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E. C. Rigby, J. S. Marshall, and Walter Hitschfeld

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.

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John V. Cortinas Jr., Ben C. Bernstein, Christopher C. Robbins, and J. Walter Strapp

Abstract

A comprehensive analysis of freezing rain, freezing drizzle, and ice pellets was conducted using data from surface observations across the United States and Canada. This study complements other studies of freezing precipitation in the United States and Canada, and provides additional information about the temporal characteristics of the distribution. In particular, it was found that during this period 1) spatial variability in the annual frequency of freezing precipitation and ice pellets is large across the United States and Canada, and these precipitation types occur most frequently across the central and eastern portions of the United States and Canada, much of Alaska, and the northern shores of Canada; 2) freezing precipitation and ice pellets occur most often from December to March, except in northern Canada and Alaska where it occurs during the warm season, as well; 3) freezing rain and freezing drizzle appear to be influenced by the diurnal solar cycle; 4) freezing precipitation is often short lived; 5) most freezing rain and freezing drizzle are not mixed with other precipitation types, whereas most reports of ice pellets included other types of precipitation; 6) freezing precipitation and ice pellets occur most frequently with a surface (2 m) temperature slightly less than 0°C; and 7) following most freezing rain events, the surface temperature remains at or below freezing for up to 10 h, and for up to 25 h for freezing drizzle.

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P. G. Dixon, D. M. Brommer, B. C. Hedquist, A. J. Kalkstein, G. B. Goodrich, J. C. Walter, C. C. Dickerson IV, S. J. Penny, and R. S. Cerveny

Studies, public reports, news reports, and Web sites cite a wide range of values associated with deaths resulting from excessive heat and excessive cold. For example, in the United States, the National Climatic Data Center's Storm Data statistics of temperature-related deaths are skewed heavily toward heat-related deaths, while the National Center for Health Statistics Compressed Mortality Database indicates the reverse—4 times more people die of “excessive cold” conditions in a given year than of “excessive heat.” In this study, we address the fundamental differences in the various temperature-related mortality databases, assess their benefits and limitations, and offer suggestions as to their use. These datasets suffer from potential incompleteness of source information, long compilation times, limited quality control, and the subjective determination of a direct versus indirect cause of death. In general, these separate mortality datasets should not be combined or compared, particularly with regard to policy determination. The use of gross mortality numbers appears to be one of the best means of determining temperature-related mortality, but those data must be detrended into order to remove a persistent winter-dominant death maximum and are difficult to obtain on a regional daily basis.

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Anthony J. Baran, Peter Hill, David Walters, Steven C. Hardiman, Kalli Furtado, Paul R. Field, and James Manners

Abstract

The impact of two different coupled cirrus microphysics–radiation parameterizations on the zonally averaged temperature and humidity biases in the tropical tropopause layer (TTL) of a Met Office climate model configuration is assessed. One parameterization is based on a linear coupling between a model prognostic variable, the ice mass mixing ratio q i, and the integral optical properties. The second is based on the integral optical properties being parameterized as functions of q i and temperature, T c, where the mass coefficients (i.e., scattering and extinction) are parameterized as nonlinear functions of the ratio between q i and T c. The cirrus microphysics parameterization is based on a moment estimation parameterization of the particle size distribution (PSD), which relates the mass moment (i.e., second moment if mass is proportional to size raised to the power of 2) of the PSD to all other PSD moments through the magnitude of the second moment and T c. This same microphysics PSD parameterization is applied to calculate the integral optical properties used in both radiation parameterizations and, thus, ensures PSD and mass consistency between the cirrus microphysics and radiation schemes. In this paper, the temperature-non-dependent and temperature-dependent parameterizations are shown to increase and decrease the zonally averaged temperature biases in the TTL by about 1 K, respectively. The temperature-dependent radiation parameterization is further demonstrated to have a positive impact on the specific humidity biases in the TTL, as well as decreasing the shortwave and longwave biases in the cloudy radiative effect. The temperature-dependent radiation parameterization is shown to be more consistent with TTL and global radiation observations.

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Rommel C. Zulueta, Walter C. Oechel, Joseph G. Verfaillie, Steven J. Hastings, Beniamino Gioli, William T. Lawrence, and Kyaw Tha Paw U

Abstract

Natural ecosystems are rarely structurally simple or functionally homogeneous. This is true for the complex coastal region of Magdalena Bay, Baja California Sur, Mexico, where the spatial variability in ecosystem fluxes from the Pacific coastal ocean, eutrophic lagoon, mangroves, and desert were studied. The Sky Arrow 650TCN environmental research aircraft proved to be an effective tool in characterizing land–atmosphere fluxes of energy, CO2, and water vapor across a heterogeneous landscape at the scale of 1 km. The aircraft was capable of discriminating fluxes from all ecosystem types, as well as between nearshore and coastal areas a few kilometers distant. Aircraft-derived average midday CO2 fluxes from the desert showed a slight uptake of −1.32 μmol CO2 m−2 s−1, the coastal ocean also showed an uptake of −3.48 μmol CO2 m−2 s−1, and the lagoon mangroves showed the highest uptake of −8.11 μmol CO2 m−2 s−1. Additional simultaneous measurements of the normalized difference vegetation index (NDVI) allowed simple linear modeling of CO2 flux as a function of NDVI for the mangroves of the Magdalena Bay region. Aircraft approaches can, therefore, be instrumental in determining regional CO2 fluxes and can be pivotal in calculating and verifying ecosystem carbon sequestration regionally when coupled with satellite-derived products and ecosystem models.

