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Abstract
An investigation of the structure and likely role of the Arabian heat low is presented in two parts. In the first paper the surface energy budget of the Arabian heat low is examined. The investigation focuses on a site within the interior of the Saudi Arabian Empty Quarter during June 1981. Automated surface stations are used to collect continuous measurements of radiative fluxes, state parameters, and the subsurface thermal profiles. These data are synthesized in order to estimate the radiation properties of the desert surface within the vortex of the Arabian heat low and to obtain an estimate of sensible heat exchange that would characterize the lower boundary of the heat low during the spring/summer transition season coinciding with the onset period of the Southwest Summer Monsoon.
Results of the analysis demonstrate how radiative exchange both controls the mean properties of the desert surface and responds to perturbations in the heat low environment. The foremost characteristic of surface energy exchange is the well-balanced diurnal regularity. It is shown how the radiation budget of the surface is modulated by basic difference in the shortwave (VIS) and new-infrared (NIR) solar spectrum. More than 2:1 differences are noted in the NIR and VIS surface albedos. Diurnal averages of the surface and parameters illustrate significant day-night differences associated with the diurnal pulsation of the heat low vortex. Day-night differences in surface temperature are extreme; close to 50°C. It is shown that the diurnal amplitude of surface skin temperature is poorly correlated with the bulk Richardson number, suggesting that surface heat exchange is largely controlled by direct radiative exchange through a modulating optical path rather than by heat diffusion. It is shown how the phase lag in subsurface heating imparts a skew in the diurnal sensible heat cycle. The amplitude of the sensible heating cycle is 220 W m−2 peaking approximately 40 minutes past local noon. In a daily averaged sense, subsurface heat storage is approximately zero—thus a first order approximation for the mean heat low at that time scale equates sensible heating to the negative value of net radiation. Finally it is shown how the surface energy budget responds to an intermittent intensification of the heat low that perturbs boundary layer moisture. In Part II, the results of this investigation are incorporated with other data sources in order to examine the bulk tropospheric heat exchange process within the overall heat low system.
Abstract
An investigation of the structure and likely role of the Arabian heat low is presented in two parts. In the first paper the surface energy budget of the Arabian heat low is examined. The investigation focuses on a site within the interior of the Saudi Arabian Empty Quarter during June 1981. Automated surface stations are used to collect continuous measurements of radiative fluxes, state parameters, and the subsurface thermal profiles. These data are synthesized in order to estimate the radiation properties of the desert surface within the vortex of the Arabian heat low and to obtain an estimate of sensible heat exchange that would characterize the lower boundary of the heat low during the spring/summer transition season coinciding with the onset period of the Southwest Summer Monsoon.
Results of the analysis demonstrate how radiative exchange both controls the mean properties of the desert surface and responds to perturbations in the heat low environment. The foremost characteristic of surface energy exchange is the well-balanced diurnal regularity. It is shown how the radiation budget of the surface is modulated by basic difference in the shortwave (VIS) and new-infrared (NIR) solar spectrum. More than 2:1 differences are noted in the NIR and VIS surface albedos. Diurnal averages of the surface and parameters illustrate significant day-night differences associated with the diurnal pulsation of the heat low vortex. Day-night differences in surface temperature are extreme; close to 50°C. It is shown that the diurnal amplitude of surface skin temperature is poorly correlated with the bulk Richardson number, suggesting that surface heat exchange is largely controlled by direct radiative exchange through a modulating optical path rather than by heat diffusion. It is shown how the phase lag in subsurface heating imparts a skew in the diurnal sensible heat cycle. The amplitude of the sensible heating cycle is 220 W m−2 peaking approximately 40 minutes past local noon. In a daily averaged sense, subsurface heat storage is approximately zero—thus a first order approximation for the mean heat low at that time scale equates sensible heating to the negative value of net radiation. Finally it is shown how the surface energy budget responds to an intermittent intensification of the heat low that perturbs boundary layer moisture. In Part II, the results of this investigation are incorporated with other data sources in order to examine the bulk tropospheric heat exchange process within the overall heat low system.
