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Plans for an intensive cirrus-cloud field experiment are described. The Cirrus Intensive Field Observations (Cirrus IFO) is a major component of the First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment (FIRE). The field campaign was conducted in Wisconsin during October 1986. Observing systems include satellites, “cloud” lidars, a very high-altitude, satellite-simulator aircraft platform, two research aircraft instrumented for detailed in situ microphysical and radiometric observations, a Doppler lidar, numerous passive surface-radiation sites, and a rawinsonde network. This is the first cirrus experiment involving such a comprehensive observing system.
Plans for an intensive cirrus-cloud field experiment are described. The Cirrus Intensive Field Observations (Cirrus IFO) is a major component of the First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment (FIRE). The field campaign was conducted in Wisconsin during October 1986. Observing systems include satellites, “cloud” lidars, a very high-altitude, satellite-simulator aircraft platform, two research aircraft instrumented for detailed in situ microphysical and radiometric observations, a Doppler lidar, numerous passive surface-radiation sites, and a rawinsonde network. This is the first cirrus experiment involving such a comprehensive observing system.
Abstract
Detailed descriptions of the rawinsonde-resolved meteorological conditions (3-hourly soundings) associated with a succession of five distinct mesoscale cirrus cloud regimes, which were observed intensely over a 36-hour period, is given. The synoptic scale systems in which these features were embedded are described and a brief overview of the experiment are given. Regional analyses of the static stability structure and vertical motion are presented and interpreted with respect to the characteristics of the corresponding cloud fields as deduced from satellite and lidar observations. The cloud fields exhibited a high degree of persistent mesoscale organization on scales of 20–500 km reflecting corresponding scales of dynamic and thermodynamic structure/variability as on the synoptic scale. Cloud generation was usually confined to layers less than 1 km deep (typically 0.5 km in depth) and cellular organization was evident in most cases irrespective of the thermal stratification. Multilayered development was prevalent (2–3 layers) and was associated with vertical structure of the temperature and moisture fields resulting primarily from vertical gradients in horizontal advection. One convective generation layer was usually present. Destabilization resulted primarily from advective processes that also led to the formation of a transient stable layer above and/or below the convective layer. Though resembling elevated frontal surfaces, the stable layers were not extensions of surface features. Cloud processes, primarily ice particle fall-out and evaporation, but also including cloud top detrainment, contributed to generating the multilayered structure. Two cases of clouds spawned from an overlying cloud deck were seen where one involved natural seeding of an ice-saturated and conditionally unstable layer in which vigorous convective development was subsequently observed. Subvisual cirrus in the lower stratosphere were found to be associated with prior tropopause features (upwind) where denser cirrus existed. Inferences are drawn with respect to the parameterization of cirrus in large-scale models. In particular, vertical resolution on the order of 0.5 km will probably be required to adequately resolve the forcing required for implementation of a physically-based parameterization. Greater understanding of the nature and causes of the observed mesoscale structure is also needed.
Abstract
Detailed descriptions of the rawinsonde-resolved meteorological conditions (3-hourly soundings) associated with a succession of five distinct mesoscale cirrus cloud regimes, which were observed intensely over a 36-hour period, is given. The synoptic scale systems in which these features were embedded are described and a brief overview of the experiment are given. Regional analyses of the static stability structure and vertical motion are presented and interpreted with respect to the characteristics of the corresponding cloud fields as deduced from satellite and lidar observations. The cloud fields exhibited a high degree of persistent mesoscale organization on scales of 20–500 km reflecting corresponding scales of dynamic and thermodynamic structure/variability as on the synoptic scale. Cloud generation was usually confined to layers less than 1 km deep (typically 0.5 km in depth) and cellular organization was evident in most cases irrespective of the thermal stratification. Multilayered development was prevalent (2–3 layers) and was associated with vertical structure of the temperature and moisture fields resulting primarily from vertical gradients in horizontal advection. One convective generation layer was usually present. Destabilization resulted primarily from advective processes that also led to the formation of a transient stable layer above and/or below the convective layer. Though resembling elevated frontal surfaces, the stable layers were not extensions of surface features. Cloud processes, primarily ice particle fall-out and evaporation, but also including cloud top detrainment, contributed to generating the multilayered structure. Two cases of clouds spawned from an overlying cloud deck were seen where one involved natural seeding of an ice-saturated and conditionally unstable layer in which vigorous convective development was subsequently observed. Subvisual cirrus in the lower stratosphere were found to be associated with prior tropopause features (upwind) where denser cirrus existed. Inferences are drawn with respect to the parameterization of cirrus in large-scale models. In particular, vertical resolution on the order of 0.5 km will probably be required to adequately resolve the forcing required for implementation of a physically-based parameterization. Greater understanding of the nature and causes of the observed mesoscale structure is also needed.
