D. Oosterhof

Passive tracer transport relevant to the TRACE A experiment

This paper explores some of the mechanisms governing the accumulation of passive tracers over the tropical southern Atlantic Ocean during the northern hemisphere fall season. There has been a pioneering observation regarding ozone maxima over the South Atlantic during austral spring. The understanding of the formation of this maxima has been the prime motivation for this study. Using a global model as a frame of reference, we have carried out three kinds of experiments during the period of the Transport and Atmospheric Chemistry Near the Equator-Atlantic (TRACE A) project of 1992. The first of these is a simple advection of total ozone (a passive tracer) in time using the Florida State University global spectral model. Integration over the period of roughly 1 week showed that the model quite closely replicates the behavior of the observed total ozone from the total ozone mapping spectrometer (TOMS). This includes many of the changes in the features of total ozone over the tropical and subtropical region of the southern Atlantic Ocean. These studies suggest a correlation of 0.8 between the observed ozone over this region and ozone modeled from “dynamics alone,” i.e., without recourse to any photochemistry. The second series of experiments invoke sustained sources of a tracer over the biomass burn region of Africa and Brazil. Furthermore, sustained sources were also introduced in the active frontal “descending air” region of the southern hemisphere and over the Asian monsoons east-west circulation. These experiments strongly suggest that air motions help to accumulate tracer elements over the tropical southern Atlantic Ocean. A third series of experiments address what may be required to improve the deficiencies of the vertical stratification of ozone predicted by the model over the flight region of the tropical southern Atlantic during TRACE A. Here we use the global model to optimally derive plausible accumulation of burn elements over the fire count regions of Brazil and Africa to provide passive tracer advections to closely match what was observed from reconnaissance aircraft-based measurements of ozone over the tropical southern Atlantic Ocean.

Coupled Atmosphere-Ocean Modeling of the El Nino of 1997-98

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 Nino 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 Nino warming of the equatorial Pacific Ocean waters commencing with the excitation of a Kelvin wave. The Nino-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 Nino made its demise by June 1998. The life cycle of the entire SS...

Predictability of low frequency modes

SummaryIn this paper we propose a procedure for the extended integration of low frequency modes of the time scale of 30 to 50 days. A major limitation of the extended integrations arise from a contamination of low frequency modes as a result of energy exchanges from the higher frequency modes. In this study we show an example on the prediction of low frequency mode to almost a month which is roughly 3 weeks beyond the conventional predictability. This was accomplished by filtering the higher frequency modes from the initial state. The initial state included a time mean state and a low frequency mode. The sea surface temperature anomalies on this time scale and the annual cycle were also prescribed.The specific experiment relates to the occurrence of a dry and a wet spell in the monsoon region. The meridional passage of an anticyclonic circulation anomaly over the lower troposphere and the eastward passage of a negative velocity potential anomaly over the upper levels of the Indian monsoon, on this time scale, are reasonably predicted. The aforementioned experiment was carried out with the 1979 data sets of the global experiment. A second example during an anomalous southward propagation of the low frequency waves over the Indian monsoon region during 1984 was also reasonably predicted by this model. Suggestions for further experimentation on the predictability of low frequency modes are proposed.

Physical Initialization and Hurricane Ensemble Forecasts

Ensemble forecasting of hurricane tracks is an emerging area in numerical weather prediction. In this paper, the spread of the ensemble of forecast tracks from a family of different First Global GARP (Global Atmospheric Research Program) Experiment analyses is illustrated. All forecasts start at the same date and use the same global prediction model. The authors have examined ensemble forecasts for three different hurricanes/typhoons of the year 1979. The authors have used eight different initial analyses to examine the spread of ensemble forecasts through 6 days from the initial state. A total of 16 forecasts were made, of which 8 of them invoked physical initialization. Physical initialization is a procedure for improving the initial rainfall rates consistent with satellite/rain gauge based measures of rainfall. The main results of this study are that useful track forecasts are obtained from physical initialization, which is shown to suppress the spread of the ensemble of track forecasts. The spread of the tracks is quite large if the rain rates are not initialized. The major issue here is how one could make use of this information on ensemble forecasts for providing guidance. Toward that end, a statistical framework that makes use of the spread of forecast tracks to provide such guidance is presented.