Abstract
An investigation of the Arabian heat low is carried out based on observations from various satellites, an experimental aircraft and a surface energy budget monitoring station. The observations suggest that during the spring period the Arabian heat low is nearly radiatively neutral and lacks the properties of an energy sink characteristic of conventional desert heat lows. Satellite derived top-of-atmosphere radiation budget analyses illustrate the high contrast properties of the radiative exchange fields over the southern Arabian Peninsula with respect to its surroundings. However, an examination of a four-month time series of daily averaged net radiative exchange over the Arabian Empty Quarter, derived from Nimbus-7 Earth Radiation Budget (ERB) measurements, indicates that the heat low region is in slight relative excess.
Combining these results with estimates of the surface energy budget inside the Arabian Empty Quarter (described in Part I), and previously estimated tropospheric radiative heating rate profiles, provide a closed set of flux terms used to evaluate the energy exchange process within the heat low region. A synthesis of these results indicates that the heat low is a total energy source region. A conceptual structure of the heat low is offered based on a three-layer stratification of the heating mechanisms. The possible role of the Arabian heat low in controlling thermodynamic conditions and forcing baroclinicity in the western Arabian Sea is discussed. It is concluded that the surplus energy properties of the heat low may serve as an important mechanism in controlling moisture transport into the southwest monsoon rainfall regions.
Abstract
An investigation of the Arabian heat low is carried out based on observations from various satellites, an experimental aircraft and a surface energy budget monitoring station. The observations suggest that during the spring period the Arabian heat low is nearly radiatively neutral and lacks the properties of an energy sink characteristic of conventional desert heat lows. Satellite derived top-of-atmosphere radiation budget analyses illustrate the high contrast properties of the radiative exchange fields over the southern Arabian Peninsula with respect to its surroundings. However, an examination of a four-month time series of daily averaged net radiative exchange over the Arabian Empty Quarter, derived from Nimbus-7 Earth Radiation Budget (ERB) measurements, indicates that the heat low region is in slight relative excess.
Combining these results with estimates of the surface energy budget inside the Arabian Empty Quarter (described in Part I), and previously estimated tropospheric radiative heating rate profiles, provide a closed set of flux terms used to evaluate the energy exchange process within the heat low region. A synthesis of these results indicates that the heat low is a total energy source region. A conceptual structure of the heat low is offered based on a three-layer stratification of the heating mechanisms. The possible role of the Arabian heat low in controlling thermodynamic conditions and forcing baroclinicity in the western Arabian Sea is discussed. It is concluded that the surplus energy properties of the heat low may serve as an important mechanism in controlling moisture transport into the southwest monsoon rainfall regions.
Abstract
The tropical radiation balance is investigated on an interannual time scale using a five-year(1979–83) dataset obtained from the Nimbus-7 Earth Radiation Budget (ERB) experiment. The study emphasizes the separate contributions to interannual fluctuations in the global radiation balance by the tropics and extratropics. An attempt is made to Identify source regions within the tropics that give rise to the fluctuations and to quantify the effect of the fluctuations on zonal heat transport.
Superimposed on the five-year global trend pattern of net radiation are large amplitude nonseasonal variations largely confined to tropical latitudes. The significant regions are the Southwest–East Asian (SW–EA) monsoon and two regions associated with the ascent and descent branches of the Pacific Walker Cell. A “cloud reciprocity index” is formulated in order to examine the degree to which extended cloud systems over the oceanic tropics can induce these interannual fluctuations in the radiation balance. The SW–EA monsoon and the eastern Pacific exhibit low-index patterns, suggesting that these are the two dominant sources of the anomalies.
