Search Results
You are looking at 1 - 10 of 13 items for :
- Author or Editor: W. L. Smith x
- Monthly Weather Review x
- Refine by Access: All Content x
Abstract
No Abstract Available.
Abstract
No Abstract Available.
Abstract
The U.S. Weather Bureau has been experimenting with a radar operating on the Doppler principle to determine whether apparatus of this type would detect and uniquely identify tornadoes. The principles of Doppler radar as applied to meteorology and results of recent experiments with equipment of this type are discussed. Calculations of anomalous wind speeds of 206 m.p.h. in a funnel cloud and 94 m.p.h. in a dust devil are presented in detail. In addition, data have been gathered from squall lines and isolated thunderstorms. Recommendations are made for an optimum Doppler radar system for the detection of tornadoes.
Abstract
The U.S. Weather Bureau has been experimenting with a radar operating on the Doppler principle to determine whether apparatus of this type would detect and uniquely identify tornadoes. The principles of Doppler radar as applied to meteorology and results of recent experiments with equipment of this type are discussed. Calculations of anomalous wind speeds of 206 m.p.h. in a funnel cloud and 94 m.p.h. in a dust devil are presented in detail. In addition, data have been gathered from squall lines and isolated thunderstorms. Recommendations are made for an optimum Doppler radar system for the detection of tornadoes.
Abstract
A composite histogram method is used to objectively derive sea-surface temperature distribution from satellite radiation measurements for the Northern and Southern Hemispheres. Comparisons with conventional observations yield root-mean-square differences of 2°–3°K. Some of the differences can be accounted for by factors such as the coherent noise introduced by the onboard tape recorder, insufficient atmospheric attenuation corrections, and basic differences between the two types of temperature measurements.
Abstract
A composite histogram method is used to objectively derive sea-surface temperature distribution from satellite radiation measurements for the Northern and Southern Hemispheres. Comparisons with conventional observations yield root-mean-square differences of 2°–3°K. Some of the differences can be accounted for by factors such as the coherent noise introduced by the onboard tape recorder, insufficient atmospheric attenuation corrections, and basic differences between the two types of temperature measurements.
Abstract
A least squares regression method is formulated for obtaining global temperature and geopotential height profiles from satellite radiation measurements, particularly those obtained by the Sate1lite Infra-Red Spectrometer (SIRS) aboard the Nimbus 3 satellite launched Apr. 14, 1969. Regression equations relating temperature and geopotential height to spectral radiance observations are derived. A method accounting for the influence of clouds, mountains, and hot terrain on the solutions is described. Results obtained from Nimbus 3 radiance data are presented.
The procedure described herein has been successfully applied to Nimbus 3 SIRS observations on a real-time basis. The temperature and geopotential heights obtained are being used operationally by the National Meteorological Center in their objective constant pressure analyses. Numerous meteorological results are given to demonstrate the usefulness of this new sounding tool.
Abstract
A least squares regression method is formulated for obtaining global temperature and geopotential height profiles from satellite radiation measurements, particularly those obtained by the Sate1lite Infra-Red Spectrometer (SIRS) aboard the Nimbus 3 satellite launched Apr. 14, 1969. Regression equations relating temperature and geopotential height to spectral radiance observations are derived. A method accounting for the influence of clouds, mountains, and hot terrain on the solutions is described. Results obtained from Nimbus 3 radiance data are presented.
The procedure described herein has been successfully applied to Nimbus 3 SIRS observations on a real-time basis. The temperature and geopotential heights obtained are being used operationally by the National Meteorological Center in their objective constant pressure analyses. Numerous meteorological results are given to demonstrate the usefulness of this new sounding tool.
Abstract
A statistical histogram method is developed to objectively determine sea-surface temperature from satellite high resolution window radiation measurements. The method involves inferring the distribution of surface radiances for the clear atmospheric case from observed histograms of generally cloud-contaminated radiances. The brightness temperature associated with the clear atmosphere modal peak radiance is the statistically most probable surface temperature. The reliability of the inferred surface temperature depends upon the number of cloud-free measurements available to define the clear mode. The method accounts for atmospheric attenuation and instrumental noise and also objectively discriminates cloud-free from cloud-contaminated observations.
The statistical histogram method is applied to 3.8 micrometer window radiation data obtained by the High Resolution Infrared Radiometer flown on the Nimbus 2 and Nimbus 3 satellites. Examples of sea temperatures inferred over both small and large areas are presented. Comparisons with conventional ship observations indicate that both bias and random errors of the inferred sea temperatures are less than 1°C.
