Search Results
You are looking at 61 - 70 of 112 items for
- Author or Editor: T. N. Krishnamurti x
- Refine by Access: All Content x
Abstract
The main theme of this paper is on the intensity forecast of a hurricane (Opal) and interpretation of factors contributing toward it. The paper illustrates the results of assimilation and prediction for Hurricane Opal of 1995 from a very high-resolution global model. The assimilation makes use of a detailed physical initialization that vastly improves the nowcasting skill of rainfall and the model-based outgoing longwave radiation. Some of the interesting aspects of Hurricane Opal’s history occurred between 1200 UTC 1 October 1995 and 1200 UTC 5 October 1995. During this period the storm made landfall over the Florida panhandle. The storm reached maximum wind speed of over 130 kt on 4 October 1995. The intensity issue of Opal has drawn much attention. Issues such as the potential vorticity impact from a middle-latitude trough, the angular momentum of the lower-tropospheric inflow layer, the warm ocean temperature anomalies of the northern Gulf of Mexico, and the possible role of mesoconvective concentric eyewall are discussed in this paper.
The main finding of this study is that a reduction of the gradient of angular momentum occurs above the regions of maximum convective heating. This contributes toward stronger cyclonic spinup of parcels that enter the storm environment from the middle latitudes. Another major contributor is the import of angular momentum along the lower-tropospheric inflow channels of the storm. These channels were found to be open, that is, uncontaminated with a plethora of deep convection and heavy rain. This permitted the high angular momentum to advance toward the storm’s interior thus contributing to its intensification.
Abstract
The main theme of this paper is on the intensity forecast of a hurricane (Opal) and interpretation of factors contributing toward it. The paper illustrates the results of assimilation and prediction for Hurricane Opal of 1995 from a very high-resolution global model. The assimilation makes use of a detailed physical initialization that vastly improves the nowcasting skill of rainfall and the model-based outgoing longwave radiation. Some of the interesting aspects of Hurricane Opal’s history occurred between 1200 UTC 1 October 1995 and 1200 UTC 5 October 1995. During this period the storm made landfall over the Florida panhandle. The storm reached maximum wind speed of over 130 kt on 4 October 1995. The intensity issue of Opal has drawn much attention. Issues such as the potential vorticity impact from a middle-latitude trough, the angular momentum of the lower-tropospheric inflow layer, the warm ocean temperature anomalies of the northern Gulf of Mexico, and the possible role of mesoconvective concentric eyewall are discussed in this paper.
The main finding of this study is that a reduction of the gradient of angular momentum occurs above the regions of maximum convective heating. This contributes toward stronger cyclonic spinup of parcels that enter the storm environment from the middle latitudes. Another major contributor is the import of angular momentum along the lower-tropospheric inflow channels of the storm. These channels were found to be open, that is, uncontaminated with a plethora of deep convection and heavy rain. This permitted the high angular momentum to advance toward the storm’s interior thus contributing to its intensification.
Abstract
The year 2009 was a major drought year for the Indian summer monsoon with a seasonal deficit of rainfall by 21.6%. Standard oceanic predictors such as ENSO and the Indian Ocean dipole are not consistent for these dry spells. There are a host of other parameters such as the Himalayan ice cover, the Eurasian snow cover, the passage of intraseasonal waves, and even accumulated effects of Asian pollution that have been considered for analysis of the dry spells of the monsoon. This paper presents another factor, the western Asian desert air incursions toward central India, and emphasizes the formation of a blocking high over western Asia as an important feature for these dry spells. The blocking high advects descending very dry air toward central India, portrayed using swaths of three-dimensional trajectories. This is a robust indicator for dry spells of the monsoon during the last several decades. This dry air above the 3-km level over central India strongly inhibits the vertical growth of deep convection. Some of the interesting antecedents of the formation of the blocking high include an eastward and somewhat northward extension of the ITCZ over North Africa, a stronger than normal local Hadley cell over North Africa, a strong subtropical jet stream over the southern Mediterranean, and strong conversions of anticyclonic shear vorticity to anticyclonic curvature vorticity. The dynamical antecedents of the aforementioned scenario in this study are related to many aspects of North African weather features. They are portrayed using both reanalysis datasets and ensemble modeling using a suite of coupled atmosphere–ocean models.
