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- Author or Editor: T. N. Krishnamurti x
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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
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
A dynamic relaxation technique is examined to update a spectral model. The technique consists of constraining selected time dependent model variables towards their predetermined space–time estimates, while the remaining variables evolve unconstrained. The scheme involves gradual assimilation of data and thus is essentially free from data insertion shocks generally associated with data assimilation schemes. The scheme can also be used to update the model variables consistent with the observed estimates of diabatic forcings. The spectral formulation is particularly suited to relax the current estimates of model variables towards their observed estimates scale-by-scale.
The scheme has been applied to initialize model variables by relaxing vorticity, divergence and total mass (surface pressure) fields through one to three observation periods using an 11-layer model with T-42 spectral resolution. In addition, the moisture field and diabatic heating rates have been relaxed consistent with the observed estimates of precipitation rates. The explicit two-day Newtonian relaxation of the streamfunction, velocity potential (consistent with rainfall estimates) and the surface pressure and an implicit treatment of the humidity (again consistent with rainfall estimates) results in a realistic initialization. Tropical rainfall, humidity analysis and the divergence field show considerable consistency and improvement. The study addresses the model initialization by this scheme and its impact on medium range forecasts using FGGE IIIb data. The reduction of the spinup time is accomplished by this procedure at the initial time. Globally averaged evaporation and precipitation exhibit an equilibration by this procedure.
A major result of this study is the ability to initialize an observed rainfall field from the use of a reverse Kuo algorithm, the Newtonian relaxation and the overall physical initialization within this model.
Abstract
A dynamic relaxation technique is examined to update a spectral model. The technique consists of constraining selected time dependent model variables towards their predetermined space–time estimates, while the remaining variables evolve unconstrained. The scheme involves gradual assimilation of data and thus is essentially free from data insertion shocks generally associated with data assimilation schemes. The scheme can also be used to update the model variables consistent with the observed estimates of diabatic forcings. The spectral formulation is particularly suited to relax the current estimates of model variables towards their observed estimates scale-by-scale.
The scheme has been applied to initialize model variables by relaxing vorticity, divergence and total mass (surface pressure) fields through one to three observation periods using an 11-layer model with T-42 spectral resolution. In addition, the moisture field and diabatic heating rates have been relaxed consistent with the observed estimates of precipitation rates. The explicit two-day Newtonian relaxation of the streamfunction, velocity potential (consistent with rainfall estimates) and the surface pressure and an implicit treatment of the humidity (again consistent with rainfall estimates) results in a realistic initialization. Tropical rainfall, humidity analysis and the divergence field show considerable consistency and improvement. The study addresses the model initialization by this scheme and its impact on medium range forecasts using FGGE IIIb data. The reduction of the spinup time is accomplished by this procedure at the initial time. Globally averaged evaporation and precipitation exhibit an equilibration by this procedure.
A major result of this study is the ability to initialize an observed rainfall field from the use of a reverse Kuo algorithm, the Newtonian relaxation and the overall physical initialization within this model.
Abstract
In this paper we examine the evolution of the low-level flow over the Arabian Sea during the onset of the summer monsoon. A detailed examination of the onset vortex that forms over the Arabian Sea just prior to the commencement of heavy rains over central India is carried out. The unique aspect of this study is the use of data sets from the Global Atmospheric Research Program (GARP) Monsoon Experiment (MONEX) from a variety of observing platforms. These include winds from geostationary satellites, constant level balloons, dropwindsonde aircraft and an enhanced World Weather Watch network. The data sets were analyzed for a 46-day period from 16 May through 30 June 1979. A number of calculations were performed with this analysis. Of major interest is a finding that the kinetic energy of the zonal flow over the central Arabian Sea increases by an order of magnitude one week prior to the commencement of monsoon rain over central India. This study provides a MONEX time-averaged analysis for the low-level flow which is an update on the well-known Findlater analysis.
A number of calculations show that the horizontal shear of the monsoon current provides substantial energy during the evolution of the onset vortex. The flow satisfies the necessary condition for the existence of instability. Corresponding stability diagrams exhibit substantial growth rates around the period of the formation of the onset vortex. The scale of maximum growth rate is closely in correspondence with the scale of the onset vortex. Finally, the conversion from zonal to eddy kinetic energy is demonstrated via a simple prediction experiment with the conservation of absolute vorticity as a constraint. A reasonable simulation of the onset vortex is shown in this experiment.
