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This paper grew out of two ad hoc conferences held last year to discuss the problem of estimating precipitation from satellite data. Although developments in an area as new as this are rapid, there was sufficient interest in the problem, progress towards a solution, and ignorance of the work of others to warrant drawing the conference summaries together in a review paper of wider circulation and general interest.
Our purpose is to examine, from the perspective of GATE rainfall requirements, methodologies in use and under development for the estimation of rainfall from satellite visible and infrared images. As an introduction to critical descriptions of each method, we touch upon the uses of rainfall observations in GATE, the advantages of satellite monitoring, and the purposes of the two conferences held to date. A discussion of calibration and accuracy requirements, including ground truth and accuracies for GATE, is followed by descriptions of two field programs—one completed, the other planned—which have provided and should provide excellent “ground truth” data. We conclude with a discussion of remaining problems and recommendations for observations in GATE.
This paper grew out of two ad hoc conferences held last year to discuss the problem of estimating precipitation from satellite data. Although developments in an area as new as this are rapid, there was sufficient interest in the problem, progress towards a solution, and ignorance of the work of others to warrant drawing the conference summaries together in a review paper of wider circulation and general interest.
Our purpose is to examine, from the perspective of GATE rainfall requirements, methodologies in use and under development for the estimation of rainfall from satellite visible and infrared images. As an introduction to critical descriptions of each method, we touch upon the uses of rainfall observations in GATE, the advantages of satellite monitoring, and the purposes of the two conferences held to date. A discussion of calibration and accuracy requirements, including ground truth and accuracies for GATE, is followed by descriptions of two field programs—one completed, the other planned—which have provided and should provide excellent “ground truth” data. We conclude with a discussion of remaining problems and recommendations for observations in GATE.
Abstract
Observations from the Meteorological Research Flight's Hercules C-130 aircraft of the microphysical characteristics of warm stratocumulus clouds have been analyzed to investigate the variation of the effective radius of cloud droplets in layer clouds. Results from experiments in the eastern Pacific, South Atlantic, subtropical regions of the North Atlantic, and the sea areas around the British Isles are presented. In situations where entrainment effects are small the (effective radius)3 is found to be a linear function of the (volume-averaged radius)3 in a given cloud and can thus be parameterized with respect to the liquid water content and the droplet number concentration in the cloud. However, the shape of the droplet size spectrum is very dependent on the cloud condensation nuclei (CCN) characteristics below cloud base, and the relationship between effective radius and volume-averaged radius varies between maritime air masses and continental air masses. This study also details comparisons that have been made in stratocumulus between the droplet number concentrations and (a) aerosol concentrations below cloud base in the size range 0.1 to 3.0 μm and (b) CCN supersaturation spectra in the boundary layer. A parameterization relating droplet concentration and aerosol concentration is suggested. The effects of nonadiabatic processes on the parameterization of effective radius are discussed. Drizzle is found to have little effect near cloud top, but in precipitating stratocumulus clouds the parameterization breaks down near cloud base. Comparisons are made between this parameterization of effective radius and others used currently or in the past.
Abstract
Observations from the Meteorological Research Flight's Hercules C-130 aircraft of the microphysical characteristics of warm stratocumulus clouds have been analyzed to investigate the variation of the effective radius of cloud droplets in layer clouds. Results from experiments in the eastern Pacific, South Atlantic, subtropical regions of the North Atlantic, and the sea areas around the British Isles are presented. In situations where entrainment effects are small the (effective radius)3 is found to be a linear function of the (volume-averaged radius)3 in a given cloud and can thus be parameterized with respect to the liquid water content and the droplet number concentration in the cloud. However, the shape of the droplet size spectrum is very dependent on the cloud condensation nuclei (CCN) characteristics below cloud base, and the relationship between effective radius and volume-averaged radius varies between maritime air masses and continental air masses. This study also details comparisons that have been made in stratocumulus between the droplet number concentrations and (a) aerosol concentrations below cloud base in the size range 0.1 to 3.0 μm and (b) CCN supersaturation spectra in the boundary layer. A parameterization relating droplet concentration and aerosol concentration is suggested. The effects of nonadiabatic processes on the parameterization of effective radius are discussed. Drizzle is found to have little effect near cloud top, but in precipitating stratocumulus clouds the parameterization breaks down near cloud base. Comparisons are made between this parameterization of effective radius and others used currently or in the past.
