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- Author or Editor: Pauline M. Austin x
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Abstract
A number of physical factors that influence the relation between measured radar reflectivity and surface rainfall are considered both theoretically and through detailed comparisons of radar and raingauge measurements. These factors include natural differences in raindrop-size distributions, enhancement of radar reflectivity by presence of hailstones or melting snow, diminution of reflectivity by downdrafts, and low-level changes in rainfall rate caused by accretion or evaporation. Results of 374 comparisons in twenty storms, which cover a wide variety of synoptic situations and rainfall patterns, are presented. Magnitudes of the effects of the different factors are estimated, and storm types where they are likely to be significant are pointed out. Also, some ways of compensating for the observed effects are suggested.
Abstract
A number of physical factors that influence the relation between measured radar reflectivity and surface rainfall are considered both theoretically and through detailed comparisons of radar and raingauge measurements. These factors include natural differences in raindrop-size distributions, enhancement of radar reflectivity by presence of hailstones or melting snow, diminution of reflectivity by downdrafts, and low-level changes in rainfall rate caused by accretion or evaporation. Results of 374 comparisons in twenty storms, which cover a wide variety of synoptic situations and rainfall patterns, are presented. Magnitudes of the effects of the different factors are estimated, and storm types where they are likely to be significant are pointed out. Also, some ways of compensating for the observed effects are suggested.
MEASUREMENT OF APPROXIMATE RAINDROP SIZE BY MICROWAVE ATTENUATION
(Paper presented 28 December 1946 at Annual Meeting, A.M..%, Cambridge, Massachusetts)
Abstract
It is not possible to deduce the size of raindrops from the average intensity of the radar signal produced by the precipitation region. However, if it is assumed that all the drops are of the same size, a relation between the diameter of the drops and the number per unit volume may be obtained. When measurable relative attenuation between radar waves of different lengths occurs, a different relation between drop size and concentration may be derived therefrom. The combination of these two types of measurement yields a single solution for the size and concentration of the drops. The solution can be considered as an approximation to the size of the largest drops present in the precipitation region, since the smaller drops contribute only slightly to the radar echo. Measurable attenuation of 3-cm waves may be expected to occur under two conditions: (a) heavy rain with large drops, and (b) moderately heavy rain with fairly uniform characteristics over a large area. Measurable attenuation of l-cm waves can be expected even with light rain.
Abstract
It is not possible to deduce the size of raindrops from the average intensity of the radar signal produced by the precipitation region. However, if it is assumed that all the drops are of the same size, a relation between the diameter of the drops and the number per unit volume may be obtained. When measurable relative attenuation between radar waves of different lengths occurs, a different relation between drop size and concentration may be derived therefrom. The combination of these two types of measurement yields a single solution for the size and concentration of the drops. The solution can be considered as an approximation to the size of the largest drops present in the precipitation region, since the smaller drops contribute only slightly to the radar echo. Measurable attenuation of 3-cm waves may be expected to occur under two conditions: (a) heavy rain with large drops, and (b) moderately heavy rain with fairly uniform characteristics over a large area. Measurable attenuation of l-cm waves can be expected even with light rain.
The development of frontal precipitation as observed by radar is illustrated by data taken on a particular storm. This storm presents a typical sequence of precipitation patterns found to be associated with a warm front and a following cold front.
The development of frontal precipitation as observed by radar is illustrated by data taken on a particular storm. This storm presents a typical sequence of precipitation patterns found to be associated with a warm front and a following cold front.
Abstract
Several sets of drop size measurements were made on ships and aircraft during GATE. The data were taken primarily as support for the radar measurements and have been analyzed to provide relations between radar reflectivity and desired meteorological quantities.
Uncertainties in the results because of instrumental difficulties and differences within and between the various data sets are examined and discussed. The overall Z-R relation based on the combined data sets is Z = 180R 1.35. Also included are relations of the reflectivity factor to rainwater content and attenuation of 5 cm radiation.
Comparison of the drop size distributions with measurements from other places suggests that tropical oceanic showers typically contain an abundance of medium-sized drops and relatively few large ones.
Abstract
Several sets of drop size measurements were made on ships and aircraft during GATE. The data were taken primarily as support for the radar measurements and have been analyzed to provide relations between radar reflectivity and desired meteorological quantities.
Uncertainties in the results because of instrumental difficulties and differences within and between the various data sets are examined and discussed. The overall Z-R relation based on the combined data sets is Z = 180R 1.35. Also included are relations of the reflectivity factor to rainwater content and attenuation of 5 cm radiation.