Due to the apparent success of this statistical histogram technique, plans have been made to use it to obtain sea-surface temperatures on a global basis daily from operational high resolution infrared radiation measurements.
Abstract
A statistical histogram method is developed to objectively determine sea-surface temperature from satellite high resolution window radiation measurements. The method involves inferring the distribution of surface radiances for the clear atmospheric case from observed histograms of generally cloud-contaminated radiances. The brightness temperature associated with the clear atmosphere modal peak radiance is the statistically most probable surface temperature. The reliability of the inferred surface temperature depends upon the number of cloud-free measurements available to define the clear mode. The method accounts for atmospheric attenuation and instrumental noise and also objectively discriminates cloud-free from cloud-contaminated observations.
The statistical histogram method is applied to 3.8 micrometer window radiation data obtained by the High Resolution Infrared Radiometer flown on the Nimbus 2 and Nimbus 3 satellites. Examples of sea temperatures inferred over both small and large areas are presented. Comparisons with conventional ship observations indicate that both bias and random errors of the inferred sea temperatures are less than 1°C.
Due to the apparent success of this statistical histogram technique, plans have been made to use it to obtain sea-surface temperatures on a global basis daily from operational high resolution infrared radiation measurements.
Abstract
Precipitation particle sizes were measured using a continuous hydrometeor sampler (foil impactor) during penetrations of hailstorms with an armored T-28 aircraft. Data have been analyzed from three penetrations of a storm near Raymer, Colorado, on 9 July 1973 at altitudes between 5.5 and 7.2 km MSL, which correspond to temperatures between about −2°C and −12°C. Other results pertinent to the Raymer storm are discussed in Parts I,II,III and elsewhere in this issue.
Most of the particles were identified as ice particles or ones containing both ice and water; however, significant amounts of liquid particles were found in the updrafts of developing cells at temperatures as cold as −12°C. Particles larger than 5 mm in diameter were typically found along the edges of the updrafts, with the precipitation concentrations being strongly dependent on these larger particles. The downdrafts were composed of ice particles.
Several particle size distributions from one of the penetrations were examined. The distributions are roughly exponential, or bi-exponential when large particles are present.
Abstract
Precipitation particle sizes were measured using a continuous hydrometeor sampler (foil impactor) during penetrations of hailstorms with an armored T-28 aircraft. Data have been analyzed from three penetrations of a storm near Raymer, Colorado, on 9 July 1973 at altitudes between 5.5 and 7.2 km MSL, which correspond to temperatures between about −2°C and −12°C. Other results pertinent to the Raymer storm are discussed in Parts I,II,III and elsewhere in this issue.
Most of the particles were identified as ice particles or ones containing both ice and water; however, significant amounts of liquid particles were found in the updrafts of developing cells at temperatures as cold as −12°C. Particles larger than 5 mm in diameter were typically found along the edges of the updrafts, with the precipitation concentrations being strongly dependent on these larger particles. The downdrafts were composed of ice particles.
Several particle size distributions from one of the penetrations were examined. The distributions are roughly exponential, or bi-exponential when large particles are present.
Abstract
Lidar and high spectral resolution infrared radiance observations taken on board the ER-2 on 28 October 1986 are used to study the radiative properties of cirrus cloud in the 8–12 μm window region. Measurements from the High-spectral resolution Interferometer Sounder (HIS) indicate that the spectral variation of the equivalent blackbody temperature across the window can be greater than 5°C for a given cirrus cloud. This difference is attributed to the presence of small particles.
A method for detecting cirrus clouds using 8 μm, 11 μm, and 12 μm bands is presented. The 8 μm band is centered on a weak water-vapor absorption line while the 11 μm and 12 μm bands are between absorption lines. The brightness temperature difference between the 8 and 11 μm bands is negative for clear regions, while for ice clouds it is positive. Differences in the 11 and 12 μm channels are positive, whether viewing a cirrus cloud or a clear region. Inclusion of the 8 μm channel therefore removes the ambiguity associated with the use of 11 and 12 μm channels alone. The method is based on the comparison of brightness temperatures observed in these three channels.
The HIS and lidar observations were combined to derive the spectral effective beam emissivity (ε) of the cirrus clouds. Fifty percent of clouds on this day displayed a spectral variation of ε from 2–10%. These differences, in conjunction with large differences in the HIS observed brightness temperatures, indicate that cirrus clouds cannot be considered gray in the 8–12 μm window region.