Abstract
In an examination of microwave data from the Nimbus 7 satellite, brightness temperatures were found that were much lower than those expected for the radiation emanating from rain-producing clouds. Every case of very cold brightness temperature coincided with heavy thunderstorm rainfall. The cold temperatures can be attributed to scattering by a layer of ice hydrometeors in the upper parts of the storms. Thus it appears that brightness temperatures observed by satellite microwave radiometers can at times distinguish heavy rain over land.
Abstract
In an examination of microwave data from the Nimbus 7 satellite, brightness temperatures were found that were much lower than those expected for the radiation emanating from rain-producing clouds. Every case of very cold brightness temperature coincided with heavy thunderstorm rainfall. The cold temperatures can be attributed to scattering by a layer of ice hydrometeors in the upper parts of the storms. Thus it appears that brightness temperatures observed by satellite microwave radiometers can at times distinguish heavy rain over land.
Abstract
Satellite images that illustrate the clearing of cumulus clouds downwind from snow-covered areas are presented. The cloud clearing resembles that occasionally observed with lakes during warm advection, supporting the suggestion that the thermal forcing associated with a uniform snow-covered area is comparable to that of a cold-water lake of similar size. Analysis of snow cover patterns in the central United States suggests that the climatological probability for situations conducive to the cloud clearing is at most once per month.
Abstract
Satellite images that illustrate the clearing of cumulus clouds downwind from snow-covered areas are presented. The cloud clearing resembles that occasionally observed with lakes during warm advection, supporting the suggestion that the thermal forcing associated with a uniform snow-covered area is comparable to that of a cold-water lake of similar size. Analysis of snow cover patterns in the central United States suggests that the climatological probability for situations conducive to the cloud clearing is at most once per month.
Abstract
On 18 September 1974, a cloud cluster growing in the GATE ship array was examined using aircraft flying close to one another at different heights, the geostationary satellite SMS-1, and radar, rawinsonde and ship data, with a view to elucidating mechanisms of convection. In this paper we concentrate analysis on cloudy convection in the moist layer.
In and above southerly surface monsoon flow approaching the cluster, clouds indigenous to the moist layer took the form of rows of tiny cumulus, and of arcs of cumulus mediocris, with patterns different from those of deeper clouds. From satellite visible images, arcs were traced for periods exceeding 2 h. Airborne photography showed that the arcs were composed of many small clouds. Radar data showed that they originated after precipitation. Apparently, throughout their life cycle, they perpetuated the pattern of an initiating dense downdraft. Eventually they yielded isolated cumulus congestus, again bearing precipitation. Aircraft recorded the distribution of thermodynamic quantities and winds at altitudes within the mixed layer, and at 537 and 1067 m. These data indicated that the arcs persisted as mesoscale circulations driven by release of latent heat in the clouds, rather than being driven by the original density current at the surface. The cloudy circulations were vigorous near and above cloud base, becoming weaker upward through altitude 1 km. The entire mesoscale circulation systems were of horizontal scale roughly 40 km.
The mesoscale cloud patterns of the moist layer appeared to play a primary role in heat transfer upward within this layer, and contributed to the forcing of showering midtropospheric clouds.
Abstract
On 18 September 1974, a cloud cluster growing in the GATE ship array was examined using aircraft flying close to one another at different heights, the geostationary satellite SMS-1, and radar, rawinsonde and ship data, with a view to elucidating mechanisms of convection. In this paper we concentrate analysis on cloudy convection in the moist layer.
In and above southerly surface monsoon flow approaching the cluster, clouds indigenous to the moist layer took the form of rows of tiny cumulus, and of arcs of cumulus mediocris, with patterns different from those of deeper clouds. From satellite visible images, arcs were traced for periods exceeding 2 h. Airborne photography showed that the arcs were composed of many small clouds. Radar data showed that they originated after precipitation. Apparently, throughout their life cycle, they perpetuated the pattern of an initiating dense downdraft. Eventually they yielded isolated cumulus congestus, again bearing precipitation. Aircraft recorded the distribution of thermodynamic quantities and winds at altitudes within the mixed layer, and at 537 and 1067 m. These data indicated that the arcs persisted as mesoscale circulations driven by release of latent heat in the clouds, rather than being driven by the original density current at the surface. The cloudy circulations were vigorous near and above cloud base, becoming weaker upward through altitude 1 km. The entire mesoscale circulation systems were of horizontal scale roughly 40 km.
The mesoscale cloud patterns of the moist layer appeared to play a primary role in heat transfer upward within this layer, and contributed to the forcing of showering midtropospheric clouds.
Abstract
Aircraft, radar, satellite and ship data, gathered on 18 September 1974 during GATE, have been brought to bear an clouds of the middle and upper troposphere associated with a cloud cluster occurring near the ridge axis of a 700 mb wave.