Comparison of the drop size distributions with measurements from other places suggests that tropical oceanic showers typically contain an abundance of medium-sized drops and relatively few large ones.
Abstract
Observations and measurements of the bright band in radar echoes are presented. It is shown that the theory of coalescence and melting of snowflakes provides an adequate explanation of the phenomenon.
Abstract
Observations and measurements of the bright band in radar echoes are presented. It is shown that the theory of coalescence and melting of snowflakes provides an adequate explanation of the phenomenon.
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Abstract
A relatively simple model of warm and cold rain microphysics in cumulus cells is developed. The proposed model avoids the large amounts of computation required in a non-parameterized treatment and yet removes some of the restrictions imposed by Kessler's microphysical parameterization. The growth of cloud droplets is bypassed, but evolution of particle size spectra for larger hydrometeors and the effects of differential fallspeeds are allowed by the growth of rain and graupel particles in a total of 25 size categories. The processes included are condensation, evaporation, accretion, collection, breakup, freezing, deposition, riming and melting. Experiments in the context of a kinematic updraft indicate results comparable to those of a stochastic model in warm rain development. It is found that a counterbalancing mechanism between auto- conversion and accretion causes the results to be relatively insensitive to assumptions about the auto- conversion process. Further sensitivity tests point out the important contributions of rain-rain interactions in the evolution of drop-size spectra, the essential role of impaction breakup as a limiting mechanism for drop growth, and the modes in which the presence of graupel affects the particle-size distributions.
Abstract
A relatively simple model of warm and cold rain microphysics in cumulus cells is developed. The proposed model avoids the large amounts of computation required in a non-parameterized treatment and yet removes some of the restrictions imposed by Kessler's microphysical parameterization. The growth of cloud droplets is bypassed, but evolution of particle size spectra for larger hydrometeors and the effects of differential fallspeeds are allowed by the growth of rain and graupel particles in a total of 25 size categories. The processes included are condensation, evaporation, accretion, collection, breakup, freezing, deposition, riming and melting. Experiments in the context of a kinematic updraft indicate results comparable to those of a stochastic model in warm rain development. It is found that a counterbalancing mechanism between auto- conversion and accretion causes the results to be relatively insensitive to assumptions about the auto- conversion process. Further sensitivity tests point out the important contributions of rain-rain interactions in the evolution of drop-size spectra, the essential role of impaction breakup as a limiting mechanism for drop growth, and the modes in which the presence of graupel affects the particle-size distributions.
Abstract
Precipitation patterns have been analyzed for six wintertime storms in New England where coastal fronts developed and for two without coastal fronts. In all of the storms the predominant precipitation features, as observed by radar, were mesoscale bands which contained convective cells, a pattern typical of extratropical cyclones. Vertical profiles of potential temperature, humidity and wind indicate that most of the condensation occurred in a layer of warm moist air lifted by synoptic-scale ascent ahead of the baroclinic disturbance. Cumulus convection was initiated in a shallow unstable region at the top of the warm moist layer. The coastal front circulations apparently developed independently of the large-scale baroclinically induced circulations and were very shallow, typically 300 m in depth. They had durations of 7–15 h and existed during most of the time when the precipitation bands were passing over eastern Massachusetts. The effect of the coastal fronts was to enhance the precipitation over an area about 80 km wide along a line between Boston, Massachusetts, and Providence, Rhode Island, with the average increase ranging from 13 to 147%. The mechanism for precipitation enhancement appears to be creation of low cloud by the coastal front circulation. The cloud droplets are then accreted by snow-flakes which originated at higher levels.
Abstract
Precipitation patterns have been analyzed for six wintertime storms in New England where coastal fronts developed and for two without coastal fronts. In all of the storms the predominant precipitation features, as observed by radar, were mesoscale bands which contained convective cells, a pattern typical of extratropical cyclones. Vertical profiles of potential temperature, humidity and wind indicate that most of the condensation occurred in a layer of warm moist air lifted by synoptic-scale ascent ahead of the baroclinic disturbance. Cumulus convection was initiated in a shallow unstable region at the top of the warm moist layer. The coastal front circulations apparently developed independently of the large-scale baroclinically induced circulations and were very shallow, typically 300 m in depth. They had durations of 7–15 h and existed during most of the time when the precipitation bands were passing over eastern Massachusetts. The effect of the coastal fronts was to enhance the precipitation over an area about 80 km wide along a line between Boston, Massachusetts, and Providence, Rhode Island, with the average increase ranging from 13 to 147%. The mechanism for precipitation enhancement appears to be creation of low cloud by the coastal front circulation. The cloud droplets are then accreted by snow-flakes which originated at higher levels.