The impact of the fluctuations is examined in terms of external entropy exchange (EEE). Paltridge's theory that climate fluctuations are controlled by a minimum EEE constraint is partially supported. The impact of tropical fluctuations on zonal heat transport is examined. The amplitudes in the year-to-year tropical transport residuals are found to be as high as 50% of, and generally out of phase with, the total global residual. The SW–EA monsoon and the eastern Pacific can explain a large portion of the total tropical residual during specific years.
Simultaneous and lagged spatial correlation analyses are used to determine the degree to which the radiative anomalies associated with the SW–EA monsoon region are coupled to other centers of variability. The simultaneous correlations with net radiation are dissimilar to those found with the albedo and outgoing longwave radiation, particularly in terms of seasonal forcing. The organization of lagged albedo anomaly correlation patterns suggest that predictive indicators of the SW–EA monsoon behavior may be found in the tropical ocean basins.
Abstract
The tropical radiation balance is investigated on an interannual time scale using a five-year(1979–83) dataset obtained from the Nimbus-7 Earth Radiation Budget (ERB) experiment. The study emphasizes the separate contributions to interannual fluctuations in the global radiation balance by the tropics and extratropics. An attempt is made to Identify source regions within the tropics that give rise to the fluctuations and to quantify the effect of the fluctuations on zonal heat transport.
Superimposed on the five-year global trend pattern of net radiation are large amplitude nonseasonal variations largely confined to tropical latitudes. The significant regions are the Southwest–East Asian (SW–EA) monsoon and two regions associated with the ascent and descent branches of the Pacific Walker Cell. A “cloud reciprocity index” is formulated in order to examine the degree to which extended cloud systems over the oceanic tropics can induce these interannual fluctuations in the radiation balance. The SW–EA monsoon and the eastern Pacific exhibit low-index patterns, suggesting that these are the two dominant sources of the anomalies.
The impact of the fluctuations is examined in terms of external entropy exchange (EEE). Paltridge's theory that climate fluctuations are controlled by a minimum EEE constraint is partially supported. The impact of tropical fluctuations on zonal heat transport is examined. The amplitudes in the year-to-year tropical transport residuals are found to be as high as 50% of, and generally out of phase with, the total global residual. The SW–EA monsoon and the eastern Pacific can explain a large portion of the total tropical residual during specific years.
Simultaneous and lagged spatial correlation analyses are used to determine the degree to which the radiative anomalies associated with the SW–EA monsoon region are coupled to other centers of variability. The simultaneous correlations with net radiation are dissimilar to those found with the albedo and outgoing longwave radiation, particularly in terms of seasonal forcing. The organization of lagged albedo anomaly correlation patterns suggest that predictive indicators of the SW–EA monsoon behavior may be found in the tropical ocean basins.
Abstract
GOES-8 thermal infrared split window measurements have been used with a simultaneous land surface temperature (LST)–spectral emissivity retrieval algorithm to examine the potential of a combined retrieval methodology cast into a variational solution for temperatures at multiple but short-term 6- to 24-h time intervals and emissivities at multiple spectral bands assumed to be invariant over the selected time intervals. Retrieved LST and emissivity quantities under differing atmospheric conditions over an annual cycle are validated and analyzed in regard to their underlying diurnal and seasonal variations over the Department of Energy’s Atmospheric Radiation Measurement–Cloud and Radiation Test Bed (ARM–CART) site in Kansas and Oklahoma.
It is shown that the accuracy of the retrieval algorithm depends primarily on GOES infrared channel detector noise and uncertainties in columnar water vapor path, in which retrieval accuracy increases as pathlength decreases. A detailed analysis is given of the characteristic temporal–spatial gradient structures of LSTs and emissivities over the ARM–CART domain at point to area space scales and diurnally to seasonally varying timescales. Emphasis is given to explaining the relationship of heterogeneous features in the retrievals in conjunction with physical attributes of the landscape, that is, ecotones and phenology, and the effects of prior cloudiness on subsequent LSTs.
