The life cycle of Supertyphoon Hope (1979) from a tropical depression stage to intensification and its eventual weakening after landfall, some 6 days later, is followed in a real-data numerical prediction experiment. The predictions are carried out with a very high-resolution global spectral model, (Spectral resolution, triangular 170 waves). The initial data for this study are obtained from the delayed reanalysis of the Global Experiment (1979). Those were the analysis of the European Centre for Medium Range Weather Forecasts (ECMWF). Initialization is based on nonlinear normal mode with physics. The model has 12 vertical layers with staggered variables. A detailed physical-dynamical model is used for these studies. This typhoon prediction is a follow-up of a recent study (Krishnamurti et al. 1989). Here we show the prediction of the life cycle at a much higher resolution. The present study illustrates a remarkable prediction of the track, structure, and intensity of the supertyphoon at the higher resolution and raises the possibility for major improvement of tropical storm prediction with real data.

Abstract

A global spectral model is used to carry out a number of short to medium range prediction experiments with global datasets. The primary objective of these studies is to examine the formation and motion of the hurricanes/typhoons with a fairly comprehensive state-of-the-art global model. Ale spectral model utilizes the usual transform method for the calculations of the nonlinear and physical processes. The physical processes include parameterizations of the planetary boundary layer, deep and shallow cumulus convection, radiative processes (including cloud feedback processes, diurnal change and surface energy balance) and large-scale condensation. ‘Envelope orography’ is used to represent steep mountains globally. Ocean temperatures are prescribed from a Preceding 10 day averaged dataset for the storm periods under investigation.

Sensitivity of storm forecasts to horizontal and vertical resolutions, datasets and representation of physical processes are addressed in this paper.

The major findings of this study are that improved results on the formation and motion of storms are achieved in several cases when (i) the surface layer fluxes are adequately resolved, (ii) the final FGGE analyzed datasets are used, (iii) very high resolution in the horizontal (106 waves triangular truncation) is used, and (iv) improved physical parameterization for the boundary layer, cumulus convection and radiative process are included.

The major limitation of this study is that in spite of the use of very high resolution the inner rain area (radius<150 km from the storm center) is not adequately represented to describe the central pressure, maximum wind and the warm core of hurricanes. Further studies to improve these areas are suggested.

Abstract

In this Paper we have examined the fluxes of latent and sensible heat between the ocean and the atmosphere utilizing primarily the observations from the global experiment. The procedure for calculation is based on the Monin-Obukhov similarity theory. The transfer coefficients are stability dependent. Some calculations are also performed utilizing several different datasets. One of these covers the region of Asian monsoon oceans during the Global Experiment. This is a daily sea surface temperature (SST) dataset. A second dataset covers the global oceans for the entire FGGE year. This is a 10-day averaged dataset prepared by the European Centre for Medium Range Weather Forecasts. A third dataset obtained from the Japan Meteorological Agency, covers a 7-year period between 1979 and 1985. This dataset was also averaged over 10-day periods. Both the daily and the 10-day averaged SST data have been used for studies of low frequency oscillations. An equivalence, on the time scale of 30 to 50 days, for the oscillations of these two datasets is demonstrated. The global distribution of SST oscillations on this time scale are explored. The region of pronounced variance in the SST oscillation lies over the equatorial western Pacific Ocean and the Bay of Bengal. Calculations of air-sea energy fluxes include (i) the total contribution, (ii) those contributed on the time scale of 30 to 50 days, and (iii) those obtained by suppressing oscillations for one or more of the variables on this time scale. The main results of these computations show that SST oscillations with an amplitude of 0.3° to 1.0°C occur on this time scale over the tropical oceans. The fluxes of latent and sensible heat on this lime scale over the tropics have mean amplitudes of the order of 30 and 3 W m⁻², respectively. The typical mean total fluxes in these regions are of the order of 200 and 30 W m⁻² respectively. The fluctuations in the wind speed and the sea surface temperatures control the heat and moisture fluxes on the time scale of 30 to 50 days. Fluctuations of surface air temperature and surface humidity do not seem to be important over most oceans. Among these variables the role of the wind oscillations is somewhat larger. Proposals for further studies towards the understanding of oscillations on this time scale are presented.

