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- Author or Editor: T. N. Krishnamurti x
- Journal of Applied Meteorology and Climatology x
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Abstract
A quasi-Lagrangian advective scheme for numerical integration of primitive equations is proposed. The advective scheme is built on a successive approximation procedure where the Fjørtoft-type of quasi-Lagrangian advection form an initial guess. The numerical program is tested by constructing some simple analytic problems containing the non-linear advective terms. It is shown that the scheme is also capable of producing very reasonable numerical forecasts for simple initial conditions for problems of mixed and filtered wave motions.
Abstract
A quasi-Lagrangian advective scheme for numerical integration of primitive equations is proposed. The advective scheme is built on a successive approximation procedure where the Fjørtoft-type of quasi-Lagrangian advection form an initial guess. The numerical program is tested by constructing some simple analytic problems containing the non-linear advective terms. It is shown that the scheme is also capable of producing very reasonable numerical forecasts for simple initial conditions for problems of mixed and filtered wave motions.
Abstract
Conventional and commerical aircraft wind observations over the global tropics have been used to prepare daily charts of the field of motion at the 200-mb level for June, July and August 1967. Computations describe the climatology of the field of motion (streamlies and isotachs) and the divergence and vorticity distributions. A number of dynamical computations are made utilizing these data. These include
1) A comparison of the strength of the rotational part of the wind with the total wind.
2) A description of the strength and geometry of the Hadley and Walker circulations.
3) An analysis of the ultralong waves (zonal wavenumbers 1 and 2). The waves, which are quasi-stationary, are shown to carry a large proportion of the total variance of the motion field and to have a very pronounced southwest to northeast tilt. The amplitude and motion of transient ultralong waves, wavenumbers 1 and 2, are studied on polar diagrams. It is shown that these waves are intermittent and rapidly moving.
4) An analysis of the amplitude and motion of shorter waves (wavenumbers 7 and 8 of the zonal spectra of the meridional wind), which are intermittent, have been studied on x−t diagrams. Their amplitudes and speeds of westward propagation vary respectively between 0 and 6 m sec−1 and 4° to 12° longitude day−1.
5) A computation of the energy exchange between the zonal flow and eddies. Quasi-periodic oscillations in time are shown with a period of roughly 3.5 days. The phenomenon is suggested to be a manifestation of pure barotropic type instability of the tropical upper level flows.
6) Computations of meridional fluxes of momentum and kinetic energy. These are shown to be related to the shapes of the quasi-stationery ultralong waves and possibly also to certain shapes of westward moving shorter waves over the Northern Hemisphere tropics.
Abstract
Conventional and commerical aircraft wind observations over the global tropics have been used to prepare daily charts of the field of motion at the 200-mb level for June, July and August 1967. Computations describe the climatology of the field of motion (streamlies and isotachs) and the divergence and vorticity distributions. A number of dynamical computations are made utilizing these data. These include
1) A comparison of the strength of the rotational part of the wind with the total wind.
2) A description of the strength and geometry of the Hadley and Walker circulations.
3) An analysis of the ultralong waves (zonal wavenumbers 1 and 2). The waves, which are quasi-stationary, are shown to carry a large proportion of the total variance of the motion field and to have a very pronounced southwest to northeast tilt. The amplitude and motion of transient ultralong waves, wavenumbers 1 and 2, are studied on polar diagrams. It is shown that these waves are intermittent and rapidly moving.
4) An analysis of the amplitude and motion of shorter waves (wavenumbers 7 and 8 of the zonal spectra of the meridional wind), which are intermittent, have been studied on x−t diagrams. Their amplitudes and speeds of westward propagation vary respectively between 0 and 6 m sec−1 and 4° to 12° longitude day−1.
5) A computation of the energy exchange between the zonal flow and eddies. Quasi-periodic oscillations in time are shown with a period of roughly 3.5 days. The phenomenon is suggested to be a manifestation of pure barotropic type instability of the tropical upper level flows.