Abstract
The year 2009 was a major drought year for the Indian summer monsoon with a seasonal deficit of rainfall by 21.6%. Standard oceanic predictors such as ENSO and the Indian Ocean dipole are not consistent for these dry spells. There are a host of other parameters such as the Himalayan ice cover, the Eurasian snow cover, the passage of intraseasonal waves, and even accumulated effects of Asian pollution that have been considered for analysis of the dry spells of the monsoon. This paper presents another factor, the western Asian desert air incursions toward central India, and emphasizes the formation of a blocking high over western Asia as an important feature for these dry spells. The blocking high advects descending very dry air toward central India, portrayed using swaths of three-dimensional trajectories. This is a robust indicator for dry spells of the monsoon during the last several decades. This dry air above the 3-km level over central India strongly inhibits the vertical growth of deep convection. Some of the interesting antecedents of the formation of the blocking high include an eastward and somewhat northward extension of the ITCZ over North Africa, a stronger than normal local Hadley cell over North Africa, a strong subtropical jet stream over the southern Mediterranean, and strong conversions of anticyclonic shear vorticity to anticyclonic curvature vorticity. The dynamical antecedents of the aforementioned scenario in this study are related to many aspects of North African weather features. They are portrayed using both reanalysis datasets and ensemble modeling using a suite of coupled atmosphere–ocean models.
Abstract
In this paper we have examined the evolution of a number of parameters we believe were important for our understanding of the drought over India during the summer of 1987. The list of parameters includes monthly means or anomalies of the following fields: sea surface temperatures, divergent circulations, outgoing longwave radiation, streamfunction of the lower and upper troposphere, and monthly precipitation (expressed as a percentage departure from a long-term mean). The El Niño related warm sea surface temperature anomaly and a weaker warm sea surface temperature anomaly over the equatorial Indian Ocean provide sustained convection, as reflected by the negative values of the outgoing longwave radiation. With the seasonal heating, a pronounced planetary-scale divergent circulation evolved with a center along the western Pacific Ocean. The monsoonal divergent circulation merged with that related to the El Niño, maintaining most of the heavy rainfall activity between the equatorial Pacific Ocean and east Asia. Persistent convective activity continued south of India during the entire monsoon season. Strong Hadley type overturnings with rising motions over these warm SST anomaly regions and descent roughly near 20° to 25°S was evident as early as April 1987. The subtropical high pressure areas near 20° to 25°S showed stronger than normal circulations. This was revealed by the presence of a counterclockwise streamfunction anomaly at 850 mb during April 1987. With the seasonal heating, this anomaly moved northwards and was located over the Arabian Sea and India. This countermonsoon circulation anomaly at the low levels was associated with a weaker than normal Somali jet and Arabian Sea circulation throughout this summer. The monsoon remained active along northeast India, Bangladesh, northern lndochina, and central China during the summer monsoon season. This was related to the eastward shift of the divergent circulation. An eastward shift of the upper tropospheric anticyclone bell near 25° to 30°N resulted in the continued presence of a westerly wind anomaly north of India. The westerly winds brought in very dry air over the tropical upper troposphere. The dry air penetrated eastwards to central Uttar Pradesh and this seemed to have a major role in inhibiting organized deep convection over most of central, northern and western parts of the Indian subcontinent. The westward extension of the planetary-scale divergent circulation over North and South Africa and the continued drought over the regions are also briefly addressed.
