Search Results
Abstract
It is shown that long-term trends in the meridional components of the trade wind circulations in both hemisphere over the Pacific are highly correlated with the precipitation falling in the intertropical convergence zone (ITCZ) over that region. The trends in the trade wind regime, on the one hand, seem to be caused by extratropical sea surface temperature (SST) anomalies in the North Pacific which influence atmospheric meridional temperature and pressure gradients. On the other hand, the release of latent heat in the ITCZ provides a self-enforcing feedback for the trade winds.
A study of the recurrence frequency of precipitation surges over the Line Islands suggests the presence of three epoch (1911–28, 1929–62, 1963 to present) during which the Hadley cell circulation and attendant meteorological and oceanographic features showed different characteristics.
Abstract
It is shown that long-term trends in the meridional components of the trade wind circulations in both hemisphere over the Pacific are highly correlated with the precipitation falling in the intertropical convergence zone (ITCZ) over that region. The trends in the trade wind regime, on the one hand, seem to be caused by extratropical sea surface temperature (SST) anomalies in the North Pacific which influence atmospheric meridional temperature and pressure gradients. On the other hand, the release of latent heat in the ITCZ provides a self-enforcing feedback for the trade winds.
A study of the recurrence frequency of precipitation surges over the Line Islands suggests the presence of three epoch (1911–28, 1929–62, 1963 to present) during which the Hadley cell circulation and attendant meteorological and oceanographic features showed different characteristics.
Abstract
A review is given of mathematical expressions for total and mean specific kinetic energies in the longitudinal, time, and mixed longitudinal-time domains. These coordinate domains differ from those defined earlier by Oort. Mathematical developments are extended into the vertical coordinate domain. A new symbolism is introduced for describing mean and eddy motions.
Abstract
A review is given of mathematical expressions for total and mean specific kinetic energies in the longitudinal, time, and mixed longitudinal-time domains. These coordinate domains differ from those defined earlier by Oort. Mathematical developments are extended into the vertical coordinate domain. A new symbolism is introduced for describing mean and eddy motions.
Abstract
Cases of explosive cyclogenesis (“bombs”) were identified over the central and eastern United States and were compared with nonexplosive cyclone development in the same region. The tendency equations for vorticity and geopotential thickness, and a modified divergence equation were used to find signatures of distinction between these two types of cyclogenesis, whose recognition might improve forecasting skills.
Bombs tend to show a marked decrease of vorticity with height during their incipient and explosive stages, whereas regular cyclones reveal only weak vertical vorticity gradients in the troposphere. The low-tropospheric spin-up in bombs precedes significantly that in the upper troposphere. Preexisting low-tropospheric vorticity maxima are associated with low-level jet streaks.
Whereas regular cyclones possess an ill-defined level of nondivergence (i.e., a broad region between 800 and 400 mb, of divergence values close to zero), incipient bombs have a well-marked zero-divergence level near 500 mb, associated with a sharp maximum of rising motions.
We observed a marked increase in large-scale latent beat release between the incipient and the explosive phases of bomb development. The convective component of latent beating shows a distinct maximum in the incipient phase of bombs and a decrease which continues through the explosive and mature phases. Regular cyclones show much less heating and much less change between the phases of development than is observed with the bombs. Static stability reveals little change as the bomb grows and mature. There also is little difference between the stability associated with bombs and regular cyclones.
Abstract
Cases of explosive cyclogenesis (“bombs”) were identified over the central and eastern United States and were compared with nonexplosive cyclone development in the same region. The tendency equations for vorticity and geopotential thickness, and a modified divergence equation were used to find signatures of distinction between these two types of cyclogenesis, whose recognition might improve forecasting skills.
Bombs tend to show a marked decrease of vorticity with height during their incipient and explosive stages, whereas regular cyclones reveal only weak vertical vorticity gradients in the troposphere. The low-tropospheric spin-up in bombs precedes significantly that in the upper troposphere. Preexisting low-tropospheric vorticity maxima are associated with low-level jet streaks.
Whereas regular cyclones possess an ill-defined level of nondivergence (i.e., a broad region between 800 and 400 mb, of divergence values close to zero), incipient bombs have a well-marked zero-divergence level near 500 mb, associated with a sharp maximum of rising motions.
We observed a marked increase in large-scale latent beat release between the incipient and the explosive phases of bomb development. The convective component of latent beating shows a distinct maximum in the incipient phase of bombs and a decrease which continues through the explosive and mature phases. Regular cyclones show much less heating and much less change between the phases of development than is observed with the bombs. Static stability reveals little change as the bomb grows and mature. There also is little difference between the stability associated with bombs and regular cyclones.
Abstract
Detailed analyses are presented of the temperature and pressure fields of the planetary boundary layer (PBL) and their seasonal variability over the western United States and over the Plateau of Tibet (Qinghai-Xizang Plateau). Over the United States these analyses rely on 850 mb data, augmented by surface data. Over Tibet the 600 mb surface adequately describes the PBL.
The effects of a “plateau monsoon” appear prominently during winter and summer over both plateaus. Together with continental monsoon effects they help to shape prominent circulation features, such as the low-level jet stream (LLJ) over Texas and Oklahoma. The complex, seasonal characteristics of precipitation regimes over the North American continent can be explained, to a large extent, by considering these monsoonal changes in the PBL, especially over the mountains.
Abstract
Detailed analyses are presented of the temperature and pressure fields of the planetary boundary layer (PBL) and their seasonal variability over the western United States and over the Plateau of Tibet (Qinghai-Xizang Plateau). Over the United States these analyses rely on 850 mb data, augmented by surface data. Over Tibet the 600 mb surface adequately describes the PBL.
The effects of a “plateau monsoon” appear prominently during winter and summer over both plateaus. Together with continental monsoon effects they help to shape prominent circulation features, such as the low-level jet stream (LLJ) over Texas and Oklahoma. The complex, seasonal characteristics of precipitation regimes over the North American continent can be explained, to a large extent, by considering these monsoonal changes in the PBL, especially over the mountains.
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
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.