6) Computations of meridional fluxes of momentum and kinetic energy. These are shown to be related to the shapes of the quasi-stationery ultralong waves and possibly also to certain shapes of westward moving shorter waves over the Northern Hemisphere tropics.
Abstract
In large-scale tropical flows an estimate of the percentage area covered by active convective cloud elements may be made according to certain theoretical formulations involving parameterization of convective-scale motions. Such an example is Kuo's model cloud which is based on replacing the ambient tropical atmosphere with a moist adiabatic cloud element over a portion of a synoptic-scale grid network. Quantities such as friction layer mass convergence, moisture convergence, and other relevant parameters like net moisture convergence over vertical columns extending over the depth of the troposphere are determined to evaluate the percentage area occupied by convective clouds.
An application to a case study of an easterly wave below a cold low is made. Some radar pictures are presented for comparison purposes.
Abstract
In large-scale tropical flows an estimate of the percentage area covered by active convective cloud elements may be made according to certain theoretical formulations involving parameterization of convective-scale motions. Such an example is Kuo's model cloud which is based on replacing the ambient tropical atmosphere with a moist adiabatic cloud element over a portion of a synoptic-scale grid network. Quantities such as friction layer mass convergence, moisture convergence, and other relevant parameters like net moisture convergence over vertical columns extending over the depth of the troposphere are determined to evaluate the percentage area occupied by convective clouds.
An application to a case study of an easterly wave below a cold low is made. Some radar pictures are presented for comparison purposes.
Abstract
A method for analysis of the horizontal and vertical distributions of the moisture field utilizing satellite, upper air and surface data is proposed in this paper. A brief overview of the microwave sensors on board Nimbus 5 and 6 is also presented. A technique is provided utilizing the radiosonde data sets to calibrate the satellite field of total precipitable water. Next, the calibrated satellite-derived field is utilized along with ship and coastal reports of moisture, and a vertical structure function to generate vertical distribution of moisture and thus provide a mapping of specific humidity at several levels in the troposphere. Utilizing these procedures, analyses for several case studies were performed. The resultant maps show detailed distribution of specific humidity along with some interesting climatological features. A reasonable acceptance of the available aerological data sets by the analysis scheme is demonstrated.
Abstract
A method for analysis of the horizontal and vertical distributions of the moisture field utilizing satellite, upper air and surface data is proposed in this paper. A brief overview of the microwave sensors on board Nimbus 5 and 6 is also presented. A technique is provided utilizing the radiosonde data sets to calibrate the satellite field of total precipitable water. Next, the calibrated satellite-derived field is utilized along with ship and coastal reports of moisture, and a vertical structure function to generate vertical distribution of moisture and thus provide a mapping of specific humidity at several levels in the troposphere. Utilizing these procedures, analyses for several case studies were performed. The resultant maps show detailed distribution of specific humidity along with some interesting climatological features. A reasonable acceptance of the available aerological data sets by the analysis scheme is demonstrated.
Abstract
Convective adjustment procedures remove conditional instability in a vertical sounding preserving the total energy (latent, internal and potential). It is shown that this procedure has very undesirable properties in the very first time step in numerical weather prediction, e.g., large-scale temperature and moisture distributions are greatly altered. If, on the other hand, convective adjustment is carried out on a mesoscale, as is the case for the Kuo parameterization procedure, then the large-scale conditional instability is preserved and the changes in initial data are small in the first (and subsequent) time steps. The latter procedure is used to evaluate convective precipitation in the vicinity of a squall line. A detailed heating function is designed for use in the ω equation and primitive equation prediction models. This function permits stable or unstable heating to occur in a given region and also allows for the simultaneous occurrence of both kinds of heating in the same region. Precipitation rate estimates show major improvement over earlier versions which followed only stable heating.
