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- Author or Editor: Walter Hitschfeld x
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Abstract
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Abstract
Well developed thunderstorms were observed by CAPPI-equipped radar to remain upright even in the presence of severe wind shear. Instead of being strongly bent by the shear, parts of the storm appeared to be carried off by the wind, forming extensive plume patterns, which trailed down toward the ground while evaporating. An analysis of the horizontal and vertical development of the plume suggested that its particles had fall speeds ranging from 0.75 to 5 m sec−1 and were thus of precipitation size.
The motion of the entire storm system is related to the wind at low levels but only in a general way. Instances where the echo motion differs substantially from the observed winds at all heights are quite common. Vertical motion and storm erosion are used to explain how active cores of large storms remain nearly vertical, almost independently of the wind shear.
Abstract
Well developed thunderstorms were observed by CAPPI-equipped radar to remain upright even in the presence of severe wind shear. Instead of being strongly bent by the shear, parts of the storm appeared to be carried off by the wind, forming extensive plume patterns, which trailed down toward the ground while evaporating. An analysis of the horizontal and vertical development of the plume suggested that its particles had fall speeds ranging from 0.75 to 5 m sec−1 and were thus of precipitation size.
The motion of the entire storm system is related to the wind at low levels but only in a general way. Instances where the echo motion differs substantially from the observed winds at all heights are quite common. Vertical motion and storm erosion are used to explain how active cores of large storms remain nearly vertical, almost independently of the wind shear.
Abstract
An equation for the rate of rainfall at a given range from the radar is derived. This is expressed in terms of the power level of the received signal (corrected for attenuation by intervening cloud and atmospheric gases) and takes account of radar attenuation due to intervening rain. The equation includes a constant which measures the performance of the radar and is determined by direct calibration.
At attenuating wavelengths (at 3 cm; to some extent at 5.6 cm) a small error in the calibration constant causes a large error in the measured rainfall. This error, which varies with range and may thus cause serious distortion, is, in fact, liable to be more serious than that caused if the attenuation were neglected entirely. Correcting for attenuation is therefore not recommended, unless the calibration error may be held within extremely narrow limits.
Very small calibration errors may be achieved by calibrating the radar by means of a rain gauge located at a point where the attenuation is appreciable. At points of smaller attenuation, a satisfactory degree of accuracy in the calculated rate of rainfall then results.
At wavelengths such as 10 cm, where the attenuation is negligible, errors in the constant still affect the measured rain, but neither so seriously, nor in a manner involving the range, thus causing no distortion.
An examination of the relative importance of the attenuation by gases and cloud at three wavelengths similarly emphasizes the difficulties associated with quantitative work at the shorter wavelengths.
Abstract
An equation for the rate of rainfall at a given range from the radar is derived. This is expressed in terms of the power level of the received signal (corrected for attenuation by intervening cloud and atmospheric gases) and takes account of radar attenuation due to intervening rain. The equation includes a constant which measures the performance of the radar and is determined by direct calibration.
At attenuating wavelengths (at 3 cm; to some extent at 5.6 cm) a small error in the calibration constant causes a large error in the measured rainfall. This error, which varies with range and may thus cause serious distortion, is, in fact, liable to be more serious than that caused if the attenuation were neglected entirely. Correcting for attenuation is therefore not recommended, unless the calibration error may be held within extremely narrow limits.
Very small calibration errors may be achieved by calibrating the radar by means of a rain gauge located at a point where the attenuation is appreciable. At points of smaller attenuation, a satisfactory degree of accuracy in the calculated rate of rainfall then results.
At wavelengths such as 10 cm, where the attenuation is negligible, errors in the constant still affect the measured rain, but neither so seriously, nor in a manner involving the range, thus causing no distortion.
An examination of the relative importance of the attenuation by gases and cloud at three wavelengths similarly emphasizes the difficulties associated with quantitative work at the shorter wavelengths.
Abstract
A rigorous study was made of the temperature profile in spherical and homogeneous hailstones falling through clear air. It is found that a stone 1 cm in radius is liable to be a dozen degrees Celsuis colder than the ambient air. For larger stones, the temperature difference becomes greater. The cooling effect of hail on the air is relatively small, if hall-size distributions of the sort commonly observed at the ground are considered.
