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H. R. Glahn and J. O. Ellis

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Andrew W. Ellis and Daniel J. Leathers

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Over the past two decades a greater emphasis has been placed on the accuracy of the representation of snow cover–atmosphere interactions in weather and climate prediction models. Much of the attention centered upon snow cover is a result of concerns associated with anthropogenic and natural causes of potential changes in the global environment that may be intensified by the snow cover climatology. As a predictive tool, the importance of the interactions between snow cover and the overlying atmosphere is recognized in areas ranging from daily and seasonal surface air temperature forecasts, to anomalies in large-scale atmospheric circulation patterns. Within this study the effects of snow cover on surface air temperatures within cold air masses moving across the U.S. Great Plains in winter were investigated.

Through the adaptation of a one-dimensional snowpack model, the thermal characteristics of the core of a cold air mass were derived from the equation governing the heat balance between the surface and the lower atmosphere. The methodology was based on the premise that the core of a cold air mass may be considered homogeneous and not subject to advection of air from outside, thereby isolating the exchange of energy between the surface and the atmosphere as the control on lower-atmospheric temperatures. The adapted model included the synergism of the air mass–snow cover relationship through time, incorporating the natural feedback process.

Simulation of surface air temperatures within four cold air masses produced results that include 1) mean daytime temperatures 6°–10°C warmer and maximum daytime temperatures 10°–15°C warmer over bare ground compared to a snow cover, 2) mean nighttime temperatures 1°–2°C warmer over bare ground compared to a snow cover, and 3) attribution of temperature differences primarily to differences in the exchange of sensible heat between the surface and the overlying air mass. Daytime differences in the energy fluxes produced by the different surface conditions were largely the result of surface temperature differences produced by variations in the albedo and the amount of solar radiation absorbed at the surface.

The authors believe that the results of this study can be used as additional guidance for more accurate forecasts of daily maximum and minimum surface air temperatures within wintertime cold air masses over, and possibly downstream from, snow cover across the North American continent. The study documents the importance of the various components of the heat balance between the lower atmosphere and surface in regard to cold airmass modification, and emphasizes the importance of an accurate representation of snow cover in forecast models.

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EARL F. ROBINSON and ELLIS J. JOSEPH

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ROBERT F. SHAW and ELLIS J. JOSEPH

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Andrew W. Ellis and Daniel J. Leathers

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Due to their mesoscale nature, forecasting lake-effect snowfall events is very difficult but extremely important to the inhabitants of those regions subject to this type of severe winter weather. Such is the case along the southern and eastern shores of Lakes Erie and Ontario in the northeastern region of the United States. Here a synoptic climatological approach is used to identify the synoptic-scale atmospheric patterns conductive to lake-induced snowfall to the lee of Lakes Erie and Ontario in the states of New York and Pennsylvania from November to March. The approach used in this study allows for a thorough investigation of the characteristics of each lake-effect synoptic type, including the intrannual and interannual variations in frequency and composite atmospheric fields of sea level pressures, 850-mb temperatures and heights, and 500-mb heights. By combining the lake-effect synoptic types with daily snowfall data for 159 stations across New York and Pennsylvania, direct associations are made between each synoptic type and the mean snowfall and snowfall frequency across that region.

Five synoptic types are identified as producing significant lake-effect snowfall in western New York and northwestern Pennsylvania. The large-scale synoptic situation is similar for each lake-effect type; however, each can be clearly distinguished by its wind components, which are important factors in the spatial pattern and intensity of lake-effect snowfall. Variations in the sea level pressure patterns, 850-mb temperatures and heights, 500-mb heights, seasonality, and overlake fetch and strength of flow result in significant differences in the location, magnitude, and frequency of the snowfalls associated with these types. Three of the lake-effect types occur most often in midwinter, while two are most frequent near the beginning and/or end of the snowfall season. Additionally, the interannual frequencies of the midwinter types indicate an upward trend that coincides with evidence of a lake-effect snowfall increase during midseason over the past century.

The authors believe that the differences in the lake-effect synoptic types outlined here can be used as additional guidance for more accurate extended forecasts of lake-effect snowfall in northwestern Pennsylvania and western New York.

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Andrew W. Ellis and Jennifer J. Johnson

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Research over the past several decades has indicated that snowfall has increased dramatically over portions of the past century across those areas of the Great Lakes region of North America that are subject to lake-effect snowfall. Within this study, time series of annual midwinter snowfall within lake-effect areas show evidence of a clear increase in both snowfall and snowfall frequency through a 40-yr period beginning in the early 1930s and ending in the early 1970s. The goal of the work presented here is to determine to what extent the apparent increases in lake-effect snowfall actually modified the winter hydroclimate of the areas.

Simple hydroclimatic analysis of midwinter precipitation to the lee of Lakes Erie and Ontario for the period of significant snowfall increases suggests that the changes were a product of 1) a shift toward more precipitation events that were snowfall rather than rainfall, 2) an associated decrease in midwinter rainfall, 3) an increase in the intensity of individual snowfall events, and 4) an increase in the snowfall/snow water equivalence ratio. The balance was a small increase in total precipitation confined to areas in close proximity to the lakes across northeastern Ohio and western New York, while areas outside the regions generally experienced an overall decrease in midwinter precipitation. While the cause(s) of the snowfall trends remains elusive, the results of the work presented here suggest that no great long-term regional change occurred in the true wintertime seasonal hydroclimate of the lake-effect areas. Rather, much of the touted snowfall increase simply came at the expense of rainfall events to produce only small changes in total precipitation over the time period of significant snowfall increase.