Abstract
Convective adjustment procedures remove conditional instability in a vertical sounding preserving the total energy (latent, internal and potential). It is shown that this procedure has very undesirable properties in the very first time step in numerical weather prediction, e.g., large-scale temperature and moisture distributions are greatly altered. If, on the other hand, convective adjustment is carried out on a mesoscale, as is the case for the Kuo parameterization procedure, then the large-scale conditional instability is preserved and the changes in initial data are small in the first (and subsequent) time steps. The latter procedure is used to evaluate convective precipitation in the vicinity of a squall line. A detailed heating function is designed for use in the ω equation and primitive equation prediction models. This function permits stable or unstable heating to occur in a given region and also allows for the simultaneous occurrence of both kinds of heating in the same region. Precipitation rate estimates show major improvement over earlier versions which followed only stable heating.
Abstract
Vertical motions in the tropics are computed for an easterly wave perturbation associated with an upper tropospheric cold core low. The winds are deduced from the pressure field by solving a five level non-geostrophic model on the IBM 7094.
The streamlines exhibit the well known properties of vortex and col points and agree quite well with the observed wind observations at the various levels. Vertical motions show the dynamical and thermodynamical interaction of the lower and upper level phenomena. Typical magnitudes of the vertical motion are around 1/10 cm sec−1. Differential vertical advection and the Laplacian of thermal advection by the rotational component of the wind contribute significantly to these organized vertical motions.
Abstract
Vertical motions in the tropics are computed for an easterly wave perturbation associated with an upper tropospheric cold core low. The winds are deduced from the pressure field by solving a five level non-geostrophic model on the IBM 7094.
The streamlines exhibit the well known properties of vortex and col points and agree quite well with the observed wind observations at the various levels. Vertical motions show the dynamical and thermodynamical interaction of the lower and upper level phenomena. Typical magnitudes of the vertical motion are around 1/10 cm sec−1. Differential vertical advection and the Laplacian of thermal advection by the rotational component of the wind contribute significantly to these organized vertical motions.
Abstract
A surface-based rainfall monitoring network reveals that summer rainfall over Taiwan exhibits a prominent diurnal variation. In this study, an attempt has been made to detect the diurnal variability of Taiwan rainfall using observations from the Tropical Rainfall Measuring Mission (TRMM) satellite. The results show that the diurnal patterns of Taiwan rainfall can be detected with TRMM Microwave Imager (TMI) observations using a satellite observation period of 36 or more days, and detected signals match reasonably with those using continuous surface observations. However, sometimes, because of the unfavorable combination of satellite sampling and the occurrence of some transient regimes in local rainfall, there is a possibility of misinterpreting the diurnal cycle. The TRMM precipitation radar sensor also reveals a diurnal cycle of convective and stratiform rainfall. The convective activity increases during the late afternoon over Taiwan, which may be the effect of convection forced by localized mass convergence caused by the sea breeze. It is of interest that TMI data indicate a significant increase of rainfall over orographic regions during the same time.
Abstract
A surface-based rainfall monitoring network reveals that summer rainfall over Taiwan exhibits a prominent diurnal variation. In this study, an attempt has been made to detect the diurnal variability of Taiwan rainfall using observations from the Tropical Rainfall Measuring Mission (TRMM) satellite. The results show that the diurnal patterns of Taiwan rainfall can be detected with TRMM Microwave Imager (TMI) observations using a satellite observation period of 36 or more days, and detected signals match reasonably with those using continuous surface observations. However, sometimes, because of the unfavorable combination of satellite sampling and the occurrence of some transient regimes in local rainfall, there is a possibility of misinterpreting the diurnal cycle. The TRMM precipitation radar sensor also reveals a diurnal cycle of convective and stratiform rainfall. The convective activity increases during the late afternoon over Taiwan, which may be the effect of convection forced by localized mass convergence caused by the sea breeze. It is of interest that TMI data indicate a significant increase of rainfall over orographic regions during the same time.