When a stone (1.1 cm in radius) falls through cloud, its heat capacity delays the commencement of wet growth by as much as 2 km. On the other hand, when hail grows in surroundings of high liquid water content, the heat capacity term of even the largest hail in the heat balance equation is quite unimportant. Such growth normally leads to mixtures of water and ice.
Abstract
A rigorous study was made of the temperature profile in spherical and homogeneous hailstones falling through clear air. It is found that a stone 1 cm in radius is liable to be a dozen degrees Celsuis colder than the ambient air. For larger stones, the temperature difference becomes greater. The cooling effect of hail on the air is relatively small, if hall-size distributions of the sort commonly observed at the ground are considered.
When a stone (1.1 cm in radius) falls through cloud, its heat capacity delays the commencement of wet growth by as much as 2 km. On the other hand, when hail grows in surroundings of high liquid water content, the heat capacity term of even the largest hail in the heat balance equation is quite unimportant. Such growth normally leads to mixtures of water and ice.
Abstract
Coalescence of raindrops and cloud droplets has been studied in the laboratory by observing the growth of water drops when falling through a three-meter column of cloud. The increase in mass of the drop is expressed in terms of a collection efficiency, defined as the fraction of the cloud water in the path swept out by the drop, which is actually picked up. Collection efficiencies for a drop of 3-mm diameter falling through three clouds of different size-distributions are found to be in agreement with the aerodynamic collision efficiencies calculated according to a theory by Langmuir (1948). The theory and his assumption that collision always results in coalescence are thus substantiated.
The effect on coalescence of charge on the drops, comparable to that observed on raindrops in nature, is found to be small.
Abstract
Coalescence of raindrops and cloud droplets has been studied in the laboratory by observing the growth of water drops when falling through a three-meter column of cloud. The increase in mass of the drop is expressed in terms of a collection efficiency, defined as the fraction of the cloud water in the path swept out by the drop, which is actually picked up. Collection efficiencies for a drop of 3-mm diameter falling through three clouds of different size-distributions are found to be in agreement with the aerodynamic collision efficiencies calculated according to a theory by Langmuir (1948). The theory and his assumption that collision always results in coalescence are thus substantiated.
The effect on coalescence of charge on the drops, comparable to that observed on raindrops in nature, is found to be small.
Abstract
Numerical methods are used to study the changes in the distribution of raindrops with size and in the radar echo, as rain falls. Changes brought about by collisions among the drops, by accretion of cloud, and by evaporation are considered. The distribution assumed aloft is that actually observed at the ground. This is justified, because the changes in the form of the distribution are found to be slight: an exponential type of distribution law would seem to be applicable at all heights. This result is taken to mean that the processes investigated cannot by themselves produce the distributions observed at the ground from distributions of a very different sort, or from the broad distributions of snow. A mechanism as yet unknown, probably involving drop break-up, would seem to be required. The work is done both for “continuous rain,” where conditions at all levels are assumed to be constant in time, and for showers, where they are assumed to be initially the same at all levels.
Unequal rates of fall of raindrops tend to increase the vertical extent of the radar echo in time. The calculated rates of motion of echo top and base depend critically on range and radar sensitivity; they are in reasonable agreement with those observed.
Where necessary, the rate of depletion of cloud due to rain falling through it is taken into account. A simple theory of this process, by Dr. K. L. S. Gunn, is described in an appendix.
Abstract
Numerical methods are used to study the changes in the distribution of raindrops with size and in the radar echo, as rain falls. Changes brought about by collisions among the drops, by accretion of cloud, and by evaporation are considered. The distribution assumed aloft is that actually observed at the ground. This is justified, because the changes in the form of the distribution are found to be slight: an exponential type of distribution law would seem to be applicable at all heights. This result is taken to mean that the processes investigated cannot by themselves produce the distributions observed at the ground from distributions of a very different sort, or from the broad distributions of snow. A mechanism as yet unknown, probably involving drop break-up, would seem to be required. The work is done both for “continuous rain,” where conditions at all levels are assumed to be constant in time, and for showers, where they are assumed to be initially the same at all levels.
Unequal rates of fall of raindrops tend to increase the vertical extent of the radar echo in time. The calculated rates of motion of echo top and base depend critically on range and radar sensitivity; they are in reasonable agreement with those observed.
Where necessary, the rate of depletion of cloud due to rain falling through it is taken into account. A simple theory of this process, by Dr. K. L. S. Gunn, is described in an appendix.
