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- Author or Editor: F. Mosher x
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Abstract
Measurements from the NOAA Boulder Atmospheric Observatory (BAO) 300 m tower, the National Center for Atmospheric Research (NCAR) Sabreliner aircraft, and the NOAA GOES-5 satellite, give evidence for the cross-front scale collapse of nonprecipitating surface cold-frontal zones to horizontal distances of ∼1 km or less. The leading edges of these frosts possess the characteristic structure of density current flows: an elevated hydraulic head followed by a turbulent wake. Vertical motions at the frontal heads exceed 5 m s−1 at 300 m (AGL). The ascent at the frontal head may act as a (∼1 km-scale) triggering mechanism for the release of potential instability and the formation of intense squall-line mesoconvection.
Abstract
Measurements from the NOAA Boulder Atmospheric Observatory (BAO) 300 m tower, the National Center for Atmospheric Research (NCAR) Sabreliner aircraft, and the NOAA GOES-5 satellite, give evidence for the cross-front scale collapse of nonprecipitating surface cold-frontal zones to horizontal distances of ∼1 km or less. The leading edges of these frosts possess the characteristic structure of density current flows: an elevated hydraulic head followed by a turbulent wake. Vertical motions at the frontal heads exceed 5 m s−1 at 300 m (AGL). The ascent at the frontal head may act as a (∼1 km-scale) triggering mechanism for the release of potential instability and the formation of intense squall-line mesoconvection.
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
Three algorithms based on geostationary visible and infrared (IR) observations are used to identify convective cells that do (or may) present a hazard to aviation over the oceans. The performance of these algorithms in detecting potentially hazardous cells is determined through verification with Tropical Rainfall Measuring Mission (TRMM) satellite observations of lightning and radar reflectivity, which provide internal information about the convective cells. The probability of detection of hazardous cells using the satellite algorithms can exceed 90% when lightning is used as a criterion for hazard, but the false-alarm ratio with all three algorithms is consistently large (∼40%), thereby exaggerating the presence of hazardous conditions. This shortcoming results in part from the algorithms’ dependence upon visible and IR observations, and can be traced to the widespread prevalence of deep cumulonimbi with weak updrafts but without lightning over tropical oceans, whose origin is attributed to significant entrainment during ascent.
Abstract
Three algorithms based on geostationary visible and infrared (IR) observations are used to identify convective cells that do (or may) present a hazard to aviation over the oceans. The performance of these algorithms in detecting potentially hazardous cells is determined through verification with Tropical Rainfall Measuring Mission (TRMM) satellite observations of lightning and radar reflectivity, which provide internal information about the convective cells. The probability of detection of hazardous cells using the satellite algorithms can exceed 90% when lightning is used as a criterion for hazard, but the false-alarm ratio with all three algorithms is consistently large (∼40%), thereby exaggerating the presence of hazardous conditions. This shortcoming results in part from the algorithms’ dependence upon visible and IR observations, and can be traced to the widespread prevalence of deep cumulonimbi with weak updrafts but without lightning over tropical oceans, whose origin is attributed to significant entrainment during ascent.
Abstract
The International Satellite Cloud Climatology Project (ISCCP) will provide a uniform global climatology of satellite-measured radiances and derive an experimental climatology of cloud radiative properties from these radiances. A pilot study to intercompare cloud analysis algorithms was initiated in 1981 to define a state-of-the-art algorithm for ISCCP. This study compared the results of applying six different algorithms to the same satellite radiance data. The results show that the performance of all current algorithms depends on how accurately the clear sky radiances are specified; much improvement in results is possible with better methods for obtaining these clear-sky radiances. A major difference between the algorithms is caused by their sensitivity to changes in the cloud size distribution and optical properties: all methods, which work well for some cloud types or climate regions, do poorly for other situations. Therefore, the ISCCP algorithm is composed of a series of steps, each of which is designed to detect some of the clouds present in the scene. This progressive analysis is used to retrieve an estimate of the clear sky radiances corresponding to each satellite image. Application of a bispectral threshold is then used as the last step to determine the cloud fraction. Cloudy radiances are interpreted in terms of a simplified model of cloud radiative effects to provide some measure of cloud radiative properties. Application of this experimental algorithm to produce a cloud climatology and field observation programs to validate the results will stimulate further research on cloud analysis techniques as part of ISCCP.
Abstract
The International Satellite Cloud Climatology Project (ISCCP) will provide a uniform global climatology of satellite-measured radiances and derive an experimental climatology of cloud radiative properties from these radiances. A pilot study to intercompare cloud analysis algorithms was initiated in 1981 to define a state-of-the-art algorithm for ISCCP. This study compared the results of applying six different algorithms to the same satellite radiance data. The results show that the performance of all current algorithms depends on how accurately the clear sky radiances are specified; much improvement in results is possible with better methods for obtaining these clear-sky radiances. A major difference between the algorithms is caused by their sensitivity to changes in the cloud size distribution and optical properties: all methods, which work well for some cloud types or climate regions, do poorly for other situations. Therefore, the ISCCP algorithm is composed of a series of steps, each of which is designed to detect some of the clouds present in the scene. This progressive analysis is used to retrieve an estimate of the clear sky radiances corresponding to each satellite image. Application of a bispectral threshold is then used as the last step to determine the cloud fraction. Cloudy radiances are interpreted in terms of a simplified model of cloud radiative effects to provide some measure of cloud radiative properties. Application of this experimental algorithm to produce a cloud climatology and field observation programs to validate the results will stimulate further research on cloud analysis techniques as part of ISCCP.
First results are presented from an experiment to sound the atmosphere's temperature and moisture distribution from a geostationary satellite. Sounding inferences in clear and partially cloudy conditions have the anticipated accuracy and horizontal and vertical resolutions. Most important is the preliminary indication that small but significant temporal variations of atmospheric temperature and moisture can be observed by the geostationary satellite sounder. Quantitative assessment of the accuracy and meteorological utility of this new sounding capability must await the accumulation of results over the coming months.
First results are presented from an experiment to sound the atmosphere's temperature and moisture distribution from a geostationary satellite. Sounding inferences in clear and partially cloudy conditions have the anticipated accuracy and horizontal and vertical resolutions. Most important is the preliminary indication that small but significant temporal variations of atmospheric temperature and moisture can be observed by the geostationary satellite sounder. Quantitative assessment of the accuracy and meteorological utility of this new sounding capability must await the accumulation of results over the coming months.