Abstract
In this paper we examine the evolution of the low-level flow over the Arabian Sea during the onset of the summer monsoon. A detailed examination of the onset vortex that forms over the Arabian Sea just prior to the commencement of heavy rains over central India is carried out. The unique aspect of this study is the use of data sets from the Global Atmospheric Research Program (GARP) Monsoon Experiment (MONEX) from a variety of observing platforms. These include winds from geostationary satellites, constant level balloons, dropwindsonde aircraft and an enhanced World Weather Watch network. The data sets were analyzed for a 46-day period from 16 May through 30 June 1979. A number of calculations were performed with this analysis. Of major interest is a finding that the kinetic energy of the zonal flow over the central Arabian Sea increases by an order of magnitude one week prior to the commencement of monsoon rain over central India. This study provides a MONEX time-averaged analysis for the low-level flow which is an update on the well-known Findlater analysis.
A number of calculations show that the horizontal shear of the monsoon current provides substantial energy during the evolution of the onset vortex. The flow satisfies the necessary condition for the existence of instability. Corresponding stability diagrams exhibit substantial growth rates around the period of the formation of the onset vortex. The scale of maximum growth rate is closely in correspondence with the scale of the onset vortex. Finally, the conversion from zonal to eddy kinetic energy is demonstrated via a simple prediction experiment with the conservation of absolute vorticity as a constraint. A reasonable simulation of the onset vortex is shown in this experiment.
Abstract
In this paper we present many examples (based on 43 years of data) of a phenomenon of downstream amplification over the monsoonal belt. The specific finding here is the following sequence of events: 1) During northern summer pressure drops in the vicinity of the North Vietnam coast (near 20°N) as a typhoon or a tropical storm arrives; 2) during the ensuing week pressure rises over Indochina and Burma by some 5–7 mb; and 3) during the following week a monsoon disturbance forms near the northern part of the Bay of Bengal. On an x-t (or Hovmöller) diagram this sequence of low-high-low formation is similar to the downstream amplification phenomenon of the middle latitudes. The following are some interesting differences: over the middle latitudes the eastward propagating phase velocity is of the order of 10° longitude day−1, while the eastward propagating group velocity (the speed of propagation of the amplification) is around 30° longitude day−1. The tropical counterparts are westward propagating, and the phase and group velocity are, respectively, around 6° and 2° longitude day−1. In meteorological literature one frequently notes reference to in situ formation of monsoon depressions over the northern part of the Bay of Bengal. Our study illustrates the superposition of stationary long waves with progressive short waves, the latter arriving from the western Pacific. This result is contrary to this notion of in situ formation. In this paper we examine some aspects of this slowly westward propagating group velocity phenomenon.
Abstract
In this paper we present many examples (based on 43 years of data) of a phenomenon of downstream amplification over the monsoonal belt. The specific finding here is the following sequence of events: 1) During northern summer pressure drops in the vicinity of the North Vietnam coast (near 20°N) as a typhoon or a tropical storm arrives; 2) during the ensuing week pressure rises over Indochina and Burma by some 5–7 mb; and 3) during the following week a monsoon disturbance forms near the northern part of the Bay of Bengal. On an x-t (or Hovmöller) diagram this sequence of low-high-low formation is similar to the downstream amplification phenomenon of the middle latitudes. The following are some interesting differences: over the middle latitudes the eastward propagating phase velocity is of the order of 10° longitude day−1, while the eastward propagating group velocity (the speed of propagation of the amplification) is around 30° longitude day−1. The tropical counterparts are westward propagating, and the phase and group velocity are, respectively, around 6° and 2° longitude day−1. In meteorological literature one frequently notes reference to in situ formation of monsoon depressions over the northern part of the Bay of Bengal. Our study illustrates the superposition of stationary long waves with progressive short waves, the latter arriving from the western Pacific. This result is contrary to this notion of in situ formation. In this paper we examine some aspects of this slowly westward propagating group velocity phenomenon.
Abstract
This paper examines how combining training-set forecasts from two separate oceanic basins affects the resulting tropical cyclone track and intensity forecasts in a particular oceanic basin. Atlantic and eastern Pacific training sets for 2002 and 2003 are combined and used to forecast 2004 eastern Pacific tropical cyclones in a real-time setting. These experiments show that the addition of Atlantic training improves the 2004 eastern Pacific forecasts. Finally, a detailed study of training-set and real-time model biases is completed in an effort to determine why cross-oceanic training may have helped in this instance.