Abstract
In this paper we have examined the evolution of a number of parameters we believe were important for our understanding of the drought over India during the summer of 1987. The list of parameters includes monthly means or anomalies of the following fields: sea surface temperatures, divergent circulations, outgoing longwave radiation, streamfunction of the lower and upper troposphere, and monthly precipitation (expressed as a percentage departure from a long-term mean). The El Niño related warm sea surface temperature anomaly and a weaker warm sea surface temperature anomaly over the equatorial Indian Ocean provide sustained convection, as reflected by the negative values of the outgoing longwave radiation. With the seasonal heating, a pronounced planetary-scale divergent circulation evolved with a center along the western Pacific Ocean. The monsoonal divergent circulation merged with that related to the El Niño, maintaining most of the heavy rainfall activity between the equatorial Pacific Ocean and east Asia. Persistent convective activity continued south of India during the entire monsoon season. Strong Hadley type overturnings with rising motions over these warm SST anomaly regions and descent roughly near 20° to 25°S was evident as early as April 1987. The subtropical high pressure areas near 20° to 25°S showed stronger than normal circulations. This was revealed by the presence of a counterclockwise streamfunction anomaly at 850 mb during April 1987. With the seasonal heating, this anomaly moved northwards and was located over the Arabian Sea and India. This countermonsoon circulation anomaly at the low levels was associated with a weaker than normal Somali jet and Arabian Sea circulation throughout this summer. The monsoon remained active along northeast India, Bangladesh, northern lndochina, and central China during the summer monsoon season. This was related to the eastward shift of the divergent circulation. An eastward shift of the upper tropospheric anticyclone bell near 25° to 30°N resulted in the continued presence of a westerly wind anomaly north of India. The westerly winds brought in very dry air over the tropical upper troposphere. The dry air penetrated eastwards to central Uttar Pradesh and this seemed to have a major role in inhibiting organized deep convection over most of central, northern and western parts of the Indian subcontinent. The westward extension of the planetary-scale divergent circulation over North and South Africa and the continued drought over the regions are also briefly addressed.
Abstract
Recently the National Aeronautics and Space Administration (NASA) Tropical Rainfall Measuring Mission (TRMM) project office made available a new product called the convective–stratiform heating (CSH). These are the datasets for vertical profiles of diabatic heating rates (the apparent heat source). These observed estimates of heating are obtained from the TRMM satellite’s microwave radiances and the precipitation radar. The importance of such datasets for defining the vertical distribution of heating was largely the initiative of Dr. W.-K. Tao from NASA’s Goddard Laboratory. The need to examine how well some of the current cumulus parameterization schemes perform toward describing the amplitude and the three-dimensional distributions of heating is addressed in this paper. Three versions of the Florida State University (FSU) global atmospheric model are run that utilize different versions of cumulus parameterization schemes; namely, modified Kuo parameterization, simple Arakawa–Schubert parameterization, and Zhang–McFarlane parameterization. The Kuo-type scheme used here relies on moisture convergence and tends to overestimate the rainfall generally compared to the TRMM estimates. The other schemes used here show only a slight overestimate of rain rates compared to TRMM; those invoke mass fluxes that are less stringent in this regard in defining cloud volumes. The mass flux schemes do carry out a total moisture budget for a vertical column model and include all components of the moisture budget and are not limited to the horizontal convergence of moisture. The authors carry out a numerical experimentation that includes over a hundred experiments from each of these models; these experiments differ only in their use of the cumulus parameterization. The rest of the model physics, resolution, and initial states are kept the same for each set of 117 forecasts. The strategy for this experimentation follows the authors’ previous studies with the FSU multimodel superensemble. This includes a 100-day training and a 17-day forecast phase, both of which include a large number of forecast experiments. The training phase provides a useful statistical database for tagging the systematic errors of the respective models. The forecast phase is designed to minimize the collective bias errors of these member models. In these forecasts the authors also include the ensemble mean and the multimodel superensemble. In this paper the authors examine model errors in their representations of the heating (amplitude, vertical level of maximum, and the geographical distributions). The main message of this study is that some cumulus parameterization schemes overestimate the amplitude of heating, whereas others carry lower values. The models also exhibit large errors in the placement of the vertical level of maximum heating. Some significant errors were also found in the geographical distributions of heating. The ensemble mean largely mimics the model features and also carries some large errors. The superensemble is more selective in reducing the three-dimensional collective bias errors of the models and provides the best short range forecasts, through hour 60, for the heating. This study shows that it is possible to diagnose some of the modeling errors in the heating for individual member models and that information can be important for correcting such features.
