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Hua-Lu Pan

Abstract

The use of a simple bucket method to parameterize the evapotranspiration over land has been found to overestimate the latent heat flux in the global spectral model at the National Meteorological Center. A new parameterization package based on the Penman-Monteith potential evapotranspiration concept is reported here. This package has been implemented to reduce the over-moistening problem. Results indicate improvements in the near surface moisture field over land.

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Jongil Han and Hua-Lu Pan

Abstract

A new physics package containing revised convection and planetary boundary layer (PBL) schemes in the National Centers for Environmental Prediction’s Global Forecast System is described. The shallow convection (SC) scheme in the revision employs a mass flux parameterization replacing the old turbulent diffusion-based approach. For deep convection, the scheme is revised to make cumulus convection stronger and deeper to deplete more instability in the atmospheric column and result in the suppression of the excessive grid-scale precipitation. The PBL model was revised to enhance turbulence diffusion in stratocumulus regions. A remarkable difference between the new and old SC schemes is seen in the heating or cooling behavior in lower-atmospheric layers above the PBL. While the old SC scheme using the diffusion approach produces a pair of layers in the lower atmosphere with cooling above and heating below, the new SC scheme using the mass-flux approach produces heating throughout the convection layers. In particular, the new SC scheme does not destroy stratocumulus clouds off the west coasts of South America and Africa as the old scheme does. On the other hand, the revised deep convection scheme, having a larger cloud-base mass flux and higher cloud tops, appears to effectively eliminate the remaining instability in the atmospheric column that is responsible for the excessive grid-scale precipitation in the old scheme. The revised PBL scheme, having an enhanced turbulence mixing in stratocumulus regions, helps prevent too much low cloud from forming. An overall improvement was found in the forecasts of the global 500-hPa height, vector wind, and continental U.S. precipitation with the revised model. Consistent with the improvement in vector wind forecast errors, hurricane track forecasts are also improved with the revised model for both Atlantic and eastern Pacific hurricanes in 2008.

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Jongil Han and Hua-Lu Pan

Abstract

A parameterization of the convection-induced pressure gradient force (PGF) in convective momentum transport (CMT) is tested for hurricane intensity forecasting using NCEP's operational Global Forecast System (GFS) and its nested Regional Spectral Model (RSM). In the parameterization the PGF is assumed to be proportional to the product of the cloud mass flux and vertical wind shear. Compared to control forecasts using the present operational GFS and RSM where the PGF effect in CMT is taken into account empirically, the new PGF parameterization helps increase hurricane intensity by reducing the vertical momentum exchange, giving rise to a closer comparison to the observations. In addition, the new PGF parameterization forecasts not only show more realistically organized precipitation patterns with enhanced hurricane intensity but also reduce the forecast track error. Nevertheless, the model forecasts with the new PGF parameterization still largely underpredict the observed intensity. One of the many possible reasons for the large underprediction may be the absence of hurricane initialization in the models.

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Song-You Hong and Hua-Lu Pan

Abstract

In this paper, the incorporation of a simple atmospheric boundary layer diffusion scheme into the NCEP Medium-Range Forecast Model is described. A boundary layer diffusion package based on the Troen and Mahrt nonlocal diffusion concept has been tested for possible operational implementation. The results from this approach are compared with those from the local diffusion approach, which is the current operational scheme, and verified against FIFE observations during 9–10 August 1987. The comparisons between local and nonlocal approaches are extended to the forecast for a heavy rain case of 15–17 May 1995. The sensitivity of both the boundary layer development and the precipitation forecast to the tuning parameters in the nonlocal diffusion scheme is also investigated. Special attention is given to the interaction of boundary layer processes with precipitation physics. Some results of parallel runs during August 1995 are also presented.

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Xiaosong Yang, Timothy DelSole, and Hua-Lu Pan

Abstract

This paper examines the extent to which an empirical correction method can improve forecasts of the National Centers for Environmental Prediction (NCEP) operational Global Forecast System. The empirical correction is based on adding a forcing term to the prognostic equations equal to the negative of the climatological tendency errors. The tendency errors are estimated by a least squares method using 6-, 12-, 18-, and 24-h forecast errors. Tests on independent verification data show that the empirical correction significantly reduces temperature biases nearly everywhere at all lead times up to at least 5 days but does not significantly reduce biases in forecast winds and humidity. Decomposing mean-square error into bias and random components reveals that the reduction in total mean-square error arises solely from reduction in bias. Interestingly, the empirical correction increases the random error slightly, but this increase is argued to be an artifact of the change in variance in the forecasts. The empirical correction also is found to reduce the bias more than traditional “after the fact” corrections. The latter result might be a consequence of the very different sample sizes available for estimation, but this difference in sample size is unavoidable in operational situations in which limited calibration data are available for a given forecast model.