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W. G. Ellis Jr. and J. T. Merrill

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Atmospheric chemistry programs often make use of retrospective back trajectories to determine the source regions of substances sampled at a particular site. Isentropic trajectories, which depict motion on hypothetical surfaces of constant potential temperature, have been used as part of the Atmosphere/Ocean Chemistry Experiment to understand the sources of aerosols and gases sampled over the North Atlantic Ocean. However, isentropic trajectories typically do not adequately describe the transport of mineral aerosol, for example, from the Sahara Desert to Barbados. Boundary layer trajectories indicated that northern Africa was the source region for 12% of the samples with significant aluminum (Al) concentrations (> 1.0 µg m−3). Upper-level isentropic trajectories (310 and 315 K) indicated transport from northern Africa for approximately half of the samples with Al concentrations greater than 1.0 µg m−3. However, at the location of the sampling site, the upper-level trajectories were well above the boundary layer, where the mineral aerosol samples were collected. Stokes's law was used in the calculation of nonisentropic trajectories that incorporated the gravitational settling of aerosol particles. These trajectories, which began in the boundary layer, were calculated from a combination of lower-level and upper-level wind fields, and more accurately represented the wind fields that transported Saharan dust to Barbados. The Stokes's law trajectories for 4-µm aerosol particles reached a maximum in transport from northern Africa during the summer, which was also when the highest Al concentrations were observed. The Stokes's law trajectories for 4-µm aerosol particles indicated northern Africa as the source region for 55% of the high Al samples, while the equivalent value was only 9% for the 2-µm aerosols trajectories. Dust samples collected at Barbados have a smaller mean radius than the calculation required, indicating that other vertical motions are important during transport in addition to gravitational settling. However, up to 20% of the dust sampled at Barbados is in the size range used (4 µm).

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Andrew W. Ellis and Daniel J. Leathers

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The presence of snow cover has been shown to modify atmospheric conditions through much of the earth’s troposphere due to its radiative effects. Snow cover has garnered much attention in recent decades as a result of concerns associated with potential changes in the global environment that may be intensified by the presence or absence of a snow cover. As a result, a greater emphasis has been placed on the representation of snow cover in weather and climate prediction models. This study investigates the effects of snow albedo and snow depth on the modification of surface air temperatures within cold air masses moving across the U.S. Great Plains in winter.

Through the adaptation of a one-dimensional snowpack model, the thermal characteristics of the core of a cold air mass were derived from the equation governing the heat balance between the surface and the lower atmosphere. The methodology was based on the premise that the core of a cold air mass may be considered homogeneous and not subject to advection of air from outside, thereby isolating the exchange of energy between the surface and the atmosphere as the control on lower-tropospheric temperatures. The adapted model included the synergism of the air mass–snow cover relationship through time, incorporating the natural feedback process.

Simulation of surface air temperatures within four cold air masses over snow cover of different albedo values and depths led to several conclusions. In testing the effects of snow albedo, results indicate 1) mean daytime air temperatures 3°–6°C higher and maximum daytime air temperatures 7°–12°C higher over snow with an albedo equal to 0.50 compared to 0.90, as a consequence of differences in sensible heat flux, and ultimately, absorbed solar radiation, and 2) little thermal inertia and therefore little difference in subsequent nighttime airmass temperatures over snow with an albedo of 0.50 compared to 0.90. In testing the effects of snow depth, results indicate 1) little difference in daytime air temperatures associated with a snow depth of 2.5 cm compared to 15.0 or 30.0 cm, 2) an increase in mean nighttime temperatures of 0.2°–0.7°C over a snow depth of 2.5 cm compared to either of the larger depths, and 3) a masking of the underlying bare soil surfaces by the snow depths of 15.0 and 30.0 cm and virtually no difference in airmass temperatures over the two snow depths.

The potential utility of the results of this study lies in their application as additional guidance for temperature forecasts within wintertime cold air masses over, and downstream from, snow cover across the U.S. Great Plains. Likewise, this study illustrates the importance of the various components of the heat balance between the lower atmosphere and snow cover as based on the physical characteristics of the snowpack, which could prove beneficial in considerations of snow cover in weather and climate models.

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T. Rossby, J. Ellis, and D. C. Webb

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To meet the expected need for wide-area acoustic navigation for Lagrangian studies of ocean circulation using RAFOS floats, a new and powerful sound source, a resonant pipe projector has been developed. It consists of a free-flooded open steel pipe with a ceramic-steel driver ring at its midsection. Conservatively demonstrated here at a source level of 195.5 dB re 1 µpa @ 1 m and with an energy conversion efficiency of 85% at resonance (260 Hz), useful operating ranges to 4000 km and beyond are possible, depending on ambient noise conditions. A successful 6-month accelerated test of the complete transducer system was recently completed near Bermuda.

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Ronald W. Fegley, Howard T. Ellis, and J. L. Heffter

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We have detected the transport of volcanic sulfate through the tropical tropopause. This is particularly noteworthy because the source volcanic eruption was only of modest intensity and, therefore, not normally thought to be of stratospheric importance. Using lidar, the veil was detected on two days during January 1977. Although the bulk of the material was in the troposphere, there was evidence of transport through the tropopause on both days. Trajectory calculations suggested that the source was Nyiragongo volcano in central Africa (1.5°S, 29.2°E). The presence of an adiabatic temperature profile through the dust layer suggests that the volcanic layer was being convected by means of radiative absorption. This mechanism may provide sulfate for the maintenance of a background level of stratospheric sulfate. During the six years of lidar observations in Hawaii (19.5°N), we have rarely observed such layers. Visual, lidar and other observations of the volcanic cloud are presented.

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