Abstract
A two-dimensional (x, z), time-dependent, numerical cloud model is developed for the purpose of investigating the role of various physical processes involved in the maintenance of cirriform clouds. In addition to accounting for dynamic and thermodynamic processes including phase changes of water, effects due to microphysical composition and radiative processes are also explicitly incorporated into the model. Diagnostic parameterizations for the local radiative properties of cloudy volumes and the gravity induced relative fall speed of the contained ice water are presented. Results of a simulation of a thin cirrostratus cloud are given. Features of the simulated cloud structure are quite realistic. Quantitative agreement is found between the simulated ice water contents and vertical motions and comparable observations. It is shown that radiative effect may be very significant in the maintenance of cirrus. The effects of the gravity-induced relative fall speed of ice crystals are found to be of critical importance in the evolution of the cloud layer.
Abstract
A two-dimensional (x, z), time-dependent, numerical cloud model is developed for the purpose of investigating the role of various physical processes involved in the maintenance of cirriform clouds. In addition to accounting for dynamic and thermodynamic processes including phase changes of water, effects due to microphysical composition and radiative processes are also explicitly incorporated into the model. Diagnostic parameterizations for the local radiative properties of cloudy volumes and the gravity induced relative fall speed of the contained ice water are presented. Results of a simulation of a thin cirrostratus cloud are given. Features of the simulated cloud structure are quite realistic. Quantitative agreement is found between the simulated ice water contents and vertical motions and comparable observations. It is shown that radiative effect may be very significant in the maintenance of cirrus. The effects of the gravity-induced relative fall speed of ice crystals are found to be of critical importance in the evolution of the cloud layer.
Abstract
The numerical cirrus cloud model of Starr and Cox is used to investigate the role of various physical processes in the formation and maintenance of cirrus. Effects due to microphysical composition, i.e., crystal habit and size distribution, are found to be quite important in determining the overall cloud water budget. Radiative processes are also shown to affect the organization and bulk properties of the cloud. Substantial differences between simulations of thin cirrus under midday and nighttime conditions are found with the cloud being less dense overall (∼20%) but more persistently cellular during the day with all other environmental factors being the same. Cloud-scale interactions and feedbacks between dynamic, thermodynamic and radiative processes and the microphysical composition are significant and strongly modulate the properties of the simulated clouds. A comparison is made between simulations of weakly forced cirrostratus and nonprecipitating altostratus (liquid phase) under comparable environmental conditions. Five times more cloud water is maintained in the altostratus case where the updraft wind speed are greater by a factor of 10. Ale role of the large-wale ascent or descent is also examined. Inferences are drawn from these results with respect to the parameterization of cirrus in large-scale forecast or climate models.
Abstract
The numerical cirrus cloud model of Starr and Cox is used to investigate the role of various physical processes in the formation and maintenance of cirrus. Effects due to microphysical composition, i.e., crystal habit and size distribution, are found to be quite important in determining the overall cloud water budget. Radiative processes are also shown to affect the organization and bulk properties of the cloud. Substantial differences between simulations of thin cirrus under midday and nighttime conditions are found with the cloud being less dense overall (∼20%) but more persistently cellular during the day with all other environmental factors being the same. Cloud-scale interactions and feedbacks between dynamic, thermodynamic and radiative processes and the microphysical composition are significant and strongly modulate the properties of the simulated clouds. A comparison is made between simulations of weakly forced cirrostratus and nonprecipitating altostratus (liquid phase) under comparable environmental conditions. Five times more cloud water is maintained in the altostratus case where the updraft wind speed are greater by a factor of 10. Ale role of the large-wale ascent or descent is also examined. Inferences are drawn from these results with respect to the parameterization of cirrus in large-scale forecast or climate models.
Abstract
Operational satellite data from GOES-8 and GOES-9 were used to make stereoscopic measurements of cloud heights during the National Aeronautics and Space Administration’s Subsonic Aircraft: Contrail and Cloud Effects Special Study program. The stereoscopic data were used to differentiate between boundary layer wave clouds and cirrus in the mid- and upper troposphere. This separation was difficult to evaluate from radiometric data alone. Stereographic cloud height analysis provided a definitive result. The technique used for calculating cloud heights is described. GOES-8 and -9 data were better suited for stereoscopic measurements than data from previous satellites.