Abstract
GOES-8 thermal infrared split window measurements have been used with a simultaneous land surface temperature (LST)–spectral emissivity retrieval algorithm to examine the potential of a combined retrieval methodology cast into a variational solution for temperatures at multiple but short-term 6- to 24-h time intervals and emissivities at multiple spectral bands assumed to be invariant over the selected time intervals. Retrieved LST and emissivity quantities under differing atmospheric conditions over an annual cycle are validated and analyzed in regard to their underlying diurnal and seasonal variations over the Department of Energy’s Atmospheric Radiation Measurement–Cloud and Radiation Test Bed (ARM–CART) site in Kansas and Oklahoma.
It is shown that the accuracy of the retrieval algorithm depends primarily on GOES infrared channel detector noise and uncertainties in columnar water vapor path, in which retrieval accuracy increases as pathlength decreases. A detailed analysis is given of the characteristic temporal–spatial gradient structures of LSTs and emissivities over the ARM–CART domain at point to area space scales and diurnally to seasonally varying timescales. Emphasis is given to explaining the relationship of heterogeneous features in the retrievals in conjunction with physical attributes of the landscape, that is, ecotones and phenology, and the effects of prior cloudiness on subsequent LSTs.
Abstract
A combined land surface temperature–emissivity retrieval algorithm is developed and tested for Geostationary Operational Environmental Satellite (GOES)-Imager and National Oceanic and Atmospheric Administration Advanced Very High Resolution Radiometer (AVHRR) split-window channels. By assuming that the spectral emissivities are constant over a short time period (12–24 h), two sets of split-window radiance measurements taken at two different times are used to retrieve two spectral emissivities and two land surface temperatures (LSTs) simultaneously. The algorithm employs an optimization scheme rather than a direct solver for a system of equations because of constraint requirements. The retrieved variables minimize the rms differences between measured satellite radiances and those predicted by a spectrally detailed radiative transfer model.
A GOES-8 version of the algorithm is validated with in situ radiometer measurements from the Department of Energy’s Atmospheric Radiation Measurement Program Cloud and Radiation Testbed (ARM CART) site. In addition, an AVHRR version is validated with in situ measurements from the First ISLSCP Field Experiment (FIFE) site, the Hydrological Atmospheric Pilot Experiment–Sahel (HAPEX–Sahel) site, and an LST validation site operated near Melbourne, Australia. The biases of the retrieved LSTs for the validation sites in the Australian, FIFE, and ARM CART study areas are approximately 0.08°, 1.7°, and 1.4°C, respectively, yielding an overall bias error of better than half the current expected accuracy limit of some ±3°C. The associated bias-adjusted rmse differences are approximately 0.78°, 4.8°, and 4.5°C, respectively, mostly driven by intercomparing in situ point measurements to area-integrated satellite pixel retrievals. The bias-adjusted rmse differences for HAPEX–Sahel are larger (5° and 11°C), resulting from incomplete characterization of site heterogeneity, insufficient radiosonde launch frequency, and poor data quality of the temperature–moisture soundings, rather than intrinsic algorithm problems. Notably, the averaged retrieved emissivities for the trouble-free sites are within the expected range of emissivities for vegetated surfaces.
The GOES-8 retrieved LSTs exhibit small amplitude, high-frequency noise, and a daily error cycle when compared to in situ measurements. The noise is attributed to random detector errors in the satellite observations for which the channel 4 noise-equivalent temperature difference is larger than that of channel 5. The systematic differences between validation measurements and retrievals are near zero during nighttime but exhibit a small semidiurnal oscillation during daytime. Notwithstanding a possible semidiurnal bias in the pyrgeometer validation measurements associated with imperfect solar dome heating corrections, plus unaccounted-for attenuation between the surface and pyrgeometer, the latter error cycle is attributed to a too-coarse sampling of the nonlinear diurnal evolution of the thermodynamic structure of the atmospheric boundary layer, particularly near the sunrise and sunset transition times. Thus, sounding frequency determines the error characteristics of the nonlinearly evolving split-window weighting functions.
