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Abstract
The diurnal variation of 850 mb heights, the detailed distribution of which could be assessed by the inclusion of surface data, and of resultant winds over, and in the vicinity of, the Great Basin reveals clearly a plateau-wind circulation during summer. This circulation reverses between day and night and appears to include the low-level jet stream over Texas and Oklahoma, as well as the time of occurrence of thunderstorms. This plateau circulation system interacts with local mountain-valley breeze systems. The thickness of the daytime inflow and nighttime outflow layer over the plateau is approximately 2 km.
Abstract
The diurnal variation of 850 mb heights, the detailed distribution of which could be assessed by the inclusion of surface data, and of resultant winds over, and in the vicinity of, the Great Basin reveals clearly a plateau-wind circulation during summer. This circulation reverses between day and night and appears to include the low-level jet stream over Texas and Oklahoma, as well as the time of occurrence of thunderstorms. This plateau circulation system interacts with local mountain-valley breeze systems. The thickness of the daytime inflow and nighttime outflow layer over the plateau is approximately 2 km.
Abstract
The heating of the Plateau of Tibet (the Qinghai-Xizang Plateau) has been deemed to play an important role in the development and movement of the South Asian anticyclone which, to a great extent, affects the Indian monsoon development. In this paper the heating effects are estimated from synoptic data from that region for April 1979. During the transition season warming over the plateau interacts with advected synoptic systems and induces the South Asian high, normally located south of the Himalayas during that season, to develop strongly and shift northward.
Abstract
The heating of the Plateau of Tibet (the Qinghai-Xizang Plateau) has been deemed to play an important role in the development and movement of the South Asian anticyclone which, to a great extent, affects the Indian monsoon development. In this paper the heating effects are estimated from synoptic data from that region for April 1979. During the transition season warming over the plateau interacts with advected synoptic systems and induces the South Asian high, normally located south of the Himalayas during that season, to develop strongly and shift northward.
Abstract
Cloud-photogrammetric studies conducted from the ground reveal the existence of wave perturbations near tropopause level, of a wavelength the same order of magnitude as is experienced in clear-air turbulence. A case study of the CAT occurrence on 13 April 1962 reveals the importance of the confluence mechanism of two jet streams, and the turning of wind with height in CAT generation.
Abstract
Cloud-photogrammetric studies conducted from the ground reveal the existence of wave perturbations near tropopause level, of a wavelength the same order of magnitude as is experienced in clear-air turbulence. A case study of the CAT occurrence on 13 April 1962 reveals the importance of the confluence mechanism of two jet streams, and the turning of wind with height in CAT generation.
Abstract
Ultralong and long planetary waves are analyzed at the 500 mb level in terms of their amplitudes, phases and stationarity characteristics, the latter described in terms of a stationarity index SI n . This index consists of the ratio between planetary-wave amplitudes computed from time-averaged 500 mb height patterns, and the wave amplitudes computed on a daily basis and averaged for the same time interval irrespective of their phase angles. The index assumes the value 1 for completely stationary waves and 0 for randomly variable waves.
Mean 500 mb height and planetary wave characteristics have been developed by calendar day. using NMC data between the years 1946 and 1979. Various significant singularities (i.e., departures from a low-order harmonic seasonal trend) in planetary-wave behavior are described. An index, SI n (d), computed by calendar day, was developed as a measure of probability that a certain planetary wave with number n would achieve its long-term mean phase angle on a certain date. This index, too, shows significant patterns with latitude and season, suggesting the presence of relatively short periods within the seasonal cycles of planetary-wave behavior, during which these waves may be extra sensitive to anomalies in their forcing parameters.
It is suggested that perturbation statistics derived from numerical general circulation models should be compared with the results presented in this study to permit an improved judgement on the veracity of such models.