Abstract
This paper examines how combining training-set forecasts from two separate oceanic basins affects the resulting tropical cyclone track and intensity forecasts in a particular oceanic basin. Atlantic and eastern Pacific training sets for 2002 and 2003 are combined and used to forecast 2004 eastern Pacific tropical cyclones in a real-time setting. These experiments show that the addition of Atlantic training improves the 2004 eastern Pacific forecasts. Finally, a detailed study of training-set and real-time model biases is completed in an effort to determine why cross-oceanic training may have helped in this instance.
Abstract
This paper provides an understanding of essential differences between developing and nondeveloping African easterly waves, which was a major goal of NAMMA, NASA’s field program in the eastern Atlantic, which functioned as an extension of the African Monsoon Multidisciplinary Analysis (AMMA) program during 2006.
Three NAMMA waves are studied in detail using FNL analysis: NAMMA wave 2, which developed into Tropical Storm Debby; NAMMA wave 7, which developed into Hurricane Helene; and NAMMA wave 4, which did not develop within the NAMMA domain. Diagnostic calculations are performed on the analyzed fields using energy transformation equations and the isentropic potential vorticity equation.
The results show that the two developing waves possess clear and robust positive barotropic energy conversion in conjunction with positive diabatic heating that includes a singular burst of heating at a particular time in the wave’s history. This positive barotropic energy conversion is facilitated in waves that have a northeast–southwest tilt to the trough axis and a wind maximum to the west of this axis. The nondeveloping wave is found to have the same singular burst of diabatic heating at one point in its history, but development of the wave does not occur due to negative barotropic energy conversion. Such conversion is facilitated by a northwest–southeast tilt to the trough axis and a wind maximum to the east of this axis.
The conclusions about wave development and nondevelopment formulated in this research are viewed as important and significant, but they require additional testing with detailed observational- and numerical-based studies.
Abstract
This paper provides an understanding of essential differences between developing and nondeveloping African easterly waves, which was a major goal of NAMMA, NASA’s field program in the eastern Atlantic, which functioned as an extension of the African Monsoon Multidisciplinary Analysis (AMMA) program during 2006.
Three NAMMA waves are studied in detail using FNL analysis: NAMMA wave 2, which developed into Tropical Storm Debby; NAMMA wave 7, which developed into Hurricane Helene; and NAMMA wave 4, which did not develop within the NAMMA domain. Diagnostic calculations are performed on the analyzed fields using energy transformation equations and the isentropic potential vorticity equation.
The results show that the two developing waves possess clear and robust positive barotropic energy conversion in conjunction with positive diabatic heating that includes a singular burst of heating at a particular time in the wave’s history. This positive barotropic energy conversion is facilitated in waves that have a northeast–southwest tilt to the trough axis and a wind maximum to the west of this axis. The nondeveloping wave is found to have the same singular burst of diabatic heating at one point in its history, but development of the wave does not occur due to negative barotropic energy conversion. Such conversion is facilitated by a northwest–southeast tilt to the trough axis and a wind maximum to the east of this axis.
The conclusions about wave development and nondevelopment formulated in this research are viewed as important and significant, but they require additional testing with detailed observational- and numerical-based studies.
Abstract
This study provides a monsoonal link to the rapid Arctic ice melt. Each year the planetary-scale African–Asian monsoonal outflow near the tropopause carries a large anticyclonic gyre that has a longitudinal spread that occupies nearly half of the entire tropics. In recent years, the South Asian summer monsoon has experienced increased rainfall over northwestern India and Pakistan and it has also contributed to more intense local anticyclonic outflows from this region. The western lobes of these intense upper-high-pressure areas carry outflows with large heat fluxes from the monsoon belt toward central Asia and eventually to the region of the rapid ice melt of the Canadian Arctic. In this study this spectacular pathway has been defined from airflow trajectories, heat content, and heat flux anomalies. Most of these show slow increasing trends in the last 20 years. The monsoonal connection to the rapid Arctic ice melt is a new contribution of this study. This is shown from the passage of a vertical column of large positive values of the heat content anomaly that can be traced from the Asian monsoon belt to the Canadian Arctic. The heat flux along these episodic and intermittently active pathways is shown to be considerably larger than the atmospheric poleward flux across latitude circles and from the oceans. This study contrasts these thermodynamic wave trains (defining this pathway) for the more conventional dynamic wave trains.
