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Abstract
Analysis of wind, pressure and cloudiness patterns shows that the weather of Southeast Asia during the southwest monsoon relates best to westward-moving equatorial waves. These waves are basically the same as those found over the western Pacific by other investigators although they are often not perceived without special analysis as they move through the vigorous monsoonal circulation. They are characterized by average wavelength of 3100 km, average period of 5½ days or speed of 6.5 m sec−1, a deep cool center from the surface to about the 300-mb level, and a deep band of convergence and cloudiness which feeds into the center from the southwest and south. Relatively good weather with lower amounts of cloudiness and areal precipitation coverage is normally found in the area of synoptic-scale subsidence between waves.
Abstract
Analysis of wind, pressure and cloudiness patterns shows that the weather of Southeast Asia during the southwest monsoon relates best to westward-moving equatorial waves. These waves are basically the same as those found over the western Pacific by other investigators although they are often not perceived without special analysis as they move through the vigorous monsoonal circulation. They are characterized by average wavelength of 3100 km, average period of 5½ days or speed of 6.5 m sec−1, a deep cool center from the surface to about the 300-mb level, and a deep band of convergence and cloudiness which feeds into the center from the southwest and south. Relatively good weather with lower amounts of cloudiness and areal precipitation coverage is normally found in the area of synoptic-scale subsidence between waves.
Abstract
In order to place cloud interpretation from the TIROS satellite pictures on a more objective basis, methods have been developed for computing radiance and albedos from the pictures. Provision was made for multiple scattering in a Rayleigh atmosphere and absorption due to 0.28 cm of ozone. Effects due to Mie scattering were not included.
Corrections have been determined for variations at the satellite across the filter-lens-videcon system, the shutter speeds and warm-up of the TV system and for variations at the ground across the cathode-ray tube-camera system. The photometry on test cases made use of rerun video tapes and controlled film sensitometry. Simultaneous with the times of satellite pictures, other pictures of the same clouds were obtained from a U-2 aircraft. Radiance and albedos of the reflecting surfaces in these pictures was determined in a manner similar to that from the satellite pictures. A comparison of albedos for the respective spectral response of the systems shows good agreement, although the values at high radiance levels appear more realistic when measured by the satellite.
The standard deviation of a series of readings, from successive pictures, on the same reflecting cloud surface averaged about 15 per cent of the radiance. From this series of pictures, variations in albedo that might be due to non-Lambertian reflecting surfaces and changes in nadir viewing angle could not be isolated from the “noise” of the system.
A summary of average albedos, determined from TIROS, for cloud types and terrestrial surfaces is presented. Typical albedos range from 32 to 36 per cent for cirrus and cirrostratus over land and from about 18 to 92 per cent for giant cumulonimbus.
The application to TIROS photography of similar corrections and techniques will permit digital computation of albedo in “real time” and its presentation in the form of rectified mosaics. These digital data are also suitable for insertion into computer forecasts, compilation of climatological data and other special applications.
Abstract
In order to place cloud interpretation from the TIROS satellite pictures on a more objective basis, methods have been developed for computing radiance and albedos from the pictures. Provision was made for multiple scattering in a Rayleigh atmosphere and absorption due to 0.28 cm of ozone. Effects due to Mie scattering were not included.
Corrections have been determined for variations at the satellite across the filter-lens-videcon system, the shutter speeds and warm-up of the TV system and for variations at the ground across the cathode-ray tube-camera system. The photometry on test cases made use of rerun video tapes and controlled film sensitometry. Simultaneous with the times of satellite pictures, other pictures of the same clouds were obtained from a U-2 aircraft. Radiance and albedos of the reflecting surfaces in these pictures was determined in a manner similar to that from the satellite pictures. A comparison of albedos for the respective spectral response of the systems shows good agreement, although the values at high radiance levels appear more realistic when measured by the satellite.
The standard deviation of a series of readings, from successive pictures, on the same reflecting cloud surface averaged about 15 per cent of the radiance. From this series of pictures, variations in albedo that might be due to non-Lambertian reflecting surfaces and changes in nadir viewing angle could not be isolated from the “noise” of the system.
A summary of average albedos, determined from TIROS, for cloud types and terrestrial surfaces is presented. Typical albedos range from 32 to 36 per cent for cirrus and cirrostratus over land and from about 18 to 92 per cent for giant cumulonimbus.
The application to TIROS photography of similar corrections and techniques will permit digital computation of albedo in “real time” and its presentation in the form of rectified mosaics. These digital data are also suitable for insertion into computer forecasts, compilation of climatological data and other special applications.
Abstract
Orographic cloud patterns photographed by satellites are classified as wave-like, isolated lenticular, fibrous plume, and large single-line. Other orographic patterns consisting of rotor or cap clouds are not included because they cannot be resolved by the present satellite systems. Characteristics of form (i.e., cumuliform or fibrous), brightness pattern (i.e., bands, eddies, etc.), size and wavelength, when applicable, are used in classifying the clouds. Interpretations of each pattern, as complete as is possible from the satellite pictures, are given in terms of cloud types, heights and thickness, lapse rates, wind velocity and vertical shear. It is not expected that further information can be deduced from the patterns photographed until the atmospheric parameters are measured within, above and below the cloud layers.