Abstract
Ultralong and long planetary waves are analyzed at the 500 mb level in terms of their amplitudes, phases and stationarity characteristics, the latter described in terms of a stationarity index SI n . This index consists of the ratio between planetary-wave amplitudes computed from time-averaged 500 mb height patterns, and the wave amplitudes computed on a daily basis and averaged for the same time interval irrespective of their phase angles. The index assumes the value 1 for completely stationary waves and 0 for randomly variable waves.
Mean 500 mb height and planetary wave characteristics have been developed by calendar day. using NMC data between the years 1946 and 1979. Various significant singularities (i.e., departures from a low-order harmonic seasonal trend) in planetary-wave behavior are described. An index, SI n (d), computed by calendar day, was developed as a measure of probability that a certain planetary wave with number n would achieve its long-term mean phase angle on a certain date. This index, too, shows significant patterns with latitude and season, suggesting the presence of relatively short periods within the seasonal cycles of planetary-wave behavior, during which these waves may be extra sensitive to anomalies in their forcing parameters.
It is suggested that perturbation statistics derived from numerical general circulation models should be compared with the results presented in this study to permit an improved judgement on the veracity of such models.
Abstract
A strong, persistent and significant oscillation of about 24-day periodicity is observed in hemispheric-scale energy parameters during the winter season. The characteristics of this cycle are defined using 9.5 years of daily NMC gridded height and temperature fields. Analysis of northward eddy heat flux strongly suggests that baroclinic processes are involved in this oscillation. The vacillation acts primarily on the planetary-wave scale and some of its synoptic characteristics are investigated. A decomposition of eddy available potential energy by zone wavenumber shows that the vacillation tends to favor different wavenumbers in different years. The concept of a vacillation in orientation of trough and ridge line tilt is not substantiated.
Abstract
A strong, persistent and significant oscillation of about 24-day periodicity is observed in hemispheric-scale energy parameters during the winter season. The characteristics of this cycle are defined using 9.5 years of daily NMC gridded height and temperature fields. Analysis of northward eddy heat flux strongly suggests that baroclinic processes are involved in this oscillation. The vacillation acts primarily on the planetary-wave scale and some of its synoptic characteristics are investigated. A decomposition of eddy available potential energy by zone wavenumber shows that the vacillation tends to favor different wavenumbers in different years. The concept of a vacillation in orientation of trough and ridge line tilt is not substantiated.
Abstract
Measurements of clear-air turbulence spectra conducted by a Canberra aircraft over Australia between July and October 1963 reveal the existence of a wavelength region from somewhat less than 1000 ft to 4000 ft, in which the atmosphere receives turbulent energy. It is suggested that this energy stems from gravitational shearing waves which break up into turbulent eddies below a critical wavelength. The energies of these turbulent eddies seem to he well represented by a proportionality to k −5/3, characteristic of the inertial subrange of turbulence.
Abstract
Measurements of clear-air turbulence spectra conducted by a Canberra aircraft over Australia between July and October 1963 reveal the existence of a wavelength region from somewhat less than 1000 ft to 4000 ft, in which the atmosphere receives turbulent energy. It is suggested that this energy stems from gravitational shearing waves which break up into turbulent eddies below a critical wavelength. The energies of these turbulent eddies seem to he well represented by a proportionality to k −5/3, characteristic of the inertial subrange of turbulence.
Abstract
Recent aircraft measurements of clear air turbulence over Australia have shown that the phenomenon of CAT in a thermally stable environment is associated with a breakdown of waves, presumably gravity waves or Helmholtz waves on a stable interface, into random turbulent eddies. The energy distribution in the wavelength range in which clear air turbulence is experienced seems to follow the “−5/3 law” postulated by Kolmogorov's similarity hypothesis. The “−5/3 law” seems to extend to much longer wavelengths than previously anticipated.
Combining these results of aircraft measurements with the theory on lee waves which has been derived by the use of perturbation equations, one finds that the energy involved in standing lee waves over mountains may “cascade” down from a wavelength range of approximately 10 km to a range near 100 m which then would be experienced as clear air turbulence, provided that the energy levels are high enough to cause any responses in an aircraft.