Clouds penetrating above 2.5 km into the middle troposphere were organized in bands about 9 km apart, aligned roughly along the direction of the wind shear in the cloud layer. Radar echoes corresponding to the cumulus congestus were of lifetime roughly 30 min, top height 6 km and peak rainfall rate 1.3 mm h−1. The number density of such echoes increased from one in 15 000 km2 to a value about three times greater, while convergence at ∼950 hPa (obtained from satellite cloud tracking) increased from about 1.5 to 3 × 10−5 s−1.
Convergence into a square box of side 150 km, circumnavigated by three aircraft in the moist layer, reached about 3.5 × 10−5 s−1 near cloud base. The upward flux of water vapor at cloud base was about 0.25 g m−2 s−1, equivalent to rain of intensity 0.9 mm h−1 with 100% conversion of the vapor. During this time mean rainfall rates over the area, and peaks averaged over 18 km2, increased from 0.2 to 0.7, and 23 to 39 mm h−1, respectively. Areas of small rainfall rate merged. High towers became taller and more numerous, but remained the same size, ∼50 km2 in area at altitude 7 km, for echo cores of intensity 29 dBZ.
A gust front at the ship Oceanographer was associated with one of the cloud bands., it was found that the band propagated discontinuously by new growth in its leading side. It featured a mesoscale pattern of updrafts in front and downdrafts behind, the downdrafts originating near altitude 2.5–3 km. A tentative conclusion is drawn that convective circulations tended to generate horizontal momentum near cloud base. The longitudinal rolls obtained theoretically by Sun (1978), for conditions of relatively strong buoyancy, match the observed bands well.
Cumulonimbus clouds reaching 15 km grew only out of an environment already moistened by lesser clouds. Cloud towers a few kilometers wide were photographed. Such towers were linked with groups of echoes identified from a high-resolution display of three-dimensional radar scans. This “wall chart” revealed that echoes were multicellular, and moved with widely differing velocities. It is deduced that individual echo groups yielded local winds of speed exceeding those in the environment. The lesser echo groups were part of a population having log-normally distributed properties. Those which yielded reflectivities as high as 46 dBZ were a different population; they were elongated norlh-northeast to south-southwest, a direction corresponding to that of a confluence asymptote at ∼950 hpa discerned from satellite data.
Abstract
Aircraft, radar, satellite and ship data, gathered on 18 September 1974 during GATE, have been brought to bear an clouds of the middle and upper troposphere associated with a cloud cluster occurring near the ridge axis of a 700 mb wave.
Clouds penetrating above 2.5 km into the middle troposphere were organized in bands about 9 km apart, aligned roughly along the direction of the wind shear in the cloud layer. Radar echoes corresponding to the cumulus congestus were of lifetime roughly 30 min, top height 6 km and peak rainfall rate 1.3 mm h−1. The number density of such echoes increased from one in 15 000 km2 to a value about three times greater, while convergence at ∼950 hPa (obtained from satellite cloud tracking) increased from about 1.5 to 3 × 10−5 s−1.
Convergence into a square box of side 150 km, circumnavigated by three aircraft in the moist layer, reached about 3.5 × 10−5 s−1 near cloud base. The upward flux of water vapor at cloud base was about 0.25 g m−2 s−1, equivalent to rain of intensity 0.9 mm h−1 with 100% conversion of the vapor. During this time mean rainfall rates over the area, and peaks averaged over 18 km2, increased from 0.2 to 0.7, and 23 to 39 mm h−1, respectively. Areas of small rainfall rate merged. High towers became taller and more numerous, but remained the same size, ∼50 km2 in area at altitude 7 km, for echo cores of intensity 29 dBZ.
A gust front at the ship Oceanographer was associated with one of the cloud bands., it was found that the band propagated discontinuously by new growth in its leading side. It featured a mesoscale pattern of updrafts in front and downdrafts behind, the downdrafts originating near altitude 2.5–3 km. A tentative conclusion is drawn that convective circulations tended to generate horizontal momentum near cloud base. The longitudinal rolls obtained theoretically by Sun (1978), for conditions of relatively strong buoyancy, match the observed bands well.
Cumulonimbus clouds reaching 15 km grew only out of an environment already moistened by lesser clouds. Cloud towers a few kilometers wide were photographed. Such towers were linked with groups of echoes identified from a high-resolution display of three-dimensional radar scans. This “wall chart” revealed that echoes were multicellular, and moved with widely differing velocities. It is deduced that individual echo groups yielded local winds of speed exceeding those in the environment. The lesser echo groups were part of a population having log-normally distributed properties. Those which yielded reflectivities as high as 46 dBZ were a different population; they were elongated norlh-northeast to south-southwest, a direction corresponding to that of a confluence asymptote at ∼950 hpa discerned from satellite data.