Abstract
Operational satellite data from GOES-8 and GOES-9 were used to make stereoscopic measurements of cloud heights during the National Aeronautics and Space Administration’s Subsonic Aircraft: Contrail and Cloud Effects Special Study program. The stereoscopic data were used to differentiate between boundary layer wave clouds and cirrus in the mid- and upper troposphere. This separation was difficult to evaluate from radiometric data alone. Stereographic cloud height analysis provided a definitive result. The technique used for calculating cloud heights is described. GOES-8 and -9 data were better suited for stereoscopic measurements than data from previous satellites.
Abstract
Generation of available potential energy (APE) is computed for the warm sector of an extratropical cyclone containing intense convection. Three hourly mesoscale rawinsonde data from the 10–11 April day of AVESESAME 1979 are used to evaluate generation by five diabatic components. Convective latent heat release is found to be the dominant diabatic term during times of intense convection, whereas stable latent heating provides a relatively small contribution. Sensible heat transfer is important near the surface during the afternoon. Solar and infrared radiative processes are quite significant in regions of low-level stratus and convective activity. Solar absorption during midday is observed to be much greater than at the standard rawinsonde observation times. The use of subjectively specified cloud data and sophisticated radiative transfer models permit more detailed resolution of cloud effects thin possible in earlier studies of this type.
Negative generation of APE is dominant during the 24 h period because convective latent heating is superimposed on areas of negative efficiency. The only consistent positive generation is due to infrared cooling. Sensible heating is the third largest generating component, while stable heating and solar absorption are least significant. Results document rapid temporal variations in generation as well as contrasts between energetics of the warm sector and those of entire cyclones.
Abstract
Generation of available potential energy (APE) is computed for the warm sector of an extratropical cyclone containing intense convection. Three hourly mesoscale rawinsonde data from the 10–11 April day of AVESESAME 1979 are used to evaluate generation by five diabatic components. Convective latent heat release is found to be the dominant diabatic term during times of intense convection, whereas stable latent heating provides a relatively small contribution. Sensible heat transfer is important near the surface during the afternoon. Solar and infrared radiative processes are quite significant in regions of low-level stratus and convective activity. Solar absorption during midday is observed to be much greater than at the standard rawinsonde observation times. The use of subjectively specified cloud data and sophisticated radiative transfer models permit more detailed resolution of cloud effects thin possible in earlier studies of this type.
Negative generation of APE is dominant during the 24 h period because convective latent heating is superimposed on areas of negative efficiency. The only consistent positive generation is due to infrared cooling. Sensible heating is the third largest generating component, while stable heating and solar absorption are least significant. Results document rapid temporal variations in generation as well as contrasts between energetics of the warm sector and those of entire cyclones.
Abstract
The structure and composition of three basic cirrus cloud types are examined through coordinated aircraft and ground-based polarization lidar and radar measurements. The cloud systems consist of a multilayered orographic cirrus, a 6-km deep cirrostratus, and a group of fibrous cirrus bands at the tropopause. The data reveal the presence of mesoscale generating regions with horizontal dimensions ranging from ∼15 km in narrow cloud bands up to ∼100 km in cirrostratus. These generating regions appear to be composed of complexes of much smaller convective structures, presumably on the ∼1-km scale of cirrus uncinus cells, and so are termed Mesoscale Uncinus Complexes (MUC). Accumulations of ice particles within cirrus, commonly referred to as precipitation trails, are associated with generating regions at or near cloud tops, but are also created by the local production of ice crystals within embedded convective impulses. Supercooled cloud droplets large enough to be detected by aircraft probes (≳5 μm diameter) were sampled in embedded convective cells near cloud base at temperatures ranging from −21° to −36°C. Ice particle nucleation at colder temperatures is assumed to involve the homogeneous freezing of haze particles too small to be detected by the aircraft probes employed, although they appear to have been detected by the polarization lidar technique under some conditions. Average ice mass contents are temperature dependent in a manner consistent with the conversion of a relatively small amount of excess water vapor (corresponding to ice supersaturations of a few percent) to ice mass.