Abstract
A combined land surface temperature–emissivity retrieval algorithm is developed and tested for Geostationary Operational Environmental Satellite (GOES)-Imager and National Oceanic and Atmospheric Administration Advanced Very High Resolution Radiometer (AVHRR) split-window channels. By assuming that the spectral emissivities are constant over a short time period (12–24 h), two sets of split-window radiance measurements taken at two different times are used to retrieve two spectral emissivities and two land surface temperatures (LSTs) simultaneously. The algorithm employs an optimization scheme rather than a direct solver for a system of equations because of constraint requirements. The retrieved variables minimize the rms differences between measured satellite radiances and those predicted by a spectrally detailed radiative transfer model.
A GOES-8 version of the algorithm is validated with in situ radiometer measurements from the Department of Energy’s Atmospheric Radiation Measurement Program Cloud and Radiation Testbed (ARM CART) site. In addition, an AVHRR version is validated with in situ measurements from the First ISLSCP Field Experiment (FIFE) site, the Hydrological Atmospheric Pilot Experiment–Sahel (HAPEX–Sahel) site, and an LST validation site operated near Melbourne, Australia. The biases of the retrieved LSTs for the validation sites in the Australian, FIFE, and ARM CART study areas are approximately 0.08°, 1.7°, and 1.4°C, respectively, yielding an overall bias error of better than half the current expected accuracy limit of some ±3°C. The associated bias-adjusted rmse differences are approximately 0.78°, 4.8°, and 4.5°C, respectively, mostly driven by intercomparing in situ point measurements to area-integrated satellite pixel retrievals. The bias-adjusted rmse differences for HAPEX–Sahel are larger (5° and 11°C), resulting from incomplete characterization of site heterogeneity, insufficient radiosonde launch frequency, and poor data quality of the temperature–moisture soundings, rather than intrinsic algorithm problems. Notably, the averaged retrieved emissivities for the trouble-free sites are within the expected range of emissivities for vegetated surfaces.
The GOES-8 retrieved LSTs exhibit small amplitude, high-frequency noise, and a daily error cycle when compared to in situ measurements. The noise is attributed to random detector errors in the satellite observations for which the channel 4 noise-equivalent temperature difference is larger than that of channel 5. The systematic differences between validation measurements and retrievals are near zero during nighttime but exhibit a small semidiurnal oscillation during daytime. Notwithstanding a possible semidiurnal bias in the pyrgeometer validation measurements associated with imperfect solar dome heating corrections, plus unaccounted-for attenuation between the surface and pyrgeometer, the latter error cycle is attributed to a too-coarse sampling of the nonlinear diurnal evolution of the thermodynamic structure of the atmospheric boundary layer, particularly near the sunrise and sunset transition times. Thus, sounding frequency determines the error characteristics of the nonlinearly evolving split-window weighting functions.
Abstract
This study addresses the retrieval of tropical open-ocean latent heating using Special Sensor Microwave Imager (SSM/I) satellite measurements. The analysis is carried out for the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) intensive observation period in the western Pacific, much of it focused on the study area of the third WCRP–GPCP Algorithm Intercomparison Project (AIP-3) situated over the TOGA COARE Inner Flux Array (IFA). The retrieval algorithm is a profile-type physical inversion scheme based on the use of multispectral passive microwave (PMW) measurements. It estimates vertically distributed rain rate and latent heating by first retrieving mixing ratio profiles of liquid and frozen hydrometeors and then calculating rain fallout rates and vertical derivatives of the liquid–ice mass fluxes. Various modifications to the existing algorithm are discussed, including a combined visible–infrared–PMW–radar screening scheme for distinguishing among “clear,” “cloud without rain,” and “cloud with rain pixels” to better delineate vertical heating structure. Validation of retrieved rain rates over the AIP-3 study area indicates acceptable accuracy/precision uncertainty levels in terms of intensity, distribution, and time variation.