Abstract
This study provides a monsoonal link to the rapid Arctic ice melt. Each year the planetary-scale African–Asian monsoonal outflow near the tropopause carries a large anticyclonic gyre that has a longitudinal spread that occupies nearly half of the entire tropics. In recent years, the South Asian summer monsoon has experienced increased rainfall over northwestern India and Pakistan and it has also contributed to more intense local anticyclonic outflows from this region. The western lobes of these intense upper-high-pressure areas carry outflows with large heat fluxes from the monsoon belt toward central Asia and eventually to the region of the rapid ice melt of the Canadian Arctic. In this study this spectacular pathway has been defined from airflow trajectories, heat content, and heat flux anomalies. Most of these show slow increasing trends in the last 20 years. The monsoonal connection to the rapid Arctic ice melt is a new contribution of this study. This is shown from the passage of a vertical column of large positive values of the heat content anomaly that can be traced from the Asian monsoon belt to the Canadian Arctic. The heat flux along these episodic and intermittently active pathways is shown to be considerably larger than the atmospheric poleward flux across latitude circles and from the oceans. This study contrasts these thermodynamic wave trains (defining this pathway) for the more conventional dynamic wave trains.
Abstract
Improved seasonal prediction of sea surface temperature (SST) anomalies over the global oceans is the theme of this paper. Using 13 state-of-the-art coupled global atmosphere–ocean models and 13 yr of seasonal forecasts, the performance of individual models, the ensemble mean, the bias-removed ensemble mean, and the Florida State University (FSU) superensemble are compared. A total of 23 400 seasonal forecasts based on 1-month lead times were available for this study. Evaluation metrics include both deterministic and probabilistic skill measures, such as verification of anomalies based on model and observed climatology, time series of specific climate indices, standard deterministic ensemble mean scores including anomaly correlations, root-mean-square (RMS) errors, and probabilistic skill measures such as equitable threat scores for seasonal SST forecasts. This study also illustrates the Niño-3.4 SST forecast skill for the equatorial Pacific Ocean and for the dipole index for the Indian Ocean. The relative skills of total SST fields and of the SST anomalies from the 13 coupled atmosphere–ocean models are presented. Comparisons of superensemble-based seasonal forecasts with recent studies on SST anomaly forecasts are also shown.
Overall it is found that the multimodel superensemble forecasts are characterized by considerable RMS error reductions and increased accuracy in the spatial distribution of SST. Superensemble SST skill also persists for El Niño and La Niña forecasts since the large comparative skill of the superensemble is retained across such years. Real-time forecasts of seasonal sea surface temperature anomalies appear to be possible.
Abstract
Improved seasonal prediction of sea surface temperature (SST) anomalies over the global oceans is the theme of this paper. Using 13 state-of-the-art coupled global atmosphere–ocean models and 13 yr of seasonal forecasts, the performance of individual models, the ensemble mean, the bias-removed ensemble mean, and the Florida State University (FSU) superensemble are compared. A total of 23 400 seasonal forecasts based on 1-month lead times were available for this study. Evaluation metrics include both deterministic and probabilistic skill measures, such as verification of anomalies based on model and observed climatology, time series of specific climate indices, standard deterministic ensemble mean scores including anomaly correlations, root-mean-square (RMS) errors, and probabilistic skill measures such as equitable threat scores for seasonal SST forecasts. This study also illustrates the Niño-3.4 SST forecast skill for the equatorial Pacific Ocean and for the dipole index for the Indian Ocean. The relative skills of total SST fields and of the SST anomalies from the 13 coupled atmosphere–ocean models are presented. Comparisons of superensemble-based seasonal forecasts with recent studies on SST anomaly forecasts are also shown.
Overall it is found that the multimodel superensemble forecasts are characterized by considerable RMS error reductions and increased accuracy in the spatial distribution of SST. Superensemble SST skill also persists for El Niño and La Niña forecasts since the large comparative skill of the superensemble is retained across such years. Real-time forecasts of seasonal sea surface temperature anomalies appear to be possible.