Abstract
The structure and composition of three basic cirrus cloud types are examined through coordinated aircraft and ground-based polarization lidar and radar measurements. The cloud systems consist of a multilayered orographic cirrus, a 6-km deep cirrostratus, and a group of fibrous cirrus bands at the tropopause. The data reveal the presence of mesoscale generating regions with horizontal dimensions ranging from ∼15 km in narrow cloud bands up to ∼100 km in cirrostratus. These generating regions appear to be composed of complexes of much smaller convective structures, presumably on the ∼1-km scale of cirrus uncinus cells, and so are termed Mesoscale Uncinus Complexes (MUC). Accumulations of ice particles within cirrus, commonly referred to as precipitation trails, are associated with generating regions at or near cloud tops, but are also created by the local production of ice crystals within embedded convective impulses. Supercooled cloud droplets large enough to be detected by aircraft probes (≳5 μm diameter) were sampled in embedded convective cells near cloud base at temperatures ranging from −21° to −36°C. Ice particle nucleation at colder temperatures is assumed to involve the homogeneous freezing of haze particles too small to be detected by the aircraft probes employed, although they appear to have been detected by the polarization lidar technique under some conditions. Average ice mass contents are temperature dependent in a manner consistent with the conversion of a relatively small amount of excess water vapor (corresponding to ice supersaturations of a few percent) to ice mass.
Abstract
Airborne observations using a downward-looking, dual-frequency, near-infrared, differential absorption lidar (DIAL) system provide the first measurements of the height-dependent pressure-perturbation field associated with a strong mesoscale gravity wave. A pressure-perturbation amplitude of 3.5 mb was measured within the lowest 1.6 km of the atmosphere over a 52-km flight line. Corresponding vertical displacements of 250–500 m were inferred from lidar-observed displacement of aerosol layers. Accounting for probable wave orientation, a horizontal wavelength of about 40 km was estimated. Satellite observations reveal wave structure of a comparable scale in concurrent cirrus cloud fields over an extended area. Smaller-scale waves were also observed. Local meteorological soundings are analyzed to confirm the existence of a suitable wave duct. Potential wave-generation mechanisms are examined and discussed. The large pressure-perturbation wave is attributed to rapid amplification or possible wave breaking of a gravity wave as it propagated offshore and interacted with a very stable marine boundary layer capped by a strong shear layer.
Abstract
Airborne observations using a downward-looking, dual-frequency, near-infrared, differential absorption lidar (DIAL) system provide the first measurements of the height-dependent pressure-perturbation field associated with a strong mesoscale gravity wave. A pressure-perturbation amplitude of 3.5 mb was measured within the lowest 1.6 km of the atmosphere over a 52-km flight line. Corresponding vertical displacements of 250–500 m were inferred from lidar-observed displacement of aerosol layers. Accounting for probable wave orientation, a horizontal wavelength of about 40 km was estimated. Satellite observations reveal wave structure of a comparable scale in concurrent cirrus cloud fields over an extended area. Smaller-scale waves were also observed. Local meteorological soundings are analyzed to confirm the existence of a suitable wave duct. Potential wave-generation mechanisms are examined and discussed. The large pressure-perturbation wave is attributed to rapid amplification or possible wave breaking of a gravity wave as it propagated offshore and interacted with a very stable marine boundary layer capped by a strong shear layer.
Abstract
In this fifth of a series of papers describing the extended-time high cloud observation program from the University of Utah Facility for Atmospheric Remote Sensing, the structural properties of cirrus clouds over Salt Lake City, Utah, are examined. Wavelet analysis is applied to a 10-yr record of cirrus cloud ruby (0.694 μm) lidar backscatter data as a function of cloud height in order to study the presence of periodic cloud structures, such as the signatures of Kelvin–Helmholtz instabilities, cirrus mammata, and uncinus cells (all with length scales of ∼1–10 km), as well as mesoscale cloud organizations generally believed to be induced by gravity waves. About 8.4% of the data display structures after passing a 95% confidence level test, but an 80% confidence level, which seems better able to resolve structures spread over long periods, yields 16.4%. The amount of identified cloud structures does not change significantly with length scale from 0.2 to 200 km, although the frequency of mesoscale cloud structures tends to increase as length scales increase. The middle-to-lower portion of cirrus clouds contains the most identified cloud structures, which seems related to the mesoscale organization of fall streaks from cloud-top-generating cells. The variability of cirrus cloud optical depth τ (defined by the standard deviation over mean τ) derived from a combined lidar and infrared radiometer (LIRAD) analysis is shown to be largely independent of τ. Because visual examination of the lidar displays also indicates that few cirrus layers can be considered horizontally homogeneous over our typical 3-h lidar data collection period, the authors conclude that the clouds in their sample are inherently inhomogeneous even though most cirrus structures are not revealed as periodic by wavelet analysis.
