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- Author or Editor: J. G. Hosking x
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Abstract
Typical results of field measurements of the fallspeeds of natural raindrops are presented and instrumental effects investigated by numerically simulating the measurement processes. Deviations between the measured fallspeeds and those expected on the basis of theory and windtunnel results are smaller than previous investigators have found, probably because of the improved instrumentation used. The random component of these deviations is consistent with estimated experimental error, but on average drops are found to fall about 5%–10% slower than expected, and this mean deviation increased with drop size; these data are consistent with the existence of local updrafts and downdrafts. The effects of the observed fallspeed fluctuations on estimates of other quantities at ground level (e.g., drop radius) are considered and, in some cases, shown to be significant.
Abstract
Typical results of field measurements of the fallspeeds of natural raindrops are presented and instrumental effects investigated by numerically simulating the measurement processes. Deviations between the measured fallspeeds and those expected on the basis of theory and windtunnel results are smaller than previous investigators have found, probably because of the improved instrumentation used. The random component of these deviations is consistent with estimated experimental error, but on average drops are found to fall about 5%–10% slower than expected, and this mean deviation increased with drop size; these data are consistent with the existence of local updrafts and downdrafts. The effects of the observed fallspeed fluctuations on estimates of other quantities at ground level (e.g., drop radius) are considered and, in some cases, shown to be significant.
Abstract
Examination of the arrival-rate distribution of raindrops from disturbed, post-cold-front air masses using the statistic k, the ratio of the variance of the drop rate to the mean, shows strong deviations from Poisson behavior toward clustering of drops. Clustering occurs predominantly for drops smaller than about 0.25 mm radius and correlates strongly with the small-drop arrival rate and, to a lesser extent, with rainfall intensity but does not appear to be caused by drop splashing, breakup or local turbulence. There is evidence to suggest that, if rapid intensity fluctuations are causing clustering, these fluctuations occur with a characteristic period of less than 10 s.
Abstract
Examination of the arrival-rate distribution of raindrops from disturbed, post-cold-front air masses using the statistic k, the ratio of the variance of the drop rate to the mean, shows strong deviations from Poisson behavior toward clustering of drops. Clustering occurs predominantly for drops smaller than about 0.25 mm radius and correlates strongly with the small-drop arrival rate and, to a lesser extent, with rainfall intensity but does not appear to be caused by drop splashing, breakup or local turbulence. There is evidence to suggest that, if rapid intensity fluctuations are causing clustering, these fluctuations occur with a characteristic period of less than 10 s.
Abstract
Preliminary data from ground-based measurements of rainfall using an optical disdrometer and high-resolution rain gage network are described. The recurrence of the rain period is shown to be non-Poisson with evidence of clustering, whereas a lognormal distribution is shown to be an approximate fit to the distribution of amount of rainfall in a rain period. About 40% of rain periods possess a total rainfall smaller than that resolvable by conventional recording rain gages; three-quarters of these were resolvable by the high-resolution rain gages used. Rapid fluctuations of rainfall intensity (∼1 mm h−1 s−1) are evident in individual rainfall intensity records and the distribution of duration of rainfall above a specified threshold intensity is shown to be approximately lognormal; these observations are consistent with a lognormal distribution of precipitation region sizes. An empirical fit to the average fractional duration of rainfall is given, although the curve is generally a poor fit to fractional duration data for individual rain periods; periods of intense rainfall tend to be more singular than expected from the curve, and the maximum intensity reached in a rain period is shown to be independent of the period's duration. A method for estimating spatial sizes and shapes of precipitation regions using a high-resolution rain gage array is demonstrated.
Abstract
Preliminary data from ground-based measurements of rainfall using an optical disdrometer and high-resolution rain gage network are described. The recurrence of the rain period is shown to be non-Poisson with evidence of clustering, whereas a lognormal distribution is shown to be an approximate fit to the distribution of amount of rainfall in a rain period. About 40% of rain periods possess a total rainfall smaller than that resolvable by conventional recording rain gages; three-quarters of these were resolvable by the high-resolution rain gages used. Rapid fluctuations of rainfall intensity (∼1 mm h−1 s−1) are evident in individual rainfall intensity records and the distribution of duration of rainfall above a specified threshold intensity is shown to be approximately lognormal; these observations are consistent with a lognormal distribution of precipitation region sizes. An empirical fit to the average fractional duration of rainfall is given, although the curve is generally a poor fit to fractional duration data for individual rain periods; periods of intense rainfall tend to be more singular than expected from the curve, and the maximum intensity reached in a rain period is shown to be independent of the period's duration. A method for estimating spatial sizes and shapes of precipitation regions using a high-resolution rain gage array is demonstrated.