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Walter A. Petersen, Lawrence D. Carey, Steven A. Rutledge, Jason C. Knievel, Nolan J. Doesken, Richard H. Johnson, Thomas B. McKee, Thomas Vonder Haar, and John F. Weaver

On the evening of 28 July 1997 the city of Fort Collins, Colorado, experienced a devastating flash flood that caused five fatalities and over 200 million dollars in damage. Maximum accumulations of rainfall in the western part of the city exceeded 10 in. in a 6-h period. This study presents a multiscale meteorological overview of the event utilizing a wide variety of instrument platforms and data including rain gauge, CSU–CHILL multiparameter radar, Next Generation Radar, National Lightning Detection Network, surface and Aircraft Communication Addressing and Reporting System observations, satellite observations, and synoptic analyses.

Many of the meteorological features associated with the Fort Collins flash flood typify those of similar events in the western United States. Prominent features in the Fort Collins case included the presence of a 500-hPa ridge axis over northeastern Colorado; a weak shortwave trough on the western side of the ridge; postfrontal easterly upslope flow at low levels; weak to moderate southwesterly flow aloft; a deep, moist warm layer in the sounding; and the occurrence of a quasi-stationary rainfall system. In contrast to previous events such as the Rapid City or Big Thompson floods, the thermodynamic environment of the Fort Collins storm exhibited only modest instability, consistent with low lightning flash rates and an absence of hail and other severe storm signatures.

Radar, rain gauge, and lightning observations provided a detailed view of the cloud and precipitation morphology. Polarimetric radar observations suggest that a coupling between warm-rain collision coalescence processes and ice processes played an important role in the rainfall production. Dual-Doppler radar and mesoscale wind analyses revealed that the low-level flow field associated with a bow echo located 60 km to the southeast of Fort Collins may have been responsible for a brief easterly acceleration in the low-level winds during the last 1.5 h of the event. The enhanced flow interacted with both topography and the convection located over Fort Collins, resulting in a quasi-stationary convective system and the heaviest rainfall of the evening.

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Steven C. Hardiman, Ian A. Boutle, Andrew C. Bushell, Neal Butchart, Mike J. P. Cullen, Paul R. Field, Kalli Furtado, James C. Manners, Sean F. Milton, Cyril Morcrette, Fiona M. O’Connor, Ben J. Shipway, Chris Smith, David N. Walters, Martin R. Willett, Keith D. Williams, Nigel Wood, N. Luke Abraham, James Keeble, Amanda C. Maycock, John Thuburn, and Matthew T. Woodhouse

Abstract

A warm bias in tropical tropopause temperature is found in the Met Office Unified Model (MetUM), in common with most models from phase 5 of CMIP (CMIP5). Key dynamical, microphysical, and radiative processes influencing the tropical tropopause temperature and lower-stratospheric water vapor concentrations in climate models are investigated using the MetUM. A series of sensitivity experiments are run to separate the effects of vertical advection, ice optical and microphysical properties, convection, cirrus clouds, and atmospheric composition on simulated tropopause temperature and lower-stratospheric water vapor concentrations in the tropics. The numerical accuracy of the vertical advection, determined in the MetUM by the choice of interpolation and conservation schemes used, is found to be particularly important. Microphysical and radiative processes are found to influence stratospheric water vapor both through modifying the tropical tropopause temperature and through modifying upper-tropospheric water vapor concentrations, allowing more water vapor to be advected into the stratosphere. The representation of any of the processes discussed can act to significantly reduce biases in tropical tropopause temperature and stratospheric water vapor in a physical way, thereby improving climate simulations.

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Panel Discussion on Forecast Verification

(Held by the District of Columbia Branch on December 12, 1951)

Mr. Roger A. Allen, Mr. Glenn W. Brier, Mr. Irving I. Gringorten, Captain J. C. S. McKillip, Mr. Conrad P. Mook, Dr. George P. Wadsworth, and Mr. Walter G. Leight
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David R. Smith, William A. Krayer, Kathryn M. Ginger, Michael A. Rosenthal, Jo Ann P. Mulvany, Walter Sanford, Juanita J. Matkins, Loisteen E. Harrell, Bonnie Smith, G. Jayne Koester, Richard L. Lees, John D. Moore, and Frankie C. Vann

Project ATMOSPHERE Atmospheric Education Resource Agents (AERAs) from the mid-Atlantic states conducted their second annual regional workshop for teachers. The focus of this conference was hazardous weather. Over 150 educators from 10 states and the District of Columbia attended this one-day event held in Silver Spring, Maryland. The workshop included presentations by meteorologists and scientists from the National Oceanic and Atmospheric Administration, the Environmental Protection Agency, private corporations, and universities as well as by the AERAs themselves. The presentations were designed to develop basic understandings about hazardous weather and to provide guidance about how to deal with its effects. The orientation of the program was hands on, including a number of activities for teachers to implement in the classroom. This conference demonstrates how educators and scientists can form partnerships to improve science education.

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atmospheric sciences and problems of society

A series of statements on the relevance of the scientific and technological areas of AMS STAC Committees to national and international problems

Earl G. Droessler, John S. Perry, Lance F. Bosart, Robert F. Dale, Walter A. Lyons, Robert E. Dickinson, Floyd C. Elder, Harold W. Baynton, J. A. Weinman, V. E. Derr, and William R. Bandeen
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