This physical model of turbulence being “fed” by mountain waves has been used in developing a forecasting scheme of CAT over mountains. Results of a preliminary, but very encouraging, study are reported in this paper.
Abstract
Recent aircraft measurements of clear air turbulence over Australia have shown that the phenomenon of CAT in a thermally stable environment is associated with a breakdown of waves, presumably gravity waves or Helmholtz waves on a stable interface, into random turbulent eddies. The energy distribution in the wavelength range in which clear air turbulence is experienced seems to follow the “−5/3 law” postulated by Kolmogorov's similarity hypothesis. The “−5/3 law” seems to extend to much longer wavelengths than previously anticipated.
Combining these results of aircraft measurements with the theory on lee waves which has been derived by the use of perturbation equations, one finds that the energy involved in standing lee waves over mountains may “cascade” down from a wavelength range of approximately 10 km to a range near 100 m which then would be experienced as clear air turbulence, provided that the energy levels are high enough to cause any responses in an aircraft.
This physical model of turbulence being “fed” by mountain waves has been used in developing a forecasting scheme of CAT over mountains. Results of a preliminary, but very encouraging, study are reported in this paper.
Abstract
Estimates of the time and space variability of the atmospheric heat source over Tibet are presented for the summer of 1979. These estimates rely on new data from the People's Republic of China allowing a better assessment of the surface heat fluxes, and on new satellite data from Nimbus-7 giving the radiation balance at the top of the atmosphere. Our estimates of the atmospheric heat source turned out to be considerably smaller than those provided earlier in the literature, mainly because of different assumptions of the drag coefficient. The atmospheric heat source over Tibet is mainly modulated by the release of latent heat. Over the southeastern and southwestern plateau regions the heat source appears to be in phase with the precipitation yield of the Indian summer monsoon, whereas central Tibet reveals an out-of-phase behavior. Over western Tibet there appears to be hardly any net import of moisture from outside the region, whereas the maintenance of the hydrological cycle over eastern Tibet requires moisture flux convergence from outside the region of up to 40% of the mean rainfall, in agreement with what is known about the surface hydrology of Tibet.
Abstract
Estimates of the time and space variability of the atmospheric heat source over Tibet are presented for the summer of 1979. These estimates rely on new data from the People's Republic of China allowing a better assessment of the surface heat fluxes, and on new satellite data from Nimbus-7 giving the radiation balance at the top of the atmosphere. Our estimates of the atmospheric heat source turned out to be considerably smaller than those provided earlier in the literature, mainly because of different assumptions of the drag coefficient. The atmospheric heat source over Tibet is mainly modulated by the release of latent heat. Over the southeastern and southwestern plateau regions the heat source appears to be in phase with the precipitation yield of the Indian summer monsoon, whereas central Tibet reveals an out-of-phase behavior. Over western Tibet there appears to be hardly any net import of moisture from outside the region, whereas the maintenance of the hydrological cycle over eastern Tibet requires moisture flux convergence from outside the region of up to 40% of the mean rainfall, in agreement with what is known about the surface hydrology of Tibet.
Abstract
The initialization of numerical prediction models usually requires the transformation of variables observed in a p-coordinate system into some other coordinate frame of reference (e.g., α-coordinates or Θ-coordinates). Such transformations require the application of interpolation or curve-fitting techniques. The present study demonstrates that the choice of an appropriate interpolation scheme can become a critical issue for the skill of a low-resolution prediction model. First we show that the interpolation scheme, when applied to more than one meteorological variable, should satisfy the balance requirements that exist between these variables. Not all of the currently used schemes meet this condition. Next we provide evidence indicating that interpolation schemes used to convert p-into α-coordinates, and then back into p-coordinates, do not necessarily replicate the original, observed field distributions of these meteorological variables. Such double transformations usually are required, because the numerical output in model coordinates has to be translated back to p-coordinates for verification of model results. Because of the limitations of certain interpolation procedures, even a correct model prediction may exhibit low predictive skill because of errors introduced in this final coordinate transformation process.