A procedure is developed for improving the initially retrieved heating profiles based on calibration to shipboard radar measurements. The modified algorithm and calibration scheme were applied to the IFA for estimating vertical profiles of latent heating. An optimum high-quality sounding period (1–17 February 1993) was selected for large-scale diagnostic calculations of apparent heating (Q 1) and moistening (Q 2) to analyze heat-moisture budgets of convective and stratiform cloud systems. Comparison and sensitivity tests indicate that the retrieved latent heating and Q 1/Q 2 calculations are representative. Moisture budget analyses over the IFA were carried out to study the detailed heating structures of clouds, particularly the cumulus scale heating process. This was accomplished by using residuals between the SSM/I-retrieved latent heating and the large scale Q 2 diagnostics. Results show that estimates of daily eddy vertical moisture flux divergence contain sizable uncertainties, however, by averaging over extended periods and vertically integrating to obtain surface latent heat flux transfer, close agreement to independently derived surface evaporation rates is found. This suggests that by combining the SSM/I retrievals with large-scale sounding data, it is possible to shed light on the role of cumulus convection on diabatic heating.
Abstract
This study addresses the retrieval of tropical open-ocean latent heating using Special Sensor Microwave Imager (SSM/I) satellite measurements. The analysis is carried out for the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) intensive observation period in the western Pacific, much of it focused on the study area of the third WCRP–GPCP Algorithm Intercomparison Project (AIP-3) situated over the TOGA COARE Inner Flux Array (IFA). The retrieval algorithm is a profile-type physical inversion scheme based on the use of multispectral passive microwave (PMW) measurements. It estimates vertically distributed rain rate and latent heating by first retrieving mixing ratio profiles of liquid and frozen hydrometeors and then calculating rain fallout rates and vertical derivatives of the liquid–ice mass fluxes. Various modifications to the existing algorithm are discussed, including a combined visible–infrared–PMW–radar screening scheme for distinguishing among “clear,” “cloud without rain,” and “cloud with rain pixels” to better delineate vertical heating structure. Validation of retrieved rain rates over the AIP-3 study area indicates acceptable accuracy/precision uncertainty levels in terms of intensity, distribution, and time variation.
A procedure is developed for improving the initially retrieved heating profiles based on calibration to shipboard radar measurements. The modified algorithm and calibration scheme were applied to the IFA for estimating vertical profiles of latent heating. An optimum high-quality sounding period (1–17 February 1993) was selected for large-scale diagnostic calculations of apparent heating (Q 1) and moistening (Q 2) to analyze heat-moisture budgets of convective and stratiform cloud systems. Comparison and sensitivity tests indicate that the retrieved latent heating and Q 1/Q 2 calculations are representative. Moisture budget analyses over the IFA were carried out to study the detailed heating structures of clouds, particularly the cumulus scale heating process. This was accomplished by using residuals between the SSM/I-retrieved latent heating and the large scale Q 2 diagnostics. Results show that estimates of daily eddy vertical moisture flux divergence contain sizable uncertainties, however, by averaging over extended periods and vertically integrating to obtain surface latent heat flux transfer, close agreement to independently derived surface evaporation rates is found. This suggests that by combining the SSM/I retrievals with large-scale sounding data, it is possible to shed light on the role of cumulus convection on diabatic heating.
Abstract
The role of the Tibetan Plateau on the behavior of the surface longwave radiation budget is examined, and the behavior of the vertical profile of longwave cooling over the plateau, including its diurnal variation, is quantified. The investigation has been conducted with the aid of datasets obtained during the 1986 Tibetan Plateau Meteorological Experiment (TIPMEX-86). A medium spectral-resolution infrared radiative transfer model using a simple modification for applications in idealized complex (valley) terrain is developed for the study. This study focuses on the clear-sky case where the surface effects are most significant.