Abstract
Using currently available operational forecast datasets on the tracks and intensities of tropical cyclones over the Pacific Ocean for the years 1998, 1999, and 2000 a multimodel superensemble has been constructed following the earlier work of the authors on the Atlantic hurricanes. The models included here include forecasts from the European Centre for Medium-Range Weather Forecasts (ECMWF), the National Centers for Environmental Prediction/Environmental Modeling Center [NCEP/EMC, the Aviation (AVN) and Medium-Range Forecast (MRF) Models], the U.S. Navy [Naval Operational Global Atmospheric Prediction System, (NOGAPS)], the U.K. Met Office (UKMO), and the Japan Meteorological Agency (JMA). The superensemble methodology includes a collective bias estimation from a training phase in which a multiple-regression-based least squares minimization principle for the model forecasts with respect to the observed measures is employed. This is quite different from a simple bias correction, whereby a mean value is simply shifted. These bias estimates are described by separate weights at every 12 h during the forecasts for each of the member models. Superensemble forecasts for track and intensity are then constructed up to 144 h into the future using these weights. Some 100 past forecasts of tropical cyclone days are used to define the training phase for each basin. The findings presented herein show a marked improvement for the tracks and intensities of forecasts from the proposed multimodel superensemble as compared to the forecasts from member models and the ensemble mean. This note includes detailed statistics on the Pacific Ocean tropical cyclone forecasts for the years 1998, 1999, and 2000.
Abstract
Using currently available operational forecast datasets on the tracks and intensities of tropical cyclones over the Pacific Ocean for the years 1998, 1999, and 2000 a multimodel superensemble has been constructed following the earlier work of the authors on the Atlantic hurricanes. The models included here include forecasts from the European Centre for Medium-Range Weather Forecasts (ECMWF), the National Centers for Environmental Prediction/Environmental Modeling Center [NCEP/EMC, the Aviation (AVN) and Medium-Range Forecast (MRF) Models], the U.S. Navy [Naval Operational Global Atmospheric Prediction System, (NOGAPS)], the U.K. Met Office (UKMO), and the Japan Meteorological Agency (JMA). The superensemble methodology includes a collective bias estimation from a training phase in which a multiple-regression-based least squares minimization principle for the model forecasts with respect to the observed measures is employed. This is quite different from a simple bias correction, whereby a mean value is simply shifted. These bias estimates are described by separate weights at every 12 h during the forecasts for each of the member models. Superensemble forecasts for track and intensity are then constructed up to 144 h into the future using these weights. Some 100 past forecasts of tropical cyclone days are used to define the training phase for each basin. The findings presented herein show a marked improvement for the tracks and intensities of forecasts from the proposed multimodel superensemble as compared to the forecasts from member models and the ensemble mean. This note includes detailed statistics on the Pacific Ocean tropical cyclone forecasts for the years 1998, 1999, and 2000.
Abstract
This paper addresses a procedure to extract error estimates for the physical and dynamical components of a forecast model. This is a two-step process in which contributions to the forecast tendencies from individual terms of the model equations are first determined using an elaborate bookkeeping of the forecast. The second step regresses these estimates of tendencies from individual terms of the model equations against the observed total tendencies. This process is executed separately for the entire horizontal and vertical transform grid points of a global model. The summary of results based on the corrections to the physics and dynamics provided by the regression coefficients highlights the component errors of the model arising from its formulation. This study provides information on geographical and vertical distribution of forecast errors contributed by features such as nonlinear advective dynamics, the rest of the dynamics, deep cumulus convection, large-scale condensation physics, radiative processes, and the rest of physics. Several future possibilities from this work are also discussed in this paper.
Abstract
This paper addresses a procedure to extract error estimates for the physical and dynamical components of a forecast model. This is a two-step process in which contributions to the forecast tendencies from individual terms of the model equations are first determined using an elaborate bookkeeping of the forecast. The second step regresses these estimates of tendencies from individual terms of the model equations against the observed total tendencies. This process is executed separately for the entire horizontal and vertical transform grid points of a global model. The summary of results based on the corrections to the physics and dynamics provided by the regression coefficients highlights the component errors of the model arising from its formulation. This study provides information on geographical and vertical distribution of forecast errors contributed by features such as nonlinear advective dynamics, the rest of the dynamics, deep cumulus convection, large-scale condensation physics, radiative processes, and the rest of physics. Several future possibilities from this work are also discussed in this paper.