Abstract
Recently the National Aeronautics and Space Administration (NASA) Tropical Rainfall Measuring Mission (TRMM) project office made available a new product called the convective–stratiform heating (CSH). These are the datasets for vertical profiles of diabatic heating rates (the apparent heat source). These observed estimates of heating are obtained from the TRMM satellite’s microwave radiances and the precipitation radar. The importance of such datasets for defining the vertical distribution of heating was largely the initiative of Dr. W.-K. Tao from NASA’s Goddard Laboratory. The need to examine how well some of the current cumulus parameterization schemes perform toward describing the amplitude and the three-dimensional distributions of heating is addressed in this paper. Three versions of the Florida State University (FSU) global atmospheric model are run that utilize different versions of cumulus parameterization schemes; namely, modified Kuo parameterization, simple Arakawa–Schubert parameterization, and Zhang–McFarlane parameterization. The Kuo-type scheme used here relies on moisture convergence and tends to overestimate the rainfall generally compared to the TRMM estimates. The other schemes used here show only a slight overestimate of rain rates compared to TRMM; those invoke mass fluxes that are less stringent in this regard in defining cloud volumes. The mass flux schemes do carry out a total moisture budget for a vertical column model and include all components of the moisture budget and are not limited to the horizontal convergence of moisture. The authors carry out a numerical experimentation that includes over a hundred experiments from each of these models; these experiments differ only in their use of the cumulus parameterization. The rest of the model physics, resolution, and initial states are kept the same for each set of 117 forecasts. The strategy for this experimentation follows the authors’ previous studies with the FSU multimodel superensemble. This includes a 100-day training and a 17-day forecast phase, both of which include a large number of forecast experiments. The training phase provides a useful statistical database for tagging the systematic errors of the respective models. The forecast phase is designed to minimize the collective bias errors of these member models. In these forecasts the authors also include the ensemble mean and the multimodel superensemble. In this paper the authors examine model errors in their representations of the heating (amplitude, vertical level of maximum, and the geographical distributions). The main message of this study is that some cumulus parameterization schemes overestimate the amplitude of heating, whereas others carry lower values. The models also exhibit large errors in the placement of the vertical level of maximum heating. Some significant errors were also found in the geographical distributions of heating. The ensemble mean largely mimics the model features and also carries some large errors. The superensemble is more selective in reducing the three-dimensional collective bias errors of the models and provides the best short range forecasts, through hour 60, for the heating. This study shows that it is possible to diagnose some of the modeling errors in the heating for individual member models and that information can be important for correcting such features.
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
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
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.
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.
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.
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.
Abstract
Using The Florida State University Global Spectral Model, hydrological budgets are calculated over the Amazon River basin for the boreal summer of 1979 with and without a complex biosphere model (BATS) coupled to the atmospheric model. Substantially increased precipitation and latent heat fluxes over the Amazon are noted for the BATS case, along with better maintenance of low-level flow patterns. Partitioning of the rainfall and latent heat flux into detailed component terms from BATS reveals evidence of “moisture recycling,” particularly in relation to the intercepted rainfall. Monthly variations in the component terms for precipitation, latent heat flux, and upper soil moisture are described. A total runoff efficiency of 75% is simulated by the model, while the surface runoff efficiency is about 30%. Model performance in the locality of two intensive field study areas (Pará and Rondônia) of the Large-Scale Biosphere–Atmosphere Experiment in Amazonia has been examined via time series from the two models and observed data. The 850-hPa temperatures and wind speeds are both overestimated by the models. However, use of BATS has reduced the temperature bias by about 30%. Most significantly, the phase of the wind speed variations over Rondônia is maintained in agreement with the observations throughout the seasonal forecast.
Abstract
Using The Florida State University Global Spectral Model, hydrological budgets are calculated over the Amazon River basin for the boreal summer of 1979 with and without a complex biosphere model (BATS) coupled to the atmospheric model. Substantially increased precipitation and latent heat fluxes over the Amazon are noted for the BATS case, along with better maintenance of low-level flow patterns. Partitioning of the rainfall and latent heat flux into detailed component terms from BATS reveals evidence of “moisture recycling,” particularly in relation to the intercepted rainfall. Monthly variations in the component terms for precipitation, latent heat flux, and upper soil moisture are described. A total runoff efficiency of 75% is simulated by the model, while the surface runoff efficiency is about 30%. Model performance in the locality of two intensive field study areas (Pará and Rondônia) of the Large-Scale Biosphere–Atmosphere Experiment in Amazonia has been examined via time series from the two models and observed data. The 850-hPa temperatures and wind speeds are both overestimated by the models. However, use of BATS has reduced the temperature bias by about 30%. Most significantly, the phase of the wind speed variations over Rondônia is maintained in agreement with the observations throughout the seasonal forecast.
