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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.
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
A method is described for calculating cumulus-scale vertical transports of mass, sensible heat and hori-zontal momentum from detailed measurements of precipitation. The basic premises are that the amount of lifting in a cumulus cell is related to the precipitation which it produces and that the temperature excess and entrainment are reflected in its vertical development. The amount of cellular precipitation may be obtained from quantitative radar data or high-resolution rain gauge records, the cell depths from radar and radiosonde data.
The mass of air transported upward within the cells is computed from the conservation of water given the amount of cellular precipitation. A relationship between measured cellular rain and condensate within cumulus cells is based on available empirical information. Entrainment rate and the shape of the mass transport curve as a function of height are specified in a manner consistent with physical considerations, experimental evidence, and one-dimensional dynamic models of cumulus cells. For the transport of sensible heat the temperature excess within the cells is computed from conservation equations for water and vertical momentum. For the transport of horizontal momentum, the difference in horizontal wind speed is calculated from the conservation of horizontal momentum and the drag force exerted by the environment on the air in the updrafts.
Uncertainties in the computed transports are obtained by estimating limiting values for the assumptions involved.
Abstract
A method is described for calculating cumulus-scale vertical transports of mass, sensible heat and hori-zontal momentum from detailed measurements of precipitation. The basic premises are that the amount of lifting in a cumulus cell is related to the precipitation which it produces and that the temperature excess and entrainment are reflected in its vertical development. The amount of cellular precipitation may be obtained from quantitative radar data or high-resolution rain gauge records, the cell depths from radar and radiosonde data.
The mass of air transported upward within the cells is computed from the conservation of water given the amount of cellular precipitation. A relationship between measured cellular rain and condensate within cumulus cells is based on available empirical information. Entrainment rate and the shape of the mass transport curve as a function of height are specified in a manner consistent with physical considerations, experimental evidence, and one-dimensional dynamic models of cumulus cells. For the transport of sensible heat the temperature excess within the cells is computed from conservation equations for water and vertical momentum. For the transport of horizontal momentum, the difference in horizontal wind speed is calculated from the conservation of horizontal momentum and the drag force exerted by the environment on the air in the updrafts.
Uncertainties in the computed transports are obtained by estimating limiting values for the assumptions involved.
