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Patrick A. Jones and Peter R. Bannon

Abstract

This study examines the diurnal behavior of the dryline system using a mixed-layer model to represent the cool moist air capped by an inversion to the east of the line. This inversion is referred to as the dry front, and the intersection of this dry front with the terrain is the dryline. The results indicate that boundary layer heating is sufficient to drive the dryline and explain its diurnal variation.

The daytime eastward propagation of the model dryline of 200 km agrees well with other numerical studies and is in approximate agreement with dryline observations. The present model results also indicate a nearly vertical inversion slope up to a height of 2 km in the early afternoon. Model simulations with sloping terrain consistently yield a nocturnal low-level jet between 0000 local time (LT) and 0100 LT, with a speed of 20–25 m s−1, located below the inversion.

The effect of each mixed-layer process, such as entrainment, surface heat flux, and nighttime cooling, is examined. Entrainment tends to steepen the slope of the dry front near the dryline but has little impact on its eastward advance. The dryline advance is most sensitive to the amplitude of the surface heat flux relative to the depth of the mixed layer and the strength of the inversion. Large heat fluxes, in combination with a shallow mixed layer and a weak inversion, produce the greatest dryline advance. The westward surge of the dryline at dusk is most sensitive to the amplitude of the nighttime cooling: larger cooling produces a larger surge.

The model simulations consistently predict a local maximum in the inversion height (called a spike) near the dryline at dusk associated with entrainment and boundary layer convergence. This process may be one of the possible triggers for the deep convection often seen just to the east of the dryline.

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Douglas M. A. Jones and Wayne M. Wendland

Abstract

Known sources of data from arrays of instantaneous precipitation intensity recorders in southern Germany, east-central Illinois, northeastern Illinois, central Florida, and Hilo, Hawaii are obtained. These data are analyzed for line averages of the percent frequency of occurrence of the exceedance of selected threshold precipitation intensities. The correlation coefficients of the precipitation intensity at sites at varying distances from a reference site are determined. The decay in correlation is found to be a function of climatic region and the type of precipitation: showery or continuous. Showery rains are found to be essentially uncorrelated about 12 km from the reference site while continuous rain exhibits no correlation beyond about 50 km.

Single-station intensity data collected at Urbana, Illinois; Paris, France; Inyanga, Zimbabwe; Bogor, Indonesia; Reading, United Kingdom; Island Beach, New Jersey; Miami, Florida; Franklin, North Carolina; and Majuro, Marshall Islands, are compared.

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Douglas M. A. Jones and Arthur L. Sims

Abstract

Raingage records from four climatic zones (maritime subtropical, continental temperate, maritime temperate and midlatitude interior) were analyzed to study instantaneous rainfall rates as defined by 1 and 4 min accumulations. The frequency distribution of rainfall rates was determined for stations in each of these climatic zones and a zonal average frequency distribution calculated. A progression in the frequency of more intense precipitation was found from the North Pole to the Equator since all of the data were taken from the Northern Hemisphere. The most intense rainfalls were recorded at stations in the maritime subtropical zones and the least intense rainfalls in the maritime temperate zones.

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Christopher J. Nowotarski and Erin A. Jones

Abstract

Self-organizing maps (SOMs) have been shown to be a useful tool in classifying meteorological data. This paper builds on earlier work employing SOMs to classify model analysis proximity soundings from the near-storm environments of tornadic and nontornadic supercell thunderstorms. A series of multivariate SOMs is produced wherein the input variables, height, dimensions, and number of SOM nodes are varied. SOMs including information regarding the near-storm wind profile are more effective in discriminating between tornadic and nontornadic storms than those limited to thermodynamic information. For the best-performing SOMs, probabilistic forecasts derived from matching near-storm environments to a SOM node may provide modest improvements in forecast skill relative to existing methods for probabilistic forecasts.

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Aubrey R. Jones and Nathaniel A. Brunsell

Abstract

A series of model runs using the University of Oklahoma’s Advanced Regional Prediction System (ARPS) were conducted to investigate the relative impacts of energy balance partitioning and net radiation on soil moisture–precipitation feedbacks in the U.S. central plains and to examine how the dominant physical processes are affected by changes in mean soil moisture and spatial resolution. Soil temperature and Bowen ratio are influenced nonlinearly by soil moisture, and by varying the mean soil moisture in the model it was possible to examine the relationship between soil moisture and the scaling characteristics of these fields using the statistical moments. Information theory metrics were used to provide an indication of the uncertainty associated with varying model resolutions. It was determined that energy balance partitioning plays a dominant role in the occurrence of soil moisture–precipitation feedback, while net radiation was not impacted by mean soil moisture. A strong relationship was seen between soil moisture and the scaling properties of Bowen ratio, while soil moisture did not appear to influence the scaling characteristics of soil temperature. Spatial resolution had a large effect on the representation of boundary layer turbulence, with coarser resolutions unable to capture turbulent motions, which are necessary for convective processes. The ability of the model to capture boundary layer turbulence will alter the dynamics of soil moisture–precipitation feedback as the horizontal transport of moisture by turbulent motions will affect the spatial and temporal scales over which feedback occurs. Higher-resolution runs are generally associated with a higher information content. This may provide a methodology for monitoring land–atmosphere feedbacks via remotely sensed soil moisture and vegetation fields through statistical knowledge of the dependency of the resulting precipitation signal on soil moisture and vegetation fields at the resolution they were observed.

