Search Results
You are looking at 1 - 10 of 13 items for
- Author or Editor: Y. Han x
- Refine by Access: All Content x
Abstract
The monthly wind stress over the global ocean has been calculated using updated climatological monthly mean wind data. The effect of the variablility of the wind has been incorporated by using the wind speed frequency estimated from the observed mean and variance in a Gaussian distribution model. The spatial and temporal data gaps in the high-latitude oceans have been filled with surface winds estimated from the monthly mean geostrophic winds.
The calculated stress data have revealed a number of interesting features that were not well resolved in earlier studies. A rapid transition from a winter to summer stress pattern in the north Indian Ocean is especially notable, and large-scale gyral patterns in the Antarctic Ocean and their seasonal variations are now well-defined. The monthly wind-stress curl calculated from the present stress data is in good agreement with earlier calculations, although the present curl field reveals a more detailed structure, especially in high latitudes and in the tropics.
These data should prove useful in global ocean general circulation models, as well as in other theoretical studies of ocean transports. They should also be useful in atmospheric general circulation studies, in view of their crucial importance to the atmospheric angular momentum balance.
Abstract
The monthly wind stress over the global ocean has been calculated using updated climatological monthly mean wind data. The effect of the variablility of the wind has been incorporated by using the wind speed frequency estimated from the observed mean and variance in a Gaussian distribution model. The spatial and temporal data gaps in the high-latitude oceans have been filled with surface winds estimated from the monthly mean geostrophic winds.
The calculated stress data have revealed a number of interesting features that were not well resolved in earlier studies. A rapid transition from a winter to summer stress pattern in the north Indian Ocean is especially notable, and large-scale gyral patterns in the Antarctic Ocean and their seasonal variations are now well-defined. The monthly wind-stress curl calculated from the present stress data is in good agreement with earlier calculations, although the present curl field reveals a more detailed structure, especially in high latitudes and in the tropics.
These data should prove useful in global ocean general circulation models, as well as in other theoretical studies of ocean transports. They should also be useful in atmospheric general circulation studies, in view of their crucial importance to the atmospheric angular momentum balance.
Abstract
Numerical integrations using a potential enstrophy conserving scheme are presented for the flow within a mixed layer over hilly terrain using the hydrostatic shallow-water equations with a quadratic drag law. The mesoscale area treated is 150 km on a side; cyclic lateral boundary conditions are used. It is found that for the idealized conditions treated (no surface heating, no entrainment and no pressure adjustments aloft), the topography quickly induces a steady state flow pattern by means of surface friction. Unsteadiness does not occur unless a surface-friction Reynolds number, R = h̄/(CDL), exceeds ∼100, where h̄h is the mean mixed-layer thickness, CD is the surface drag coefficient and L is a representative horizontal terrain length scale. Effects of varying the Rossby number, Froude number and terrain-height parameter are examined.
Abstract
Numerical integrations using a potential enstrophy conserving scheme are presented for the flow within a mixed layer over hilly terrain using the hydrostatic shallow-water equations with a quadratic drag law. The mesoscale area treated is 150 km on a side; cyclic lateral boundary conditions are used. It is found that for the idealized conditions treated (no surface heating, no entrainment and no pressure adjustments aloft), the topography quickly induces a steady state flow pattern by means of surface friction. Unsteadiness does not occur unless a surface-friction Reynolds number, R = h̄/(CDL), exceeds ∼100, where h̄h is the mean mixed-layer thickness, CD is the surface drag coefficient and L is a representative horizontal terrain length scale. Effects of varying the Rossby number, Froude number and terrain-height parameter are examined.
Abstract
A two-stage retrieval technique is presented for deriving water vapor profiles from data provided by a Raman lidar, a microwave radiometer, a radio acoustic sounding system, and surface in situ instruments. In the first stage, a Kalman filtering algorithm is applied to derive water vapor profiles using surface in situ and current and past Raman measurements. In the second stage, a statistical inversion technique is applied to combine the Kalman retrieval with radiometric and climatological data. This retrieval method is tested using data collected during the First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment II experiment. The method is demonstrated to provide accurate profiles at altitudes above which the Raman lidar technique is limited.
Abstract
A two-stage retrieval technique is presented for deriving water vapor profiles from data provided by a Raman lidar, a microwave radiometer, a radio acoustic sounding system, and surface in situ instruments. In the first stage, a Kalman filtering algorithm is applied to derive water vapor profiles using surface in situ and current and past Raman measurements. In the second stage, a statistical inversion technique is applied to combine the Kalman retrieval with radiometric and climatological data. This retrieval method is tested using data collected during the First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment II experiment. The method is demonstrated to provide accurate profiles at altitudes above which the Raman lidar technique is limited.