Abstract
In this fifth of a series of papers describing the extended-time high cloud observation program from the University of Utah Facility for Atmospheric Remote Sensing, the structural properties of cirrus clouds over Salt Lake City, Utah, are examined. Wavelet analysis is applied to a 10-yr record of cirrus cloud ruby (0.694 μm) lidar backscatter data as a function of cloud height in order to study the presence of periodic cloud structures, such as the signatures of Kelvin–Helmholtz instabilities, cirrus mammata, and uncinus cells (all with length scales of ∼1–10 km), as well as mesoscale cloud organizations generally believed to be induced by gravity waves. About 8.4% of the data display structures after passing a 95% confidence level test, but an 80% confidence level, which seems better able to resolve structures spread over long periods, yields 16.4%. The amount of identified cloud structures does not change significantly with length scale from 0.2 to 200 km, although the frequency of mesoscale cloud structures tends to increase as length scales increase. The middle-to-lower portion of cirrus clouds contains the most identified cloud structures, which seems related to the mesoscale organization of fall streaks from cloud-top-generating cells. The variability of cirrus cloud optical depth τ (defined by the standard deviation over mean τ) derived from a combined lidar and infrared radiometer (LIRAD) analysis is shown to be largely independent of τ. Because visual examination of the lidar displays also indicates that few cirrus layers can be considered horizontally homogeneous over our typical 3-h lidar data collection period, the authors conclude that the clouds in their sample are inherently inhomogeneous even though most cirrus structures are not revealed as periodic by wavelet analysis.
Abstract
The evolution of synoptic-scale dynamics associated with a middle and upper tropospheric cloud event that occurred on 26 November 1991 is examined. The case under consideration occurred during the FIRE Cirrus-II Intensive Field Observing Period held in Coffeyville, Kansas, during November–December 1991. Using data from the wind profiler demonstration network and a temporarlly and spatially augmented radiosonde array, emphasis is given to explaining the evolution of the kinematically derived ageostrophic vertical circulations and correlating the circulation with the forcing of an extensively sampled cloud field. This is facilitated by decomposing the horizontal divergence into its component parts through a natural coordinate representation of the flow. Ageostrophic vertical circulations are inferred and compared to the circulation forcing arising from geostrophic confluence and shearing deformation derived from the Sawyer–Eliassen equation. It is found that a thermodynamically indirect vertical circulation existed in association with a jet streak exit region. The circulation was displaced to the cyclonic side of the jet axis due to the orientation of the jet exit between a deepening diffluent trough and a building ridge. The cloud line formed in the ascending branch of the vertical circulation, with the most concentrated cloud development occurring in conjunction with the maximum large-scale vertical motion. The relationship between the large-scale dynamics and the parameterization of middle and upper tropospheric clouds in large-scale models is discussed, and an example of ice water contents derived from a parameterization forced by the diagnosed vertical motions and observed water vapor contents is presented.
Abstract
The evolution of synoptic-scale dynamics associated with a middle and upper tropospheric cloud event that occurred on 26 November 1991 is examined. The case under consideration occurred during the FIRE Cirrus-II Intensive Field Observing Period held in Coffeyville, Kansas, during November–December 1991. Using data from the wind profiler demonstration network and a temporarlly and spatially augmented radiosonde array, emphasis is given to explaining the evolution of the kinematically derived ageostrophic vertical circulations and correlating the circulation with the forcing of an extensively sampled cloud field. This is facilitated by decomposing the horizontal divergence into its component parts through a natural coordinate representation of the flow. Ageostrophic vertical circulations are inferred and compared to the circulation forcing arising from geostrophic confluence and shearing deformation derived from the Sawyer–Eliassen equation. It is found that a thermodynamically indirect vertical circulation existed in association with a jet streak exit region. The circulation was displaced to the cyclonic side of the jet axis due to the orientation of the jet exit between a deepening diffluent trough and a building ridge. The cloud line formed in the ascending branch of the vertical circulation, with the most concentrated cloud development occurring in conjunction with the maximum large-scale vertical motion. The relationship between the large-scale dynamics and the parameterization of middle and upper tropospheric clouds in large-scale models is discussed, and an example of ice water contents derived from a parameterization forced by the diagnosed vertical motions and observed water vapor contents is presented.