Abstract
A series of observing system simulation experiments (OSSES) was conducted to assess the potential impact of the Laser Atmospheric Wind Sounder (LAWS) instrument on a 5-day forecast using the Florida State University (FSU) primitive equation multilevel spectral global circulation model. This proposed Earth Observing System satelliteborne instrument is a CO2 Doppler lidar wind sounding system. The instrument's requirement for usable measurements is that clouds or high concentrations of tropospheric aerosols must exist within the sample volume.
Two different orbits, a 55° inclined and a 98° sun synchronous, were examined by adding simulated LAWS wind profiles into a global four-dimensional data assimilation system and comparing the analyses and forecasts to a control experiment. Also, two different sets of simulations were examined for the 55° inclined orbit. The first set followed the assumption of other previous Doppler lidar wind sounding OSSES; a global concentration of aerosols exists such that observations will be usable at each pulse location. The second set of simulations incorporated the effects of subvisual cirrus, aerosols, molecular attenuation, and sampling-scale turbulence. All simulations of LAWS wind observations are degraded when the lidar pulse encounters the earth's topography and when the downward-integrated cloud amount reaches a critical threshold.
The four-dimensional data assimilation system consists of a multivariate optimum interpolation analysis and a nonlinear normal-mode initialization using the aforementioned FSU global circulation model. In this set of assimilations only upper-air data was used with the exclusion of temperature sounder data, which may reduce the overall skill of the forecasts in the largely data-void Southern Hemisphere, which has been seen in temperature sounder OSSES.
The inclusion of LAWS wind observations exhibits an overall improvement of the forecast skill for this study. The greatest increase in skill is in the Southern Hemisphere, as can be seen in both the motion and mass fields. The 98° sun-synchronous orbit resolved the polar meteorological features much better than the 55° inclined orbit. Otherwise, the two different orbits were very similar, with the 55° inclined showing a better analysis of the tropics.
A comparison of the two different sets of simulations for the 55° inclined orbit is also revealing. The incorporation of physical effects into the LAWS simulations shows a reduced forecast skill as compared with the other set of simulated LAWS observations. However, a global increase in forecast skill can still be seen over that of the control experiment. This is an attempt at creating more “realistic” simulations, and it is difficult to draw robust conclusions without completing a large number of forecasts, examining many different meteorological phenomena.
In the future, the atmospheric physical effects will be much better incorporated into the LAWS simulated observations. The analysis of Global Backscatter Experiment data will aid in a better definition of global concentrations of aerosols and will attempt to relate specific concentrations of aerosols to chemical composition and associated large-scale meteorological features.
Abstract
A series of observing system simulation experiments (OSSES) was conducted to assess the potential impact of the Laser Atmospheric Wind Sounder (LAWS) instrument on a 5-day forecast using the Florida State University (FSU) primitive equation multilevel spectral global circulation model. This proposed Earth Observing System satelliteborne instrument is a CO2 Doppler lidar wind sounding system. The instrument's requirement for usable measurements is that clouds or high concentrations of tropospheric aerosols must exist within the sample volume.
Two different orbits, a 55° inclined and a 98° sun synchronous, were examined by adding simulated LAWS wind profiles into a global four-dimensional data assimilation system and comparing the analyses and forecasts to a control experiment. Also, two different sets of simulations were examined for the 55° inclined orbit. The first set followed the assumption of other previous Doppler lidar wind sounding OSSES; a global concentration of aerosols exists such that observations will be usable at each pulse location. The second set of simulations incorporated the effects of subvisual cirrus, aerosols, molecular attenuation, and sampling-scale turbulence. All simulations of LAWS wind observations are degraded when the lidar pulse encounters the earth's topography and when the downward-integrated cloud amount reaches a critical threshold.
The four-dimensional data assimilation system consists of a multivariate optimum interpolation analysis and a nonlinear normal-mode initialization using the aforementioned FSU global circulation model. In this set of assimilations only upper-air data was used with the exclusion of temperature sounder data, which may reduce the overall skill of the forecasts in the largely data-void Southern Hemisphere, which has been seen in temperature sounder OSSES.
