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- Author or Editor: J. Wang x
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Abstract
Information on atmospheric constituents is contained in the remotely measured spectral radiances. Two iteration methods, linear and nonlinear, are presented to demonstrate the possibility of inferring the water vapor profile from ground-based measurements. The linear inversion method which linearizes the radiative transfer equation is found to have a narrow range of convergence. A study of the vertical resolution of the inferred profile through the linear inversion technique indicates that fine-scale detailed structure of the profile cannot be reconstructed. The nonlinear iteration procedure, which minimizes the root-mean-squares residual of the random noise along the direction of “steepest” descent, is found capable of inferring a reasonably stable solution with wide range of convergence and is proven in numerical stability superior to the linear technique. The effects of the errors both in radiance measurements and in temperature profile on the inferred profile are also presented.
Abstract
Information on atmospheric constituents is contained in the remotely measured spectral radiances. Two iteration methods, linear and nonlinear, are presented to demonstrate the possibility of inferring the water vapor profile from ground-based measurements. The linear inversion method which linearizes the radiative transfer equation is found to have a narrow range of convergence. A study of the vertical resolution of the inferred profile through the linear inversion technique indicates that fine-scale detailed structure of the profile cannot be reconstructed. The nonlinear iteration procedure, which minimizes the root-mean-squares residual of the random noise along the direction of “steepest” descent, is found capable of inferring a reasonably stable solution with wide range of convergence and is proven in numerical stability superior to the linear technique. The effects of the errors both in radiance measurements and in temperature profile on the inferred profile are also presented.
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A previously published method by Wang et al. for predicting subsurface velocities and density from sea surface buoyancy and surface height is extended by incorporating analytical solutions to make the vertical projection. One solution employs exponential stratification and the second has a weakly stratified surface layer, approximating a mixed layer. The results are evaluated using fields from a numerical simulation of the North Atlantic. The simple exponential solution yields realistic subsurface density and vorticity fields to nearly 1000 m in depth. Including a mixed layer improves the response in the mixed layer itself and at high latitudes where the mixed layer is deeper. It is in the mixed layer that the surface quasigeostrophic approximation is most applicable. Below that the first baroclinic mode dominates, and that mode is well approximated by the analytical solution with exponential stratification.
Abstract
A previously published method by Wang et al. for predicting subsurface velocities and density from sea surface buoyancy and surface height is extended by incorporating analytical solutions to make the vertical projection. One solution employs exponential stratification and the second has a weakly stratified surface layer, approximating a mixed layer. The results are evaluated using fields from a numerical simulation of the North Atlantic. The simple exponential solution yields realistic subsurface density and vorticity fields to nearly 1000 m in depth. Including a mixed layer improves the response in the mixed layer itself and at high latitudes where the mixed layer is deeper. It is in the mixed layer that the surface quasigeostrophic approximation is most applicable. Below that the first baroclinic mode dominates, and that mode is well approximated by the analytical solution with exponential stratification.
Abstract
Radiometric measurements were made by a millimeter-wave imaging radiometer (MIR) at the frequencies of 89, 150, 183.3 ± 1, 183.3 ± 3, 183.3 ± 7, and 220 GHz aboard the NASA ER-2 aircraft at an altitude of about 20 km over two rainstorms: one in the western Pacific Ocean on 19 January 1993 and another in southern Florida on 5 October 1993. These measurements were complemented by nearly simultaneous observations by other sensors aboard the same aircraft and another aircraft flying along the same path. Analysis of data from these measurements, aided by radiative transfer and radar reflectivity calculations of hydrometeor profiles, which are generated by a general cloud ensemble model, demonstrates the utility of these frequencies for studying the structure of frozen hydrometeors associated with storms. Particular emphasis is placed on the three water vapor channels near 183.3 GHz. Results show that the radiometric signatures measured by these channels over the storm-associated scattering media bear a certain resemblance to those previously observed over a clear and fairly dry atmosphere with a cold ocean background. Both of these atmospheric conditions are characterized by a small amount of water vapor above a cold background. Radiative transfer calculations were made at these water vapor channels for a number of relative humidity profiles characterizing dry atmospheres over an ocean surface. The results are compared with the measurements to infer some characteristics of the environment near the scattering media. Furthermore, radiometric signatures from these channels display unique features for towering deep convective cells that could be used to identify the presence of such cells in storms.