Mesoscale signatures within the tropics generated by physical initialization

Abstract This paper presents some recent results on physical initialization from the use of a very high resolution global model. Fundamentally this procedure improves the model-based initial rainfall, surface fluxes, and diagnostic cloud amount. Physical initialization is a useful procedure for the nowcasting of rainfall. Correlation between model-based initialized rain and satellite/rain gauge-based rain over the Tropics (for 6-h averages and averaged over transform grid squares) is of the order 0.85. This compares with a correlation of around 0.3 for models that do not include physical initialization. The day 1 tropical rainfall forecast skill is also relatively high for the physically initialized experiments; the correlation is of the order 0.55. It should be noted that the lifetime of mesoconvective systems is approximately 1 day, whereas more organized tropical disturbances may last substantially longer. A major portion of the tropical rainfall is associated with these short-lived systems, hence the ...

Sensitivity of Tropical Storm Forecast to Radiative Destabilization

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.

Sensitivity of hurricane intensity forecasts to physical initialization

SummaryIntensity forecasts of a hurricane are shown to be quite sensitive to the initial meso-convective scale precipitation distributions. These are included within the data assimilation using a physical initialization that was developed at Florida State University. We show a case study of a hurricane forecast where the inclusion of the ‘observed’ precipitation did provide reasonable intensity forecasts. Further experimentation with the inclusion or exclusion of individual meso-convective rainfall elements, around and over the storm, shows that the intensity forecasts were quite sensitive to these initial rainfall distributions. The exclusion of initial rain in the inner rain area of a hurricane leads to a much reduced intensity forecast, whereas that impact is less if the rainfall of an outer rain band was initially excluded.Intensity forecasts of hurricanes may be sensitive to a number of factors such as sea surface temperature anomalies, presence or absence of concentric eye walls, potential vorticity interactions in the upper troposphere and other environmental factors.This paper is a sequel to a recent study, Krishnamurti et al., 1997, on the prediction of hurricane OPAL of 1995 that was a category III storm over the Gulf of Mexico. In that study we showed successful forecasts of the storm intensity from the inclusion of observed rainfall distributions within physical initialization. In that paper we examined the issues of diabatic potential vorticity and the angular momentum in order to diagnose the storm intensity. All of the terms of the complete Ertel potential vorticity equation were evaluated and it was concluded that the diabatic contributions to the potential vorticity were quite important for the diagnosis of the storms intensity. The present paper addresses some sensitivity issues related to the individual mesoconvective precipitating elements.

Recurvature dynamics of a typhoon

SummaryIn this paper we present some recent work on typhoon prediction with a high resolution global model. The emphasis of this paper is on typhoon recurvature. Here we include examples of successful typhoon recurvature track forecasts made from a very high resolution global spectral model. The main objective of this study however is to go beyond the forecasts, i.e. to interrogate the history tapes and to diagnose residue-free budgets of the divergence and vorticity. The premise of this paper is that the recurvature of the typhoons depends on both the usual advection of vorticity by the layer mean winds and the advection of divergence in the outflow layers of the storm.The region immediately outside the heavy rain area of the storm experiences large values of divergent outflows which contribute a significant advection of divergence. Through the Dines compensation this region must, in consort, experience an enhancement of low level convergence and of deep convection, thus contributing to the storm motion. We distinguish two facets of storm motion and recurvature, one based on the conventional steering that invokes the advection of vorticity by a vertical integrated flow, the other is the generation mechanism proposed here. During recurvature the storm appears to move in a direction which is influenced by the rotational and the divergent flow dynamics. Increased vertical resolution in the outflow layer is shown to resolve stronger amplitudes in the outflow layer divergence and thus to contribute to improved forecasts of recurvature. A number of processes seem to simultaneously evolve, these include the strong advection of divergence part of the wind, enhancement of cumulus convection over this region, an enhancement of lower tropospheric convergence, generation of vorticity of the lower troposphere and the attendant recurvature.

Partitioning of the Seasonal Simulation of a Monsoon Climate

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.

Hurricane Forecasts in the FSU Models

A brief account of our studies on the hurricane forecast problem is presented here. This covers recent prediction results from the Florida State University (FSU) regional and global numerical weather prediction models. The re-gions covered are the Indian and the Pacific Oceans. The life cycle of the onset vortex (a hurricane) of the summer monsoon, typhoons over the western Pacific Ocean and tropical cyclones over the Bay of Bengal (Andhra Pradesh and the Bangladesh storms) are covered here. The essential elements in the storm formaton are the strong horizontal shear in the cyclogenetic areas, a lack of vertical shear and warn sea surface temperatures. The storm motion has a steering component largely described by the advection of vorticity by a vertically averaged layer mean wind, the recurvature of a storm appears to invoke physical processes via the advection of divergence by the divergent part of the wind especially in the outflow layers of the storm. Very high resolution global models seem to be able to handle the motion and structure during the entire life of typhoons quite reasonably. The scope for better diagnosis of the storms life cycle appears very promising in view of the realistic simulation of the life cycle.