Abstract

This study follows a recent paper on the predictability of low-frequency modes on the time scale of 30–50 days. By filtering out the high-frequency modes, we are able to delay the contamination of low-frequency modes for periods of the order of 1 month in global forecasts. A multilevel global model forecast is carried out to predict a wet spell over central China. It is shown that an initial state consisting of time-mean state, a low-frequency mode, and a specification of the sea surface temperature anomaly provides useful forecasts for the occurrence of dry or wet spells. All of these tests are carried out with a global model; however, only the monsoon region is examined in some detail. This study includes the results of a number of experiments where the sensitivity to the definitions of the mean state, the sea surface temperatures, and initial datasets are explored. The main finding of this paper is that the prediction of monsoonal low-frequency modes and the related dry and wet spells can be extended beyond the usual numerical weather prediction (NWP) predictability limit of 6 or 7 days. It appears that if the contamination from high-frequency modes is suppressed by an initial filtering, then the prediction of low-frequency motion through one cycle, a period of roughly a month, is possible. The forecasts are shown to be quite sensitive to the definition of the initial time-mean state and the sea surface temperature anomaly.

Abstract

This paper examines the medium-range forecast of a typhoon using a global model. The focus of this study is on a comparison of two longwave radiative transfer calculations, one is based on an emissivity formulation while the other utilizes a band model. A more realistic prediction of low clouds in the storm environment by the band model leads to stronger cooling rates and the resulting destabilization contributes to the maintenance of conditional instability. The inflowing air supplies this instability for the maintenance of a longer-lasting storm. The emissivity model fails to predict a sufficient abundance of low clouds resulting in weaker cooling rates hence the resulting destabilization is weak and leads to decay of the storm. The important role of radiatively active shallow clouds in maintaining the conditional instability of the storm environment is illustrated for a long-range integration. An analysis of these aspects of storm environment destabilization is presented in this paper.

Abstract

The emphasis of this paper is on residue-free budgets of seasonal climate forecasts. It is possible to ask the following question: given a seasonal mean geopotential height simulation from a climate model, what is a breakdown of that contribution from different areas of the model physics and dynamics? In that context, the authors have examined the maintenance of a monsoonal 500-mb ridge, the eastward shift of the Tibetan anticyclone during an El Nin˜o year, and the Pacific-North American pattern. The salient results of this study include a substantial contribution from the advective nonlinear dynamics toward the maintenance (positive or negative) of the seasonal climate.

Abstract

This study is based on a global coupled atmosphere–ocean model climate prediction that was designed to include 14 layers over the atmosphere and 17 layers within the ocean. In this model an 11-yr data assimilation includes physical initialization of the daily rainfall estimates. No flux corrections are included in the seasonal and annual forecasts of this coupled model. It is first shown that intraseasonal oscillation on the Madden–Julian timescale was an important feature during the onset of the El Niño of 1997. It is shown that this feature is retained in the model’s data assimilation and in the forecasts. The forecasts commence on 1 April 1997. The model forecasts showed an El Niño warming of the equatorial Pacific Ocean waters commencing with the excitation of a Kelvin wave. The Niño-3.4 region acquired above-normal sea surface temperature anomalies (SSTAs) by 15 May. The warm SSTs reached a peak by around January 1998. The El Niño made its demise by June 1998. The life cycle of the entire SSTA shows remarkable agreement to the observed anomalies over the Pacific Ocean. The subsurface temperature anomalies exhibit eastward propagating subsurface warm and cold water that are in phase with the El Niño and the La Niña features at the surface. Phenomenologically, this study is quite successful in showing the following.

• Velocity potential anomalies at the 200-hPa level are good indicators for long-lasting dry spells. In particular the authors have remarkable success in predicting the long-lasting dry spell over Florida (which resulted in major fires over Florida during June 1998, some 14 months into the forecast) and over Indonesia (which resulted in major fires over Indonesia during September and October 1997). This was by far the most promising result of the coupled modeling study. This study also enumerates several areas of the climate of 1997–98 that were not reasonably simulated at the present resolution of the coupled model. The model does not exhibit very high skill in prediction of precipitation anomalies over the Asian–Australian monsoon world, which is most likely due to the resolution and organization of convection issues.

• A realistic picture is shown of the North American monsoon system (the Mexico–Arizona monsoon) with wet conditions along 110°W, dry conditions along 95°W, and wet conditions along 80°W during the summers of 1997 and 1998. Furthermore, the model successfully shows a stronger North American monsoon system during the post–El Niño year 1998 compared to the El Niño year 1997. This is in accordance with the climatological and observational findings.

• California rainfall during January and February 1998, arising from the eastward passage of disturbances from the Pacific Ocean, was successfully simulated, although the rainfall amounts at the model resolution were roughly one-third of the observed peak estimates.