Abstract
The effect of horizontal drag forces on raindrop trajectories is shown to influence strongly the detection efficiencies computed for drop cameras and disdrometers. Two examples of sheared flow involving real instruments are studied and it is found that whereas detection efficiencies remain high for both large and small drops, low detection efficiencies occur for drops of radius near 0.2 mm. These predictions are tested against field results from one of the instruments and it is shown that the theory of Rinehart is inadequate for small drops for this instrument.
Abstract
The effect of horizontal drag forces on raindrop trajectories is shown to influence strongly the detection efficiencies computed for drop cameras and disdrometers. Two examples of sheared flow involving real instruments are studied and it is found that whereas detection efficiencies remain high for both large and small drops, low detection efficiencies occur for drops of radius near 0.2 mm. These predictions are tested against field results from one of the instruments and it is shown that the theory of Rinehart is inadequate for small drops for this instrument.
Abstract
Evaluation of an earlier raingage design based on counting drops formed on the tip of a small-diameter stainless-steel tube shows a defect due to resonant oscillation of the water column in the dropper unit. The defect causes nonlinearity in the drop rate-flow rate relationship, precluding useful integration of the gage output. An improved design is presented which eliminates this defect. Laboratory tests on the new design show linear performance over two orders of magnitude of rainfall intensity with time resolution of better than 5 s and accuracy limited by sampling errors. The new gage is also shown to perform well in field testing.
Abstract
Evaluation of an earlier raingage design based on counting drops formed on the tip of a small-diameter stainless-steel tube shows a defect due to resonant oscillation of the water column in the dropper unit. The defect causes nonlinearity in the drop rate-flow rate relationship, precluding useful integration of the gage output. An improved design is presented which eliminates this defect. Laboratory tests on the new design show linear performance over two orders of magnitude of rainfall intensity with time resolution of better than 5 s and accuracy limited by sampling errors. The new gage is also shown to perform well in field testing.
Abstract
The Amundsen Sea low (ASL) is a climatological low pressure center that exerts considerable influence on the climate of West Antarctica. Its potential to explain important recent changes in Antarctic climate, for example, in temperature and sea ice extent, means that it has become the focus of an increasing number of studies. Here, the authors summarize the current understanding of the ASL, using reanalysis datasets to analyze recent variability and trends, as well as ice-core chemistry and climate model projections, to examine past and future changes in the ASL, respectively. The ASL has deepened in recent decades, affecting the climate through its influence on the regional meridional wind field, which controls the advection of moisture and heat into the continent. Deepening of the ASL in spring is consistent with observed West Antarctic warming and greater sea ice extent in the Ross Sea. Climate model simulations for recent decades indicate that this deepening is mediated by tropical variability while climate model projections through the twenty-first century suggest that the ASL will deepen in some seasons in response to greenhouse gas concentration increases.
Abstract
The Amundsen Sea low (ASL) is a climatological low pressure center that exerts considerable influence on the climate of West Antarctica. Its potential to explain important recent changes in Antarctic climate, for example, in temperature and sea ice extent, means that it has become the focus of an increasing number of studies. Here, the authors summarize the current understanding of the ASL, using reanalysis datasets to analyze recent variability and trends, as well as ice-core chemistry and climate model projections, to examine past and future changes in the ASL, respectively. The ASL has deepened in recent decades, affecting the climate through its influence on the regional meridional wind field, which controls the advection of moisture and heat into the continent. Deepening of the ASL in spring is consistent with observed West Antarctic warming and greater sea ice extent in the Ross Sea. Climate model simulations for recent decades indicate that this deepening is mediated by tropical variability while climate model projections through the twenty-first century suggest that the ASL will deepen in some seasons in response to greenhouse gas concentration increases.