Abstract
Orographic cloud patterns photographed by satellites are classified as wave-like, isolated lenticular, fibrous plume, and large single-line. Other orographic patterns consisting of rotor or cap clouds are not included because they cannot be resolved by the present satellite systems. Characteristics of form (i.e., cumuliform or fibrous), brightness pattern (i.e., bands, eddies, etc.), size and wavelength, when applicable, are used in classifying the clouds. Interpretations of each pattern, as complete as is possible from the satellite pictures, are given in terms of cloud types, heights and thickness, lapse rates, wind velocity and vertical shear. It is not expected that further information can be deduced from the patterns photographed until the atmospheric parameters are measured within, above and below the cloud layers.
Abstract
No abstract available.
Abstract
No abstract available.
Abstract
Cirrus bands tend to parallel the jet stream. When bands occur, 50 per cent of the time they are found within 370 km (200 n mi) of the jet core, on the warm side and in the entrance or neutral areas. Sharp northern edges of cirrus sheets are generally found about 100 km south of the jet core. Cirrus is often found on both sides of the jet when the jet is weak or when the jet is south of another jet. The high-level temperature discontinuities tend to “wrap around” the jets from about 4 km below the jet core to the level of the core 50 to 400 km to the north.
The observed cirrus bands have been classified into three categories according to average spacings between bands: large scale, 185 km apart, major bands, 4.2 km apart, and minor bands 0.37 km apart. Known maximum band lengths are 650 km, 148 km, and a few kilometers, respectively. Cloud-cell spacings along the major bands average 1.3 km.
The major bands are created between parallel horizontal vortices through which the air flows in a helical motion. The vortices rotate in opposite directions and cloud forms where the combined motion is upward. The resulting clouds indicate conditional instability in all cases, and their form apparently depends on the steepness of the lapse rate and prevailing vertical shear. Maximum lateral components of motion measured across the tops of the vortices ranges from 0.6 to 6.0 m sec−1. When these components are combined with the forward speeds, a median angle of 4.4 deg from the large-scale wind direction is found across the tops of the vortices.
Abstract
Cirrus bands tend to parallel the jet stream. When bands occur, 50 per cent of the time they are found within 370 km (200 n mi) of the jet core, on the warm side and in the entrance or neutral areas. Sharp northern edges of cirrus sheets are generally found about 100 km south of the jet core. Cirrus is often found on both sides of the jet when the jet is weak or when the jet is south of another jet. The high-level temperature discontinuities tend to “wrap around” the jets from about 4 km below the jet core to the level of the core 50 to 400 km to the north.
The observed cirrus bands have been classified into three categories according to average spacings between bands: large scale, 185 km apart, major bands, 4.2 km apart, and minor bands 0.37 km apart. Known maximum band lengths are 650 km, 148 km, and a few kilometers, respectively. Cloud-cell spacings along the major bands average 1.3 km.
The major bands are created between parallel horizontal vortices through which the air flows in a helical motion. The vortices rotate in opposite directions and cloud forms where the combined motion is upward. The resulting clouds indicate conditional instability in all cases, and their form apparently depends on the steepness of the lapse rate and prevailing vertical shear. Maximum lateral components of motion measured across the tops of the vortices ranges from 0.6 to 6.0 m sec−1. When these components are combined with the forward speeds, a median angle of 4.4 deg from the large-scale wind direction is found across the tops of the vortices.
Abstract
Satellite cloud photography occasionally reveals anomalous cloud lines in the form of plumes often 500 km long and up to 25 km wide. These lines, found over the ocean, are composed of liquid particles. Hollow polygonal, convective or thin stratiform, cloud patterns in varying amounts are associated with the lines.
Numerous lines within a 10-deg square have been observed which cover up to 4 per cent of the total area.
The most likely cause of the cloud lines stems from the exhaust of ocean going vessels. Large numbers of Aitken nuclei form in this exhaust. These are carried upward by the buoyancy of the hot gases and “ships air wake” to form droplets at slight supersaturation. The phenomenon does not appear related to special characteristics of the vessel's power plant but to a critical condition of the atmosphere. As far as is known, this condition may be described as having 1) a convectively unstable layer from the surface to a low-level stable layer, 2) saturation or slight supersaturation near the top of the convective layer, and 3) a convective layer, presumably deficient in cloud forming nuclei.
Nuclei are produced in sufficient quantity by an average size ship, moving at 10 m sec−1 relative to the wind, to generate 200 droplets cm−3 throughout a depth of 200 m and width of 1.9 km. The theoretical increase in albedo of the new cloud over that of the old maritime cloud is about 25 per cent, an amount comparable to that indicated in the photographs.