Abstract
A four-layer three-dimensional model whose lowest layer is a time and space-dependent, well-mixed boundary layer is employed over artificial, irregular terrain on the mesoscale during a daytime heating cycle. Only if the surface heating and mixed-layer entrainment am suppressed does the Row field become steady as found previously using a shallow-water model. Unsteadiness is due both to diurnal effects, especially the relaxation of the frictional force as the mixed layer deepens irregularly, and to the presence of horizontal vacations in potential temperature. The latter can develop with time due to the negative feedback between mixed-layer depth and warming rate; after the early morning hours, however, this feedback causes a damping of the temperature anomalies to much smaller values by late afternoon.
Cool-air anomalies in the mixed layer are found to develop lesser mixed-layer depths than warm anomalies, yet to be accompanied by greater “reduced” surface pressures. As a result, a thermal-anomaly form drag occurs of very significant amplitude, since the cool air pools spend most of the day moving upslope, on the average, and the warm air pockets downslope. The thermal-anomaly form-drag coefficients are typically of greater magnitude than the shallow-water form-drag coefficients associated with a mixed layer of uniform potential temperature capped by a temperature jump. However, the former can on occasion become negative. Parameterizations for both types of form drag are offered as a function of terrain heights and slopes, mixed-layer wind speed and inversion strength, and horizontal temperature variability.
Abstract
A four-layer three-dimensional model whose lowest layer is a time and space-dependent, well-mixed boundary layer is employed over artificial, irregular terrain on the mesoscale during a daytime heating cycle. Only if the surface heating and mixed-layer entrainment am suppressed does the Row field become steady as found previously using a shallow-water model. Unsteadiness is due both to diurnal effects, especially the relaxation of the frictional force as the mixed layer deepens irregularly, and to the presence of horizontal vacations in potential temperature. The latter can develop with time due to the negative feedback between mixed-layer depth and warming rate; after the early morning hours, however, this feedback causes a damping of the temperature anomalies to much smaller values by late afternoon.
Cool-air anomalies in the mixed layer are found to develop lesser mixed-layer depths than warm anomalies, yet to be accompanied by greater “reduced” surface pressures. As a result, a thermal-anomaly form drag occurs of very significant amplitude, since the cool air pools spend most of the day moving upslope, on the average, and the warm air pockets downslope. The thermal-anomaly form-drag coefficients are typically of greater magnitude than the shallow-water form-drag coefficients associated with a mixed layer of uniform potential temperature capped by a temperature jump. However, the former can on occasion become negative. Parameterizations for both types of form drag are offered as a function of terrain heights and slopes, mixed-layer wind speed and inversion strength, and horizontal temperature variability.
Abstract
The observed zonally averaged column ozone shows a maximum at 90°N during the northern winter and spring and at 60°S throughout the southern winter and spring. This asymmetry is explained in the context of a zonally averaged model with coupled radiation, dynamics, and chemistry, together with consistently parameterized planetary wave driving and wave transport. It is shown that in the presence of weak wave driving, the penetration of the tropospheric circulation into the lower stratosphere and the characteristics of ozone chemistry are such that they produce a column ozone maximum at subpolar latitudes. The effect of increased wave driving is to intensify the residual circulation and extend it farther poleward, resulting in an ozone maximum at the pole. The role of the mesospheric drag is to further enhance these column ozone maxima. Model calculations show that the positions of the observed column ozone maxima are consistent with intensities of wave driving in the two hemispheres derived from data.
Abstract
The observed zonally averaged column ozone shows a maximum at 90°N during the northern winter and spring and at 60°S throughout the southern winter and spring. This asymmetry is explained in the context of a zonally averaged model with coupled radiation, dynamics, and chemistry, together with consistently parameterized planetary wave driving and wave transport. It is shown that in the presence of weak wave driving, the penetration of the tropospheric circulation into the lower stratosphere and the characteristics of ozone chemistry are such that they produce a column ozone maximum at subpolar latitudes. The effect of increased wave driving is to intensify the residual circulation and extend it farther poleward, resulting in an ozone maximum at the pole. The role of the mesospheric drag is to further enhance these column ozone maxima. Model calculations show that the positions of the observed column ozone maxima are consistent with intensities of wave driving in the two hemispheres derived from data.
Abstract
Two techniques for deriving low-altitude temperature profiles were evaluated in an experiment conducted from November 1996 to January 1997 at the Boulder Atmospheric Observatory (BAO). The first used a scanning, single wavelength, 5-mm (60 GHz) microwave radiometer to measure vertical temperature profiles. Two radiometers were operated simultaneously; one used a discrete scan, the other scanned continuously. The second technique was a Radio Acoustic Sounding System (RASS) that operated at 915 MHz. Typically, radiometric profiles were produced every 15 min; those from RASS were 5-min segments taken every hour. Ground truth for the experiment was available from in situ measurements at the BAO. The BAO has an instrumented 300-m tower with 5-min measurements of temperature and relative humidity available at the surface and at altitudes of 10, 50, 100, 200, and 300 m. The tower measurements were occasionally supplemented with radiosonde releases and with hand-held meteorological measurements taken on the tower elevator.