Abstract
The Tropical Rainfall Measuring Mission (TRMM) satellite supplemented with the Defense Meteorological Satellites Program (DMSP) microwave dataset provides accurate rain-rate estimates. Furthermore, infrared radiances from the geostationary satellites provide the possibility for mapping the diurnal change of tropical rainfall. Modeling of the phase and amplitude of the tropical rainfall is the theme of this paper. The present study utilizes a suite of global multimodels that are identical in all respects except for their cumulus parameterization algorithms. Six different cumulus parameterizations are tested in this study. These include the Florida State University (FSU) Modified Kuo Scheme (KUO), Goddard Space Flight Center (GSFC) Relaxed Arakawa–Schubert Scheme (RAS1), Naval Research Laboratory–Navy Operational Global Atmospheric Prediction System (NRL–NOGAPS) Relaxed Arakawa–Schubert Scheme (RAS2), NCEP Simplified Arakawa–Schubert Scheme (SAS), NCAR Zhang–McFarlane Scheme (ZM), and NRL–NOGAPS Emanuel Scheme (ECS). The authors carried out nearly 600 experiments with these six versions of the T170 Florida State University global spectral model. These are 5-day NWP experiments where the diurnal change datasets were archived at 3-hourly intervals. This study includes the estimation of skills of the phase and amplitudes of the diurnal rain using these member models, their ensemble mean, a multimodel superensemble, and those from a single unified model. Test results are presented for the global tropics and for some specific regions where the member models show difficulty in predicting the diurnal change of rainfall. The main contribution is the considerable improvement of the modeling of diurnal rain by deploying a multimodel superensemble and by constructing a single unified model. The authors also present a comparison of these findings on the modeling of diurnal rain from another suite of multimodels that utilized different versions of cloud radiation algorithms (instead of different cumulus parameterization schemes) toward defining the suite of multimodels. The principal result is that the superensemble does provide a future forecast for the total daily rain and for the diurnal change of rain through day 5 that is superior to forecasts provided by the best model. The training of the superensemble with good observed estimates of rain, such as those from TRMM, is necessary for such forecasts.
Abstract
The Tropical Rainfall Measuring Mission (TRMM) satellite supplemented with the Defense Meteorological Satellites Program (DMSP) microwave dataset provides accurate rain-rate estimates. Furthermore, infrared radiances from the geostationary satellites provide the possibility for mapping the diurnal change of tropical rainfall. Modeling of the phase and amplitude of the tropical rainfall is the theme of this paper. The present study utilizes a suite of global multimodels that are identical in all respects except for their cumulus parameterization algorithms. Six different cumulus parameterizations are tested in this study. These include the Florida State University (FSU) Modified Kuo Scheme (KUO), Goddard Space Flight Center (GSFC) Relaxed Arakawa–Schubert Scheme (RAS1), Naval Research Laboratory–Navy Operational Global Atmospheric Prediction System (NRL–NOGAPS) Relaxed Arakawa–Schubert Scheme (RAS2), NCEP Simplified Arakawa–Schubert Scheme (SAS), NCAR Zhang–McFarlane Scheme (ZM), and NRL–NOGAPS Emanuel Scheme (ECS). The authors carried out nearly 600 experiments with these six versions of the T170 Florida State University global spectral model. These are 5-day NWP experiments where the diurnal change datasets were archived at 3-hourly intervals. This study includes the estimation of skills of the phase and amplitudes of the diurnal rain using these member models, their ensemble mean, a multimodel superensemble, and those from a single unified model. Test results are presented for the global tropics and for some specific regions where the member models show difficulty in predicting the diurnal change of rainfall. The main contribution is the considerable improvement of the modeling of diurnal rain by deploying a multimodel superensemble and by constructing a single unified model. The authors also present a comparison of these findings on the modeling of diurnal rain from another suite of multimodels that utilized different versions of cloud radiation algorithms (instead of different cumulus parameterization schemes) toward defining the suite of multimodels. The principal result is that the superensemble does provide a future forecast for the total daily rain and for the diurnal change of rain through day 5 that is superior to forecasts provided by the best model. The training of the superensemble with good observed estimates of rain, such as those from TRMM, is necessary for such forecasts.
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 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.