Abstract
Decoupling of the marine boundary layer beneath stratocumulus clouds and the formation of cumulus clouds at the top of a surface-based mixed layer (SML) have frequently been observed and modeled. When such cumulus clouds penetrate the overlying stratocumulus layer, the cloud microphysics and hence the radiative properties of the cloud are altered locally. Observations made during a series of Lagrangian experiments in the Azores as part of the Atlantic Stratocumulus Transition Experiment (ASTEX, June 1992) have been analyzed to ascertain how the properties of a stratocumulus layer with which cumulus clouds are interacting differ from those of an unaffected cloud layer. The results suggest that in regions where cumulus clouds penetrate the cloud layer, the stratocumulus is thickened as the cumuli spread out into its base. Transport of air from the SML into the cloud by convective updrafts is observed, and the increase in available moisture within the penetrating cumulus clouds results in increased liquid water content and hence changes in the droplet size spectra. The greater liquid water path results in a larger cloud optical depth, so that regions where cumulus are interesting with the stratocumulus layer can be observed in satellite measurements. Therefore, it is likely that the surface energy budget may be significantly altered by this process, and it may be necessary to parameterize these effects in large-scale numerical models.
Abstract
Decoupling of the marine boundary layer beneath stratocumulus clouds and the formation of cumulus clouds at the top of a surface-based mixed layer (SML) have frequently been observed and modeled. When such cumulus clouds penetrate the overlying stratocumulus layer, the cloud microphysics and hence the radiative properties of the cloud are altered locally. Observations made during a series of Lagrangian experiments in the Azores as part of the Atlantic Stratocumulus Transition Experiment (ASTEX, June 1992) have been analyzed to ascertain how the properties of a stratocumulus layer with which cumulus clouds are interacting differ from those of an unaffected cloud layer. The results suggest that in regions where cumulus clouds penetrate the cloud layer, the stratocumulus is thickened as the cumuli spread out into its base. Transport of air from the SML into the cloud by convective updrafts is observed, and the increase in available moisture within the penetrating cumulus clouds results in increased liquid water content and hence changes in the droplet size spectra. The greater liquid water path results in a larger cloud optical depth, so that regions where cumulus are interesting with the stratocumulus layer can be observed in satellite measurements. Therefore, it is likely that the surface energy budget may be significantly altered by this process, and it may be necessary to parameterize these effects in large-scale numerical models.
Abstract
We present the evolving fields of a Saharan dust outbreak and its associated air mass, as the outbreak and the air mass moved across the Atlantic over a nine-day period. New details are revealed from high-resolution satellite measurements and ship soundings from GATE [Global Atmospheric Research Program (GARP) Atlantic Tropical Experiment].
A McIDAS (Man Computer Interactive Data Access System) turbidity analysis was produced for each day following dust exit from the African coast. The digital fields are subjected to local space and time continuity checks in order to minimize contamination from small clouds and other error sources. Quantitative turbidity fields show the systematic spreading of the dusty air, its gradual depletion, and its relation to synoptic cloud systems.
A clearer picture of the changing structure of the elevated Saharan air lens is made possible by the GATE analyses. High-resolution thermodynamic analyses in vertical planes and the time domain demonstrate the sharp southern boundary and detailed variations in passage across the ocean. Taken together with wind data, the results show the transit of dust and air mass together across the Atlantic, with an unsteady relation to the adjacent easterly wave circulations.
Abstract
We present the evolving fields of a Saharan dust outbreak and its associated air mass, as the outbreak and the air mass moved across the Atlantic over a nine-day period. New details are revealed from high-resolution satellite measurements and ship soundings from GATE [Global Atmospheric Research Program (GARP) Atlantic Tropical Experiment].
A McIDAS (Man Computer Interactive Data Access System) turbidity analysis was produced for each day following dust exit from the African coast. The digital fields are subjected to local space and time continuity checks in order to minimize contamination from small clouds and other error sources. Quantitative turbidity fields show the systematic spreading of the dusty air, its gradual depletion, and its relation to synoptic cloud systems.
A clearer picture of the changing structure of the elevated Saharan air lens is made possible by the GATE analyses. High-resolution thermodynamic analyses in vertical planes and the time domain demonstrate the sharp southern boundary and detailed variations in passage across the ocean. Taken together with wind data, the results show the transit of dust and air mass together across the Atlantic, with an unsteady relation to the adjacent easterly wave circulations.