The inclusion of LAWS wind observations exhibits an overall improvement of the forecast skill for this study. The greatest increase in skill is in the Southern Hemisphere, as can be seen in both the motion and mass fields. The 98° sun-synchronous orbit resolved the polar meteorological features much better than the 55° inclined orbit. Otherwise, the two different orbits were very similar, with the 55° inclined showing a better analysis of the tropics.
A comparison of the two different sets of simulations for the 55° inclined orbit is also revealing. The incorporation of physical effects into the LAWS simulations shows a reduced forecast skill as compared with the other set of simulated LAWS observations. However, a global increase in forecast skill can still be seen over that of the control experiment. This is an attempt at creating more “realistic” simulations, and it is difficult to draw robust conclusions without completing a large number of forecasts, examining many different meteorological phenomena.
In the future, the atmospheric physical effects will be much better incorporated into the LAWS simulated observations. The analysis of Global Backscatter Experiment data will aid in a better definition of global concentrations of aerosols and will attempt to relate specific concentrations of aerosols to chemical composition and associated large-scale meteorological features.
Abstract
Mid-tropospheric cyclones are an important part of the tropical general circulation of the summer season. These are synoptic-scale disturbances which appear in the daily and monthly mean circulation maps with greatest intensity at levels near 500 mb. The structure and energetics of this type over southeast Asia are discussed in this paper. Interesting features include a warm anomaly above the cyclone and a cold anomaly below.
A five-level non-geostrophic balanced model is used in this study to obtain the distribution of vertical motion. The model includes a parameterization of cumulus-scale convection. In the middle levels, rising motions are found west of the cyclone and sinking motions to the east. This is primarily due to the thermal structure of the atmosphere and associated advection of colder air from the oceanic regions and warmer air from land areas. A marked diurnal change in the vertical velocity is noted in the computations; magnitudes are large at 0700 local time compared to 1900. This diurnal change is primarily due to changes in the wind direction and speed. The important result of this study is that both the cumulus- and synoptic-scale motions exhibit the following dual roles in the maintenance of this midlevel system:
1) Both scales contribute to a net warming of the air above the cyclone, diabatic warming by cumulus-scale motions and adiabatic warming by the descent of synoptic-scale motions.
2) The two scales oppose each other in the transformation eddy available potential energy into eddy kinetic energy. Cumulus-scale motions contribute to a net generation, while synoptic-scale motions transform kinetic energy into potential energy. This latter result is consistent with 1) because the areas of descent are somewhat closer to the warm temperature anomaly than are the areas of ascent.
Abstract
Mid-tropospheric cyclones are an important part of the tropical general circulation of the summer season. These are synoptic-scale disturbances which appear in the daily and monthly mean circulation maps with greatest intensity at levels near 500 mb. The structure and energetics of this type over southeast Asia are discussed in this paper. Interesting features include a warm anomaly above the cyclone and a cold anomaly below.
A five-level non-geostrophic balanced model is used in this study to obtain the distribution of vertical motion. The model includes a parameterization of cumulus-scale convection. In the middle levels, rising motions are found west of the cyclone and sinking motions to the east. This is primarily due to the thermal structure of the atmosphere and associated advection of colder air from the oceanic regions and warmer air from land areas. A marked diurnal change in the vertical velocity is noted in the computations; magnitudes are large at 0700 local time compared to 1900. This diurnal change is primarily due to changes in the wind direction and speed. The important result of this study is that both the cumulus- and synoptic-scale motions exhibit the following dual roles in the maintenance of this midlevel system:
1) Both scales contribute to a net warming of the air above the cyclone, diabatic warming by cumulus-scale motions and adiabatic warming by the descent of synoptic-scale motions.
2) The two scales oppose each other in the transformation eddy available potential energy into eddy kinetic energy. Cumulus-scale motions contribute to a net generation, while synoptic-scale motions transform kinetic energy into potential energy. This latter result is consistent with 1) because the areas of descent are somewhat closer to the warm temperature anomaly than are the areas of ascent.
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.