The derived spectral transmittance of the cloud is used to infer the effective radii of the particle size distribution, assuming ice spheres. For 28 October 1986 the effective radius of cirrus cloud particle size distribution (r eff) was generally within the 30–40 μm range with 8% of the cases where 10 < r eff < 30 μm and 12% of the cases corresponding to r ref > 40 μm.
Abstract
Lidar and high spectral resolution infrared radiance observations taken on board the ER-2 on 28 October 1986 are used to study the radiative properties of cirrus cloud in the 8–12 μm window region. Measurements from the High-spectral resolution Interferometer Sounder (HIS) indicate that the spectral variation of the equivalent blackbody temperature across the window can be greater than 5°C for a given cirrus cloud. This difference is attributed to the presence of small particles.
A method for detecting cirrus clouds using 8 μm, 11 μm, and 12 μm bands is presented. The 8 μm band is centered on a weak water-vapor absorption line while the 11 μm and 12 μm bands are between absorption lines. The brightness temperature difference between the 8 and 11 μm bands is negative for clear regions, while for ice clouds it is positive. Differences in the 11 and 12 μm channels are positive, whether viewing a cirrus cloud or a clear region. Inclusion of the 8 μm channel therefore removes the ambiguity associated with the use of 11 and 12 μm channels alone. The method is based on the comparison of brightness temperatures observed in these three channels.
The HIS and lidar observations were combined to derive the spectral effective beam emissivity (ε) of the cirrus clouds. Fifty percent of clouds on this day displayed a spectral variation of ε from 2–10%. These differences, in conjunction with large differences in the HIS observed brightness temperatures, indicate that cirrus clouds cannot be considered gray in the 8–12 μm window region.
The derived spectral transmittance of the cloud is used to infer the effective radii of the particle size distribution, assuming ice spheres. For 28 October 1986 the effective radius of cirrus cloud particle size distribution (r eff) was generally within the 30–40 μm range with 8% of the cases where 10 < r eff < 30 μm and 12% of the cases corresponding to r ref > 40 μm.
Abstract
Observations of four cold-frontal systems traversing the coastal region of southeast Australia in late spring and early summer are described in terms of process occurring on the mesoscale. A conceptual model is presented which summarizes the main results of the data analysis. Features found in common with other studies of cold fronts include:
(i) the multiple-line nature of the frontal transition zone (FTZ);
(ii) concentration of cyclonic relative vorticity at a height z≈1 to 1.5 km in the rear of the FTZ; and
(iii) the existence of a prefrontal jet at z≈1.5 km, northerly in our case, southerly in the Northern Hemisphere.
The change lines within the FTZ (and at the leading edge if there is no sea breeze) are most probably convective instability lines whose alignment and movement depend on the large-scale, cloud-layer winds. The lines are evident as mesoscale cloud bands from satellite imagery and as rainbands from radar. At least one of these develops into a vigorous squall line whose cold outflow produces a pressure jump, and related wind-shift line. Movement of the pressure-jump line depends both on the gravity-current nature of the cold outflow and the environmental wind field. The squall line and pressure-jump line are associated with mesoscale high and low pressure features to which the boundary-layer wind field responds.
The structure of the FTZ up to z=2 km appears to be dominated by the presence of the squall line, with upwards motion ahead and downwards behind. On a horizontal scale of 100 km, cyclonic vorticity reaches twice the Coriolis parameter f in the vicinity of the squall line. Frontogenesis occurs largely within the FTZ with horizontal convergence and deformation processes being of comparable importance.
The prefrontal jet is broadly in thermal wind balance with the horizontal temperature gradient which is, itself, determined by the fact that prefrontal air closest to the FTZ originates farther to the north and is therefore hotter than prefrontal air more distant from the zone.
Abstract
Observations of four cold-frontal systems traversing the coastal region of southeast Australia in late spring and early summer are described in terms of process occurring on the mesoscale. A conceptual model is presented which summarizes the main results of the data analysis. Features found in common with other studies of cold fronts include:
(i) the multiple-line nature of the frontal transition zone (FTZ);
(ii) concentration of cyclonic relative vorticity at a height z≈1 to 1.5 km in the rear of the FTZ; and
(iii) the existence of a prefrontal jet at z≈1.5 km, northerly in our case, southerly in the Northern Hemisphere.
The change lines within the FTZ (and at the leading edge if there is no sea breeze) are most probably convective instability lines whose alignment and movement depend on the large-scale, cloud-layer winds. The lines are evident as mesoscale cloud bands from satellite imagery and as rainbands from radar. At least one of these develops into a vigorous squall line whose cold outflow produces a pressure jump, and related wind-shift line. Movement of the pressure-jump line depends both on the gravity-current nature of the cold outflow and the environmental wind field. The squall line and pressure-jump line are associated with mesoscale high and low pressure features to which the boundary-layer wind field responds.