Abstract
The initialization of numerical prediction models usually requires the transformation of variables observed in a p-coordinate system into some other coordinate frame of reference (e.g., α-coordinates or Θ-coordinates). Such transformations require the application of interpolation or curve-fitting techniques. The present study demonstrates that the choice of an appropriate interpolation scheme can become a critical issue for the skill of a low-resolution prediction model. First we show that the interpolation scheme, when applied to more than one meteorological variable, should satisfy the balance requirements that exist between these variables. Not all of the currently used schemes meet this condition. Next we provide evidence indicating that interpolation schemes used to convert p-into α-coordinates, and then back into p-coordinates, do not necessarily replicate the original, observed field distributions of these meteorological variables. Such double transformations usually are required, because the numerical output in model coordinates has to be translated back to p-coordinates for verification of model results. Because of the limitations of certain interpolation procedures, even a correct model prediction may exhibit low predictive skill because of errors introduced in this final coordinate transformation process.
Abstract
An investigation of the transition between spring and summer seasons of the surface energy budget in the Gobi desert is presented. The motivation behind this study is to determine eventually the degree to which changes in a desert system can be monitored over a short-term climate time scale (decadel) by remote means. A seasonal transition is used to evaluate the control factors involved in a variational process. The measurements incorporated in the analysis were obtained in 1984 from a specialized surface energy budget monitoring system deployed at a site in the western Gobi desert, just north of the northeastern edge of the Tibet Plateau in western Gansu province, P.R.C. The data were collected during the spring and summer periods in 1984 by a joint team of United States and Chinese scientists.
Results of the analysis reveal an interesting feature of the seasonal transition which had not been expected of a midlatitude desert. That is, although radiative forcing at the surface is altered between spring and summer through the diurnal net radiation heating function, the total radiative energy integral available for heating is largely unchanged. In some sense, the partitioning of the radiative heat supply at the surface can be viewed as a principal ingredient in defining the seasonal cycle. In terms of the Gobi desert, it may well be the only important ingredient.
Both similarities and differences in the spring and summer surface energy budgets arise from differences imparted to the system by an increase in the summertime atmospheric moisture content. Changes in the near-surface mixing ratio are shown to alter the effectiveness of the desert surface in absorbing radiative energy and redistributing it to the lower atmosphere through sensible and latent heat exchange.
Abstract
An investigation of the transition between spring and summer seasons of the surface energy budget in the Gobi desert is presented. The motivation behind this study is to determine eventually the degree to which changes in a desert system can be monitored over a short-term climate time scale (decadel) by remote means. A seasonal transition is used to evaluate the control factors involved in a variational process. The measurements incorporated in the analysis were obtained in 1984 from a specialized surface energy budget monitoring system deployed at a site in the western Gobi desert, just north of the northeastern edge of the Tibet Plateau in western Gansu province, P.R.C. The data were collected during the spring and summer periods in 1984 by a joint team of United States and Chinese scientists.
Results of the analysis reveal an interesting feature of the seasonal transition which had not been expected of a midlatitude desert. That is, although radiative forcing at the surface is altered between spring and summer through the diurnal net radiation heating function, the total radiative energy integral available for heating is largely unchanged. In some sense, the partitioning of the radiative heat supply at the surface can be viewed as a principal ingredient in defining the seasonal cycle. In terms of the Gobi desert, it may well be the only important ingredient.
Both similarities and differences in the spring and summer surface energy budgets arise from differences imparted to the system by an increase in the summertime atmospheric moisture content. Changes in the near-surface mixing ratio are shown to alter the effectiveness of the desert surface in absorbing radiative energy and redistributing it to the lower atmosphere through sensible and latent heat exchange.