Abstract
Radiometric measurements were made by a millimeter-wave imaging radiometer (MIR) at the frequencies of 89, 150, 183.3 ± 1, 183.3 ± 3, 183.3 ± 7, and 220 GHz aboard the NASA ER-2 aircraft at an altitude of about 20 km over two rainstorms: one in the western Pacific Ocean on 19 January 1993 and another in southern Florida on 5 October 1993. These measurements were complemented by nearly simultaneous observations by other sensors aboard the same aircraft and another aircraft flying along the same path. Analysis of data from these measurements, aided by radiative transfer and radar reflectivity calculations of hydrometeor profiles, which are generated by a general cloud ensemble model, demonstrates the utility of these frequencies for studying the structure of frozen hydrometeors associated with storms. Particular emphasis is placed on the three water vapor channels near 183.3 GHz. Results show that the radiometric signatures measured by these channels over the storm-associated scattering media bear a certain resemblance to those previously observed over a clear and fairly dry atmosphere with a cold ocean background. Both of these atmospheric conditions are characterized by a small amount of water vapor above a cold background. Radiative transfer calculations were made at these water vapor channels for a number of relative humidity profiles characterizing dry atmospheres over an ocean surface. The results are compared with the measurements to infer some characteristics of the environment near the scattering media. Furthermore, radiometric signatures from these channels display unique features for towering deep convective cells that could be used to identify the presence of such cells in storms.
Abstract
Using a composite procedure, North American Mesoscale Model (NAM) forecast and observed environments associated with zonally oriented, quasi-stationary surface fronts for 64 cases during July–August 2006–08 were examined for a large region encompassing the central United States. NAM adequately simulated the general synoptic features associated with the frontal environments (e.g., patterns in the low-level wind fields) as well as the positions of the fronts. However, kinematic fields important to frontogenesis such as horizontal deformation and convergence were overpredicted. Surface-based convective available potential energy (CAPE) and precipitable water were also overpredicted, which was likely related to the overprediction of the kinematic fields through convergence of water vapor flux. In addition, a spurious coherence between forecast deformation and precipitation was found using spatial correlation coefficients. Composite precipitation forecasts featured a broad area of rainfall stretched parallel to the composite front, whereas the composite observed precipitation covered a smaller area and had a WNW–ESE orientation relative to the front, consistent with mesoscale convective systems (MCSs) propagating at a slight right angle relative to the thermal gradient. Thus, deficiencies in the NAM precipitation forecasts may at least partially result from the inability to depict MCSs properly. It was observed that errors in the precipitation forecasts appeared to lag those of the kinematic fields, and so it seems likely that deficiencies in the precipitation forecasts are related to the overprediction of the kinematic fields such as deformation. However, no attempts were made to establish whether the overpredicted kinematic fields actually contributed to the errors in the precipitation forecasts or whether the overpredicted kinematic fields were simply an artifact of the precipitation errors. Regardless of the relationship between such errors, recognition of typical warm-season environments associated with these errors should be useful to operational forecasters.
Abstract
Using a composite procedure, North American Mesoscale Model (NAM) forecast and observed environments associated with zonally oriented, quasi-stationary surface fronts for 64 cases during July–August 2006–08 were examined for a large region encompassing the central United States. NAM adequately simulated the general synoptic features associated with the frontal environments (e.g., patterns in the low-level wind fields) as well as the positions of the fronts. However, kinematic fields important to frontogenesis such as horizontal deformation and convergence were overpredicted. Surface-based convective available potential energy (CAPE) and precipitable water were also overpredicted, which was likely related to the overprediction of the kinematic fields through convergence of water vapor flux. In addition, a spurious coherence between forecast deformation and precipitation was found using spatial correlation coefficients. Composite precipitation forecasts featured a broad area of rainfall stretched parallel to the composite front, whereas the composite observed precipitation covered a smaller area and had a WNW–ESE orientation relative to the front, consistent with mesoscale convective systems (MCSs) propagating at a slight right angle relative to the thermal gradient. Thus, deficiencies in the NAM precipitation forecasts may at least partially result from the inability to depict MCSs properly. It was observed that errors in the precipitation forecasts appeared to lag those of the kinematic fields, and so it seems likely that deficiencies in the precipitation forecasts are related to the overprediction of the kinematic fields such as deformation. However, no attempts were made to establish whether the overpredicted kinematic fields actually contributed to the errors in the precipitation forecasts or whether the overpredicted kinematic fields were simply an artifact of the precipitation errors. Regardless of the relationship between such errors, recognition of typical warm-season environments associated with these errors should be useful to operational forecasters.