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Song-You Hong and Hua-Lu Pan

Abstract

A precipitation physics package for the National Centers for Environmental Prediction Regional Spectral Model designed to improve the skill of precipitation forecasts is proposed. The package incorporates a prognostic grid-resolvable precipitation scheme and a parameterized convection scheme with a convective trigger function that explicitly couples boundary layer and convective precipitation processes. Comprehensive sensitivity experiments were conducted with a grid spacing of approximately 25 km for a heavy rain case over the United States during 15–17 May 1995. In this paper, the trigger function setup in the convective parameterization scheme and its impact on the predicted precipitation are discussed. Special attention is given to the interaction of cloud properties in the parameterized convection with the evolution of grid-resolvable precipitation physics. The impact of convective forcing due to different convective triggers on the large-scale pattern downstream is also discussed. The implementation of the prognostic cloud scheme and performance are presented in a companion paper.

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Alan K. Betts, Song-You Hong, and Hua-Lu Pan

Abstract

Data from the FIFE experiment of the summer of 1987 are used to assess the diurnal and seasonal cycles of the surface energy budget and boundary layer in the NCEP-NCAR reanalysis, which used the summer 1995 version of the Medium-Range Forecast model. The seasonal agreement is quite good, reflecting the improvements in land-surface parameterizations in recent years. Detailed studies, however, identify several places where still further improvements in model parameterizations are possible. Clear-sky shortwave absorption and cloudiness may be underestimated in this model as has been noticed in other global models. More frequent updates of the model cloud cover (currently every 3 h) would improve the land-surface interaction after the initial onset of precipitation. The model produces a realistic well-mixed boundary layer, but underestimates boundary layer deepening by entrainment. For much of the summer, precipitation is close to that observed: however, for a period in June and early July, the reanalysis model has excess precipitation, which comes from daytime interactions between the surface evaporation, boundary layer, and convection schemes. The model, which has a deep 2-m soil reservoir, has adequate soil moisture storage for several weeks without rain, although surface evaporative fraction fluctuates more strongly after rain than is observed. Surface evaporation is generally too high at night, especially in high winds. This analysis suggests several improvements to the model parameterizations, some of which have already been implemented in the operational MRF model.

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T. N. Krishnamurti, Charles E. Levy, and Hua-Lu Pan

Abstract

In this note we present an interesting phenomenon involving the near-simultaneous intensification and weakening of the trades. The major data source for this study are low-cloud motion vectors obtained from the geosynchronous satellites ATS III and SMS GOES. The observations show that there is a predominant oscillation in the intensity of the trades with a period of roughly two weeks. This strengthening and weakening in this intensity of the trades is noticed throughout the 100 days of the GATE period. A more detailed analysis of the surge in the trades is in preparation.

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T. N. Krishnamurti, John Molinari, and Hua Lu Pan

Abstract

In this study we show that many of the observed features of the cross-equatorial low-level jet of the Arabian Sea, Indian Ocean and Somalia can be numerically simulated by including 1) the cast African and Madagascar mountains, 2) the beta effect and 3) a lateral forcing from the east around 75°E. This lateral forcing at 75°E is, in fact, a solution of another numerical model–one where the land-ocean contrast heating in the meridional direction is incorporated in much detail to simulate the zonally symmetric monsoons, essentially following Murakami et al. (1970). This zonally symmetric solution of a very long-term numerical integration from a state of rest exhibits many of the observed characteristics of the broad-scale monsoons at 75°E. This later solution is used as a lateral forcing for the low-level jet simulations over the Arabian Sea-Indian Ocean.

The numerical model presented here is a one-level primitive equation model with a detailed bottom topography and a one-degree latitude grid size.

Results of several controlled numerical experiments suppressing or including orography, the beta effect and the broad-scale lateral monsoon forcing at 75°E are discussed in this paper. When all the three above-mentioned parameters are included, features such as strong winds just downstream from the Madagascar mountains, an equatorial relative speed minimum, an intense jet off the Somali coast and a split of the jet over the northern Arabian Sea are simulated from an initial state of rest. The Ethiopian highland appears crucial for the simulation of the Somali coast strong winds; the Madagascar mountains are most important for the strong winds just downstream from Madagascar. The split in the jet over the Arabian Sea is analyzed as a barotropic instability problem. The beta effect is essential for the simulation of the observed geometry. Experiments with a weak broad-scale monsoon forcing at 75°E fall to produce strong winds over cast Africa. The implications of this forcing are analyzed in this paper and some relevant observations are presented.

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Naomi Surgi, Hua-Lu Pan, and Stephen J. Lord

Abstract

An evaluation of the performance of the National Centers for Environmental Prediction Medium-Range Forecast Model was made for the large-scale tropical forecasts and hurricane track forecasts during the 1995 hurricane season. The assessment of the model was based on changes to the deep convection and planetary boundary layer parameterizations to determine their impact on some of the model deficiencies identified during previous hurricane seasons. Some of the deficiencies in the hurricane forecasts included a weakening of the storm circulation with time that seriously degraded the track forecasts. In the larger-scale forecasts, an upper-level easterly wind bias was identified in association with the failure of the model to maintain the midoceanic upper-tropical upper-tropospheric trough.

An overall modest improvement is shown in the large-scale upper-level tropical winds from root-mean-square-error calculations. Within a diagnostic framework, an improved simulation of the midoceanic tropical trough has contributed to a better forecast of the upper-level westerly flow. In the hurricane forecasts, enhanced diabatic heating in the model vortex has significantly improved the vertical structure of the forecast storm. This is shown to contribute to a substantial improvement in the track forecasts.

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