The structure of the FTZ up to z=2 km appears to be dominated by the presence of the squall line, with upwards motion ahead and downwards behind. On a horizontal scale of 100 km, cyclonic vorticity reaches twice the Coriolis parameter f in the vicinity of the squall line. Frontogenesis occurs largely within the FTZ with horizontal convergence and deformation processes being of comparable importance.
The prefrontal jet is broadly in thermal wind balance with the horizontal temperature gradient which is, itself, determined by the fact that prefrontal air closest to the FTZ originates farther to the north and is therefore hotter than prefrontal air more distant from the zone.
Abstract
A technique is presented for discriminating different cloud types through an image subtraction of visible and infrared SMS/GOES picture pairs. The technique emphasizes how one could separate snow from clouds and identify cirrus by the subtraction method. Quantitative threshold values are shown which can be used in an objective manner to make this separation.
Use is made of an all-digital image display device allowing such mathematical operations to be performed on satellite data. Techniques such as this can be made operational through the interfacing of the image analysis system with a direct-readout SMS/GOES ground station and distribution network.
Abstract
A technique is presented for discriminating different cloud types through an image subtraction of visible and infrared SMS/GOES picture pairs. The technique emphasizes how one could separate snow from clouds and identify cirrus by the subtraction method. Quantitative threshold values are shown which can be used in an objective manner to make this separation.
Use is made of an all-digital image display device allowing such mathematical operations to be performed on satellite data. Techniques such as this can be made operational through the interfacing of the image analysis system with a direct-readout SMS/GOES ground station and distribution network.
Abstract
A 3-h intermittent data assimilation system (Mesoscale Analysis and Prediction System—MAPS) configured in isentropic coordinates was developed and implemented in real-time operation. The major components of the system are data ingest, objective quality control of the observation, objective analysis, and a primitive equation forecast model, all using isentropic coordinates to take advantage of the improved resolution near frontal zones and greater spatial coherence of data that this coordinate system provides. Each 3-h forecast becomes the background for the subsequent analysis; in this manner, a four-dimensional set of observations can be assimilated.
The primary asynoptic data source used in current real-time operation of this system is air-craft data, most of it automated. Data from wind profilers, surface observations, and radiosondes are also included in MAPS.
Statistics were collected over the last half of 1989 and into 1990 to study the performance of MAPS and compare it with that of the Regional Analysis and Forecast System (RAFS), which is run operationally at the National Meteorological Center (NMC). Analyses generally fit mandatory-level observations more closely in MAPS than in RAFS. Three-hour forecasts from MAPS, incorporating asynoptic aircraft reports, improve on 12-h MAPS forecasts valid at the same time for all levels and variables, and also improve on 12-h RAFS forecasts of upper-level winds. This result is due to the quality and volume of the aircraft data as well as the effectiveness of the isentropic data assimilation used. Forecast fields at other levels are slightly poorer than those from RAFS. This may be largely due to the lack of diabatic and boundary-layer physics for the MAPS model used in this test period.
Abstract
A 3-h intermittent data assimilation system (Mesoscale Analysis and Prediction System—MAPS) configured in isentropic coordinates was developed and implemented in real-time operation. The major components of the system are data ingest, objective quality control of the observation, objective analysis, and a primitive equation forecast model, all using isentropic coordinates to take advantage of the improved resolution near frontal zones and greater spatial coherence of data that this coordinate system provides. Each 3-h forecast becomes the background for the subsequent analysis; in this manner, a four-dimensional set of observations can be assimilated.
The primary asynoptic data source used in current real-time operation of this system is air-craft data, most of it automated. Data from wind profilers, surface observations, and radiosondes are also included in MAPS.
Statistics were collected over the last half of 1989 and into 1990 to study the performance of MAPS and compare it with that of the Regional Analysis and Forecast System (RAFS), which is run operationally at the National Meteorological Center (NMC). Analyses generally fit mandatory-level observations more closely in MAPS than in RAFS. Three-hour forecasts from MAPS, incorporating asynoptic aircraft reports, improve on 12-h MAPS forecasts valid at the same time for all levels and variables, and also improve on 12-h RAFS forecasts of upper-level winds. This result is due to the quality and volume of the aircraft data as well as the effectiveness of the isentropic data assimilation used. Forecast fields at other levels are slightly poorer than those from RAFS. This may be largely due to the lack of diabatic and boundary-layer physics for the MAPS model used in this test period.