The differences between the radiometers and the tower sensors were about 1°C rms. The accuracy using an in situ temperature measurement at the radiometer height as a predictor was also evaluated; at the 200- and 300-m levels, only about 4°C rms accuracies resulted. During the experiment, the RASS occasionally experienced radio frequency interference; to eliminate these effects, a quality-control algorithm for the RASS system was developed and evaluated. In addition, an experiment was held in September 1996 at the Department of Energy’s Atmospheric Radiation Program Southern Great Plains site in north central Oklahoma. For this experiment, evaluations of a scanning 5-mm radiometer relative to 3-hourly radiosondes are presented. Quality control on the radiosondes was provided by comparisons with independent in situ surface and 60-m tower observations. The agreement between the radiometric profiles and the quality-controlled radiosondes was better than 1°C up to 800 m. Plans for future deployments of these instruments are discussed.
In addition to the in situ comparisons, theoretical analyses of the scanning radiometer systems were also conducted. The effects of angular resolution of the current system, noise level, prediction from in situ measurements, and vertical resolution were examined.
Abstract
Two techniques for deriving low-altitude temperature profiles were evaluated in an experiment conducted from November 1996 to January 1997 at the Boulder Atmospheric Observatory (BAO). The first used a scanning, single wavelength, 5-mm (60 GHz) microwave radiometer to measure vertical temperature profiles. Two radiometers were operated simultaneously; one used a discrete scan, the other scanned continuously. The second technique was a Radio Acoustic Sounding System (RASS) that operated at 915 MHz. Typically, radiometric profiles were produced every 15 min; those from RASS were 5-min segments taken every hour. Ground truth for the experiment was available from in situ measurements at the BAO. The BAO has an instrumented 300-m tower with 5-min measurements of temperature and relative humidity available at the surface and at altitudes of 10, 50, 100, 200, and 300 m. The tower measurements were occasionally supplemented with radiosonde releases and with hand-held meteorological measurements taken on the tower elevator.
The differences between the radiometers and the tower sensors were about 1°C rms. The accuracy using an in situ temperature measurement at the radiometer height as a predictor was also evaluated; at the 200- and 300-m levels, only about 4°C rms accuracies resulted. During the experiment, the RASS occasionally experienced radio frequency interference; to eliminate these effects, a quality-control algorithm for the RASS system was developed and evaluated. In addition, an experiment was held in September 1996 at the Department of Energy’s Atmospheric Radiation Program Southern Great Plains site in north central Oklahoma. For this experiment, evaluations of a scanning 5-mm radiometer relative to 3-hourly radiosondes are presented. Quality control on the radiosondes was provided by comparisons with independent in situ surface and 60-m tower observations. The agreement between the radiometric profiles and the quality-controlled radiosondes was better than 1°C up to 800 m. Plans for future deployments of these instruments are discussed.
In addition to the in situ comparisons, theoretical analyses of the scanning radiometer systems were also conducted. The effects of angular resolution of the current system, noise level, prediction from in situ measurements, and vertical resolution were examined.
Abstract
Simultaneous multiwavelength measurements of a developing cloud system were obtained by NOAA Doppler lidar, Doppler radar, Fourier transform infrared interferometer, and microwave and infrared radiometers on 26 November 1991. The evolution of the cloud system is described in terms of lidar backscatter, radar reflectivity and velocity, interferometer atmospheric spectra, and radiometer brightness temperature, integrated liquid water, and water vapor paths. Utilizing the difference in wavelength between the radar and lidar, and therefore their independent sensitivity to different regions of the same cloud, the cloud top, base, depth, and multiple layer heights can he determined with better accuracy than with either instrument alone. Combining the radar, lidar, and radiometer measurements using two different techniques allows an estimation of the vertical profile of cloud microphysical properties such as particle sizes. Enhancement of lidar backscatter near zenith revealed when highly oriented ice crystals were present. The authors demonstrate that no single instrument is sufficient to accurately describe cirrus clouds and that measurements in combination can provide important details on their geometric, radiative, and microphysical properties.
Abstract
Simultaneous multiwavelength measurements of a developing cloud system were obtained by NOAA Doppler lidar, Doppler radar, Fourier transform infrared interferometer, and microwave and infrared radiometers on 26 November 1991. The evolution of the cloud system is described in terms of lidar backscatter, radar reflectivity and velocity, interferometer atmospheric spectra, and radiometer brightness temperature, integrated liquid water, and water vapor paths. Utilizing the difference in wavelength between the radar and lidar, and therefore their independent sensitivity to different regions of the same cloud, the cloud top, base, depth, and multiple layer heights can he determined with better accuracy than with either instrument alone. Combining the radar, lidar, and radiometer measurements using two different techniques allows an estimation of the vertical profile of cloud microphysical properties such as particle sizes. Enhancement of lidar backscatter near zenith revealed when highly oriented ice crystals were present. The authors demonstrate that no single instrument is sufficient to accurately describe cirrus clouds and that measurements in combination can provide important details on their geometric, radiative, and microphysical properties.