The TIPMEX-86 data, obtained during the spring-summer transition into the East Asian monsoon season, are used to help validate the surface longwave radiation budget at two sites of varying elevation: Lasa (3650 m) and Naqu (4500 m). Based on the degree to which skin-temperature boundary conditions control the magnitude of infrared cooling, we define the concept of relative longwave heating and explain its influence on the vertical infrared cooling-rate profile. Relative longwave radiative heating at the higher-elevation Naqu site is found to be twice as large as that corresponding to the lower-elevation Lasa site located within a valley. Besides reducing the infrared cooling rates, it is shown that relative longwave heating extends the period of the day over which the plateau acts as a direct heat source to the atmosphere. Computational results from the infrared model help substantiate observational analyses that indicate surface longwave net radiation at the high-elevation site, on clear days, exceeds 300 W m−2; this is an order of magnitude greater than typical of sea-level oceanic conditions. As a result of the unique meteorological and surface conditions, total infrared flux convergence occurs within the deep planetary boundary layer (i.e., infrared heating of the cloud-free lower atmosphere) at the high-elevation site during the afternoon. An important characteristic of the daytime longwave heating process of the lower layers is how it turns off like a switch at approximately 1800 MST, transforming almost immediately to maximum cooling of the lower layers.
Atmospheric longwave cooling is significantly influenced by variations in the biophysical composition of the surface and the associated thermal diurnal cycle. It is estimated that natural variations of surface emissivity could modulate longwave cooling by up to 40%. The largest impact would occur at a time when the surface temperature is high and the relative longwave radiative heating of the lower atmosphere by the surface reaches its maximum value.
Abstract
The role of the Tibetan Plateau on the behavior of the surface longwave radiation budget is examined, and the behavior of the vertical profile of longwave cooling over the plateau, including its diurnal variation, is quantified. The investigation has been conducted with the aid of datasets obtained during the 1986 Tibetan Plateau Meteorological Experiment (TIPMEX-86). A medium spectral-resolution infrared radiative transfer model using a simple modification for applications in idealized complex (valley) terrain is developed for the study. This study focuses on the clear-sky case where the surface effects are most significant.
The TIPMEX-86 data, obtained during the spring-summer transition into the East Asian monsoon season, are used to help validate the surface longwave radiation budget at two sites of varying elevation: Lasa (3650 m) and Naqu (4500 m). Based on the degree to which skin-temperature boundary conditions control the magnitude of infrared cooling, we define the concept of relative longwave heating and explain its influence on the vertical infrared cooling-rate profile. Relative longwave radiative heating at the higher-elevation Naqu site is found to be twice as large as that corresponding to the lower-elevation Lasa site located within a valley. Besides reducing the infrared cooling rates, it is shown that relative longwave heating extends the period of the day over which the plateau acts as a direct heat source to the atmosphere. Computational results from the infrared model help substantiate observational analyses that indicate surface longwave net radiation at the high-elevation site, on clear days, exceeds 300 W m−2; this is an order of magnitude greater than typical of sea-level oceanic conditions. As a result of the unique meteorological and surface conditions, total infrared flux convergence occurs within the deep planetary boundary layer (i.e., infrared heating of the cloud-free lower atmosphere) at the high-elevation site during the afternoon. An important characteristic of the daytime longwave heating process of the lower layers is how it turns off like a switch at approximately 1800 MST, transforming almost immediately to maximum cooling of the lower layers.
Atmospheric longwave cooling is significantly influenced by variations in the biophysical composition of the surface and the associated thermal diurnal cycle. It is estimated that natural variations of surface emissivity could modulate longwave cooling by up to 40%. The largest impact would occur at a time when the surface temperature is high and the relative longwave radiative heating of the lower atmosphere by the surface reaches its maximum value.
Abstract
During the summer east Asian monsoon transition period in 1979, a meteorological field experiment entitled the Qinghai-Xizang Plateau Meteorological Experiment (QXPMEX-79) was conducted over the entire Tibetan Plateau. Data collected on and around the plateau during this period, in conjunction with a medium spectral-resolution infrared radiative transfer model, are used to gain an understanding of how elevation and surface biophysical factors, which are highly variable over the large-scale plateau domain, regulate the spatial distribution of clear-sky infrared cooling during the transition phase of the summer monsoon.
