in tall mountain regions.

The global RSW comparisons show that the CS data from the reanalysis is in very good agreement with ERBE, while the total-sky RSW data overestimate ERBE by 12.6 W m−2 (∼10%) globally. Persistent overestimates of RSW throughout the period indicate that the global energy budget for the reanalysis is not balanced. This result also is consistent with the finding in OLR suggesting that the reanalysis GDAS contains shortcomings in the cloud/moisture parameterizations. Another possibility for the difference in RSW is deficiencies in the GDAS shortwave parameterizations.

Over the Sahara Desert, the reanalysis underestimates RSW, and overestimates OLR, both in the clear-sky and total-sky conditions. Comparison with the Staylor and Wilber ERBE-derived surface albedo suggests that GDAS surface albedo in this region should be increased by up to 0.1 (in albedo units). A comparison with the interannual variations of the satellite data for the boreal summer illustrates that the radiation budget data of the reanalysis contains a realistic climate signal.

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