Abstract
Upwelling radiometric measurements at 90 GHz and three side bands near 183 GHz are used to retrieve water vapor profiles over the ocean surface. An algorithm incorporating a new technique of handling moderate cloud cover is illustrated for the profiling of both relative humidity and water vapor burden. It is shown that the retrieved relative humidity profiles reflect gross features of the corresponding profiles recorded by the radiosondes. However, the retrieval generally cannot produce fine details of the observed profiles at altitudes where a rapid change in relative humidity occurs. For this reason, comparison of retrieved and observed values at a given altitude often yields an appreciable rms error. Profiling of water vapor burden, a parameter equivalent to total integrated water vapor above a certain altitude, results in much better agreement, as expected. The rms error obtained from the results of the retrieval at the surface is comparable to that derived from the combination of measurements at 18 GHz and 21 GHz channels of the Scanning Multichannel Microwave Radiometer aboard the Nimbus 7 satellite.
Abstract
Upwelling radiometric measurements at 90 GHz and three side bands near 183 GHz are used to retrieve water vapor profiles over the ocean surface. An algorithm incorporating a new technique of handling moderate cloud cover is illustrated for the profiling of both relative humidity and water vapor burden. It is shown that the retrieved relative humidity profiles reflect gross features of the corresponding profiles recorded by the radiosondes. However, the retrieval generally cannot produce fine details of the observed profiles at altitudes where a rapid change in relative humidity occurs. For this reason, comparison of retrieved and observed values at a given altitude often yields an appreciable rms error. Profiling of water vapor burden, a parameter equivalent to total integrated water vapor above a certain altitude, results in much better agreement, as expected. The rms error obtained from the results of the retrieval at the surface is comparable to that derived from the combination of measurements at 18 GHz and 21 GHz channels of the Scanning Multichannel Microwave Radiometer aboard the Nimbus 7 satellite.
Abstract
The dynamics of the movement of an initially axisymmetric baroclinic vortex embedded in an environment at rest on a beta plane is investigated with a three-dimensional primitive equation model. The study focuses on the motion and evolution of an adiabatic vortex and especially the manner in which vertical coupling of a tilted vortex influences its motion. The authors find that the vortex movement is determined by both the asymmetric flow over the vortex core associated with beta gyres and the flow associated with vertical projection of the tilted potential vorticity anomaly. The effects of vortex tilt can be large and complex. The secondary divergent circulation is found to be associated with the development of potential temperature anomalies required to maintain a balanced state. The processes involved strongly depend on the vertical structure, size, and intensity of the vortex together with external parameters such as the earth rotation and static stability of the environment. As a result, simple relationships between vortex motion and the vertical mean relative angular momentum are not always applicable.
Abstract
The dynamics of the movement of an initially axisymmetric baroclinic vortex embedded in an environment at rest on a beta plane is investigated with a three-dimensional primitive equation model. The study focuses on the motion and evolution of an adiabatic vortex and especially the manner in which vertical coupling of a tilted vortex influences its motion. The authors find that the vortex movement is determined by both the asymmetric flow over the vortex core associated with beta gyres and the flow associated with vertical projection of the tilted potential vorticity anomaly. The effects of vortex tilt can be large and complex. The secondary divergent circulation is found to be associated with the development of potential temperature anomalies required to maintain a balanced state. The processes involved strongly depend on the vertical structure, size, and intensity of the vortex together with external parameters such as the earth rotation and static stability of the environment. As a result, simple relationships between vortex motion and the vertical mean relative angular momentum are not always applicable.
Abstract
A model for numerical prediction of the 1000-mb surface is developed which includes a term expressing the interchange of sensible heat between the air and the underlying surface as well as the effect of terrain-induced vertical motion. In spite of the crudeness of the non-adiabatic representation, the model shows a definite improvement over a similar adiabatic model when the two are compared in a series of prognoses. Moreover, when monthly mean isotherms may be used to represent the temperature of the underlying surface, the non-adiabatic term may be combined with the orographic term and the earth's vorticity so that there is no work added to the prognostic routine.