Abstract
The microphysical characteristics, radiative impact, and life cycle of a long-lived, surface-based mixed-layer, mixed-phase cloud with an average temperature of approximately −20°C are presented and discussed. The cloud was observed during the Surface Heat Budget of the Arctic experiment (SHEBA) from 1 to 10 May 1998. Vertically resolved properties of the liquid and ice phases are retrieved using surface-based remote sensors, utilize the adiabatic assumption for the liquid component, and are aided by and validated with aircraft measurements from 4 and 7 May. The cloud radar ice microphysical retrievals, originally developed for all-ice clouds, compare well with aircraft measurements despite the presence of much greater liquid water contents than ice water contents. The retrieved time-mean liquid cloud optical depth of 10.1 ± 7.8 far surpasses the mean ice cloud optical depth of 0.2, so that the liquid phase is primarily responsible for the cloud’s radiative (flux) impact. The ice phase, in turn, regulates the overall cloud optical depth through two mechanisms: sedimentation from a thin upper ice cloud, and a local ice production mechanism with a time scale of a few hours, thought to reflect a preferred freezing of the larger liquid drops. The liquid water paths replenish within half a day or less after their uptake by ice, attesting to strong water vapor fluxes. Deeper boundary layer depths and higher cloud optical depths coincide with large-scale rising motion at 850 hPa, but the synoptic activity is also associated with upper-level ice clouds. Interestingly, the local ice formation mechanism appears to be more active when the large-scale subsidence rate implies increased cloud-top entrainment. Strong cloud-top radiative cooling rates promote cloud longevity when the cloud is optically thick. The radiative impact of the cloud upon the surface is significant: a time-mean positive net cloud forcing of 41 W m−2 with a diurnal amplitude of ∼20 W m−2. This is primarily because a high surface reflectance (0.86) reduces the solar cooling influence. The net cloud forcing is primarily sensitive to cloud optical depth for the low-optical-depth cloudy columns and to the surface reflectance for the high-optical-depth cloudy columns. Any projected increase in the springtime cloud optical depth at this location (76°N, 165°W) is not expected to significantly alter the surface radiation budget, because clouds were almost always present, and almost 60% of the cloudy columns had optical depths >6.
Abstract
The microphysical characteristics, radiative impact, and life cycle of a long-lived, surface-based mixed-layer, mixed-phase cloud with an average temperature of approximately −20°C are presented and discussed. The cloud was observed during the Surface Heat Budget of the Arctic experiment (SHEBA) from 1 to 10 May 1998. Vertically resolved properties of the liquid and ice phases are retrieved using surface-based remote sensors, utilize the adiabatic assumption for the liquid component, and are aided by and validated with aircraft measurements from 4 and 7 May. The cloud radar ice microphysical retrievals, originally developed for all-ice clouds, compare well with aircraft measurements despite the presence of much greater liquid water contents than ice water contents. The retrieved time-mean liquid cloud optical depth of 10.1 ± 7.8 far surpasses the mean ice cloud optical depth of 0.2, so that the liquid phase is primarily responsible for the cloud’s radiative (flux) impact. The ice phase, in turn, regulates the overall cloud optical depth through two mechanisms: sedimentation from a thin upper ice cloud, and a local ice production mechanism with a time scale of a few hours, thought to reflect a preferred freezing of the larger liquid drops. The liquid water paths replenish within half a day or less after their uptake by ice, attesting to strong water vapor fluxes. Deeper boundary layer depths and higher cloud optical depths coincide with large-scale rising motion at 850 hPa, but the synoptic activity is also associated with upper-level ice clouds. Interestingly, the local ice formation mechanism appears to be more active when the large-scale subsidence rate implies increased cloud-top entrainment. Strong cloud-top radiative cooling rates promote cloud longevity when the cloud is optically thick. The radiative impact of the cloud upon the surface is significant: a time-mean positive net cloud forcing of 41 W m−2 with a diurnal amplitude of ∼20 W m−2. This is primarily because a high surface reflectance (0.86) reduces the solar cooling influence. The net cloud forcing is primarily sensitive to cloud optical depth for the low-optical-depth cloudy columns and to the surface reflectance for the high-optical-depth cloudy columns. Any projected increase in the springtime cloud optical depth at this location (76°N, 165°W) is not expected to significantly alter the surface radiation budget, because clouds were almost always present, and almost 60% of the cloudy columns had optical depths >6.