The spatial distribution of longwave cooling over the plateau is significantly influenced by variations in biophysical composition, topography, and elevation, the surface thermal diurnal cycle, and various climatological factors. An important factor is soil moisture. Bulk clear-sky longwave cooling rates are larger in the southeast sector of the plateau than in the north. This is because rainfall is greatest in the southeast, whereas the north is highly desertified and relative longwave radiative heating by the surface is greatest. Another important phenomenon is that the locale of a large-scale east-west-aligned spatial gradient in radiative cooling propagates northward with time. During the premonsoon period (May–June), the location of the strong spatial gradient is found in the southeastern margin of the plateau. Due to changes in surface and atmospheric conditions after the summer monsoon commences, the high gradient sector is shifted to the central Qinghai region. Furthermore, an overall decrease in longwave cooling takes place in the lower atmosphere immediately prior to the arrival of the active monsoon.
The magnitude of longwave cooling is significantly affected by skin-temperature boundary conditions at plateau altitudes. If skin-temperature discontinuities across the surface-atmosphere interface are neglected, bulk cooling rates will be in error up to 1°C day−1. The high surface skin temperatures, particularly in the afternoon, lead to significant relative longwave radiative heating in the lower atmosphere for which the impact in terms of vertical depth is shown to increase rather dramatically as a function of the elevation of the terrain. The significance of these results in the context of previous heat budget studies of the plateau suggest that the radiative heating term (QR ) used by previous investigators contains far too much longwave cooling, and thus in a classic formulation of the Yanai Q 1 balance equation, would lead to underestimation of sensible heating into the atmospheric column.
Abstract
During the summer east Asian monsoon transition period in 1979, a meteorological field experiment entitled the Qinghai-Xizang Plateau Meteorological Experiment (QXPMEX-79) was conducted over the entire Tibetan Plateau. Data collected on and around the plateau during this period, in conjunction with a medium spectral-resolution infrared radiative transfer model, are used to gain an understanding of how elevation and surface biophysical factors, which are highly variable over the large-scale plateau domain, regulate the spatial distribution of clear-sky infrared cooling during the transition phase of the summer monsoon.
The spatial distribution of longwave cooling over the plateau is significantly influenced by variations in biophysical composition, topography, and elevation, the surface thermal diurnal cycle, and various climatological factors. An important factor is soil moisture. Bulk clear-sky longwave cooling rates are larger in the southeast sector of the plateau than in the north. This is because rainfall is greatest in the southeast, whereas the north is highly desertified and relative longwave radiative heating by the surface is greatest. Another important phenomenon is that the locale of a large-scale east-west-aligned spatial gradient in radiative cooling propagates northward with time. During the premonsoon period (May–June), the location of the strong spatial gradient is found in the southeastern margin of the plateau. Due to changes in surface and atmospheric conditions after the summer monsoon commences, the high gradient sector is shifted to the central Qinghai region. Furthermore, an overall decrease in longwave cooling takes place in the lower atmosphere immediately prior to the arrival of the active monsoon.
The magnitude of longwave cooling is significantly affected by skin-temperature boundary conditions at plateau altitudes. If skin-temperature discontinuities across the surface-atmosphere interface are neglected, bulk cooling rates will be in error up to 1°C day−1. The high surface skin temperatures, particularly in the afternoon, lead to significant relative longwave radiative heating in the lower atmosphere for which the impact in terms of vertical depth is shown to increase rather dramatically as a function of the elevation of the terrain. The significance of these results in the context of previous heat budget studies of the plateau suggest that the radiative heating term (QR ) used by previous investigators contains far too much longwave cooling, and thus in a classic formulation of the Yanai Q 1 balance equation, would lead to underestimation of sensible heating into the atmospheric column.