Abstract
A model for numerical prediction of the 1000-mb surface is developed which includes a term expressing the interchange of sensible heat between the air and the underlying surface as well as the effect of terrain-induced vertical motion. In spite of the crudeness of the non-adiabatic representation, the model shows a definite improvement over a similar adiabatic model when the two are compared in a series of prognoses. Moreover, when monthly mean isotherms may be used to represent the temperature of the underlying surface, the non-adiabatic term may be combined with the orographic term and the earth's vorticity so that there is no work added to the prognostic routine.
Abstract
The motion and the evolution of tropical cyclone-like vortices in an environmental flow with vertical shear are investigated using a baroclinic primitive equation model. The study focuses on the fundamental dynamics of a baroclinic vortex in vertical shear, the influence of vortex structure, and the role of diabatic heating. The results show that the initial response of the vortex to the vertical shear is to tilt downshear. As soon as the tilt develops, the upper-level anticyclonic and lower-level cyclonic circulations begin to interact with each other. As a result of these interactions, the tilted axis of the vortex reaches a stable state after an initial adjustment, which varies with the structure of the vortex, its environmental flow shear, and the cumulus convective heating.
The motion of an adiabatic vortex in vertical shear is controlled by both the steering of the environmental flow and vertical coupling mechanisms. Most of the vortices move with the environmental flow at about 650 hPa or with the layer mean between 350 and 900 hPa, but the stronger tropical cyclone vortices move with a relatively deeper layer mean flow. In addition to advection by the environmental flow, most vortices propagate to the left of the vertical shear due to downward penetration of the circulation associated with the upper-level anticyclonic potential vorticity (PV) anomalies that are displaced downshear.
Diabatic and moist processes can substantially modify the adiabatic vortex motion by both the vertical transport of potential vorticity associated with diabatic heating and the development of convective asymmetries within the vortex core region. Diabatic heating can either substantially enhance the leftward motion tendency or result in a rightward motion relative to the vertical shear, depending on the vertical structure and intensity of the vortex and its environment. This occurs by transport of anticyclonic PV to the upper troposphere and cyclonic PV to the right of the vortex center relative to the vertical shear in the lower troposphere. A rightward motion tendency to the boundary layer flow is found to arise from enhanced heat fluxes from the ocean on the higher wind side of the vortex center. Cumulus convection is substantially enhanced on the downshear side of the vortex center due to the interaction between the vortex circulation and the vertical shear in the environmental flow. The asymmetric divergent flow associated with these convective asymmetries affects the vortex motion by deflecting the vortex to the region with maximum convection.
Abstract
The motion and the evolution of tropical cyclone-like vortices in an environmental flow with vertical shear are investigated using a baroclinic primitive equation model. The study focuses on the fundamental dynamics of a baroclinic vortex in vertical shear, the influence of vortex structure, and the role of diabatic heating. The results show that the initial response of the vortex to the vertical shear is to tilt downshear. As soon as the tilt develops, the upper-level anticyclonic and lower-level cyclonic circulations begin to interact with each other. As a result of these interactions, the tilted axis of the vortex reaches a stable state after an initial adjustment, which varies with the structure of the vortex, its environmental flow shear, and the cumulus convective heating.
The motion of an adiabatic vortex in vertical shear is controlled by both the steering of the environmental flow and vertical coupling mechanisms. Most of the vortices move with the environmental flow at about 650 hPa or with the layer mean between 350 and 900 hPa, but the stronger tropical cyclone vortices move with a relatively deeper layer mean flow. In addition to advection by the environmental flow, most vortices propagate to the left of the vertical shear due to downward penetration of the circulation associated with the upper-level anticyclonic potential vorticity (PV) anomalies that are displaced downshear.
Diabatic and moist processes can substantially modify the adiabatic vortex motion by both the vertical transport of potential vorticity associated with diabatic heating and the development of convective asymmetries within the vortex core region. Diabatic heating can either substantially enhance the leftward motion tendency or result in a rightward motion relative to the vertical shear, depending on the vertical structure and intensity of the vortex and its environment. This occurs by transport of anticyclonic PV to the upper troposphere and cyclonic PV to the right of the vortex center relative to the vertical shear in the lower troposphere. A rightward motion tendency to the boundary layer flow is found to arise from enhanced heat fluxes from the ocean on the higher wind side of the vortex center. Cumulus convection is substantially enhanced on the downshear side of the vortex center due to the interaction between the vortex circulation and the vertical shear in the environmental flow. The asymmetric divergent flow associated with these convective asymmetries affects the vortex motion by deflecting the vortex to the region with maximum convection.