Abstract
Satellite cloud photography occasionally reveals anomalous cloud lines in the form of plumes often 500 km long and up to 25 km wide. These lines, found over the ocean, are composed of liquid particles. Hollow polygonal, convective or thin stratiform, cloud patterns in varying amounts are associated with the lines.
Numerous lines within a 10-deg square have been observed which cover up to 4 per cent of the total area.
The most likely cause of the cloud lines stems from the exhaust of ocean going vessels. Large numbers of Aitken nuclei form in this exhaust. These are carried upward by the buoyancy of the hot gases and “ships air wake” to form droplets at slight supersaturation. The phenomenon does not appear related to special characteristics of the vessel's power plant but to a critical condition of the atmosphere. As far as is known, this condition may be described as having 1) a convectively unstable layer from the surface to a low-level stable layer, 2) saturation or slight supersaturation near the top of the convective layer, and 3) a convective layer, presumably deficient in cloud forming nuclei.
Nuclei are produced in sufficient quantity by an average size ship, moving at 10 m sec−1 relative to the wind, to generate 200 droplets cm−3 throughout a depth of 200 m and width of 1.9 km. The theoretical increase in albedo of the new cloud over that of the old maritime cloud is about 25 per cent, an amount comparable to that indicated in the photographs.
The new Foxboro dew-point recorder, “Dewcel,” which was built for air-conditioning control, has been adapted to meteorological use at Blue Hill Observatory and tested over a 21-month period. These tests indicate that a rugged recorder, having an accuracy within ± 3F° of that of an Assmann ventilated psychrometer in the natural wind (usually unstable conditions) is now available for station use. The moisture-sensitive element consists of two parallel electrical conductors with an A.C. potential of 25 volts between them, wound about a tube covered with wicking containing an excess of lithium chloride crystals. Dew point is indicated by the temperature of the cell as equilibrium between the ambient vapor pressure and lithium-chloride vapor pressure is maintained, rather than by the Diamond-Hinman principle of recording electrical resistance to maintain vapor-pressure equilibrium. A satisfactory shield for the sensitive element against wind, rain, snow and fog has been devised after considerable experiment. Other than re-doping the cell with a lithium-chloride solution every 90–100 days no servicing is required. Dry and wet thermocouple tests revealed rapid fluctuations of the dew point whenever the air is unstable; therefore, the lag of the instrument, 2 to 4 minutes to make 98% of the change to a sudden new value, is advantageous when dew points for synoptic purposes are required.
Of the three general types of dew-point recorders the Foxboro Dewcel appears to be best for routine synoptic weather reports.
The simpler mechanical type of thermal element and hair hygrometer linked has the disadvantages of several linkages and those inherent in the hair hygrometer. It is not readily adaptable to indicating or recording at a distance. Instruments maintaining a thermal element in a wetted condition fail during prolonged periods of freezing. The direct reading photoelectric type dew-point recorder operates over a wider range and with greater sensitivity although there is always an uncertainty between 15° and 32°F whether the dew or frost point is being measured. The photoelectric type requires special care and frequent attention during operation.
The new Foxboro dew-point recorder, “Dewcel,” which was built for air-conditioning control, has been adapted to meteorological use at Blue Hill Observatory and tested over a 21-month period. These tests indicate that a rugged recorder, having an accuracy within ± 3F° of that of an Assmann ventilated psychrometer in the natural wind (usually unstable conditions) is now available for station use. The moisture-sensitive element consists of two parallel electrical conductors with an A.C. potential of 25 volts between them, wound about a tube covered with wicking containing an excess of lithium chloride crystals. Dew point is indicated by the temperature of the cell as equilibrium between the ambient vapor pressure and lithium-chloride vapor pressure is maintained, rather than by the Diamond-Hinman principle of recording electrical resistance to maintain vapor-pressure equilibrium. A satisfactory shield for the sensitive element against wind, rain, snow and fog has been devised after considerable experiment. Other than re-doping the cell with a lithium-chloride solution every 90–100 days no servicing is required. Dry and wet thermocouple tests revealed rapid fluctuations of the dew point whenever the air is unstable; therefore, the lag of the instrument, 2 to 4 minutes to make 98% of the change to a sudden new value, is advantageous when dew points for synoptic purposes are required.
Of the three general types of dew-point recorders the Foxboro Dewcel appears to be best for routine synoptic weather reports.
The simpler mechanical type of thermal element and hair hygrometer linked has the disadvantages of several linkages and those inherent in the hair hygrometer. It is not readily adaptable to indicating or recording at a distance. Instruments maintaining a thermal element in a wetted condition fail during prolonged periods of freezing. The direct reading photoelectric type dew-point recorder operates over a wider range and with greater sensitivity although there is always an uncertainty between 15° and 32°F whether the dew or frost point is being measured. The photoelectric type requires special care and frequent attention during operation.