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J. E. Jones and A. M. Davies

Abstract

Although the problem of predicting storm surge elevations has received significant attention, the simulation of currents has suffered because of lack of current observations during surges. Current measurements made during surge conditions are presented here and are used in combination with three-dimensional models to understand processes producing storm currents in the Irish Sea. A coarse-grid (resolution of order 7 km) model of the west coast of Britain together with a fine-grid (of order 1 km) model of the eastern Irish Sea is used to examine the processes, namely, open boundary forcing of the west coast model and wind fields, that produced flows within the eastern Irish Sea during the storm surge of November 1977. Simulations of the surge show that the fine-grid model nested within the west coast model can reproduce observed coastal changes in surge elevation. However, an observed major inflow that was recorded by current meters in the region, prior to a storm surge elevation peak, is not represented, although subsequent inflows and outflows are reproduced. The flow fields in the west coast model giving rise to these currents are analyzed in detail. Also, computations are performed with idealized open boundary forcing and wind fields to understand their role in determining the circulation within the region. An analysis of computed flows shows that outflows from the eastern Irish Sea following major storm events are determined by sea surface elevation gradients in the region and topographic effects. Observed flows under these conditions are reproduced by the model. Inflows, however, are more difficult to compute and depend upon a delicate balance of northern and southern boundary forcing of the west coast model and wind fields over the region. The first observed inflow event, which was not reproduced in the model, was associated with a current from the south. A second inflow event that was reproduced arose from a combination of an inflow from north and south, and a third event was again reproduced in the model due to a current from the north. Without a more comprehensive observational dataset, it was not possible to determine the exact reason why the first inflow was not reproduced.

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Thomas A. Jones, Sundar A. Christopher, and Walt Petersen

Abstract

Dual-polarimetric microwave wavelength radar observations of an apartment fire in Huntsville, Alabama, on 3 March 2008 are examined to determine the radar-observable properties of ash and fire debris lofted into the atmosphere. Dual-polarimetric observations are collected at close range (<20 km) by the 5-cm (C band) Advanced Radar for Meteorological and Operational Research (ARMOR) radar operated by the University of Alabama in Huntsville. Precipitation radars, such as ARMOR, are not sensitive to aerosol-sized (D < 10 μm) smoke particles, but they are sensitive to the larger ash and burnt debris embedded within the smoke plume. The authors also assess if turbulent eddies caused by the heat of the fire cause Bragg scattering to occur at the 5-cm wavelength.

In this example, the mean reflectivity within the debris plume from the 1.3° elevation scan was 9.0 dBZ, with a few values exceeding 20 dBZ. The plume is present more than 20 km downstream of the fire, with debris lofted at least 1 km above ground level into the atmosphere. Velocities up to 20 m s−1 are present within the plume, indicating that the travel time for the debris from its source to the maximum range of detection is less than 20 min. Dual-polarization observations show that backscattered radiation is dominated by nonspherical, large, oblate targets as indicated by nonzero differential reflectivity values (mean = 1.7 dB) and low correlation coefficients (0.49). Boundary layer convective rolls are also observed that have very low reflectivity values (−6.0 dBZ); however, differential reflectivity is much larger (3.2 dB). This is likely the result of noise, because ARMOR differential reflectivity is not reliable for reflectivity values <0 dBZ. Also, copolar correlation is even lower compared to the debris plume (0.42). The remainder of the data mainly consists of atmospheric and ground-clutter noise. The large differential phase values coupled with positive differential reflectivity strongly indicate that the source of much of the return from the debris plume is particle scattering. However, given the significant degree of noise present, a substantial contribution from Bragg scattering cannot be entirely ruled out.

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Robert Davies-Jones, Vincent T. Wood, and Mark A. Askelson

Abstract

Two accepted postulates for applications of ground-based weather radars are that Earth’s surface is a perfect sphere and that all the rays launched at low-elevation angles have the same constant small curvature. To accommodate a straight vertically launched ray, we amend the second postulate by making the ray curvature dependent on the cosine of the launch angle. A standard atmospheric stratification determines the ray-curvature value at zero launch angle. Granted this amended postulate, we develop exact formulas for ray height, ground range, and ray slope angle as functions of slant range and launch angle on the real Earth. Standard practice assumes a hypothetical equivalent magnified earth, for which the rays become straight while ray height above radar level remains virtually the same function of the radar coordinates. The real-Earth and equivalent-earth formulas for height agree to within 1 m. Our ultimate goal is to place a virtual Doppler radar within a numerical or analytical model of a supercell and compute virtual signatures of simulated storms for development and testing of new warning algorithms. Since supercell models have a flat lower boundary, we must first compute the ray curvature that preserves the height function as the earth curvature tends to zero. Using an approximate height formula, we find that keeping planetary curvature minus the ray curvature at zero launch angle constant preserves ray height to within 5 m. For standard refraction the resulting ray curvature is negative, indicating that rays bend concavely upward relative to a flat earth.

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Ryan L. Fogt, Megan E. Jones, Chad A. Goergens, Susan Solomon, and Julie M. Jones
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Ryan L. Fogt, Megan E. Jones, Susan Solomon, Julie M. Jones, and Chad A. Goergens

Abstract

The meteorological conditions during the Amundsen and Scott South Pole expeditions in 1911/12 are examined using a combination of observations collected during the expeditions as well as modern reanalysis and reconstructed pressure datasets. It is found that over much of this austral summer, pressures were exceptionally high (more than two standard deviations above the climatological mean) at both main bases, as well as along the sledging journeys, especially in December 1911. In conjunction with the anomalously high pressures, Amundsen and his crew experienced temperatures that peaked above –16°C on the polar plateau on 6 December 1911, which is extremely warm for this region. While Scott also encountered unusually warm conditions at this time, the above-average temperatures were accompanied by a wet snowstorm that slowed his progress across the Ross Ice Shelf. Although January 1912 was marked with slightly below-average temperatures and pressure, high temperatures and good conditions were observed in early February 1912, when Scott and his companions were at the top of the Beardmore Glacier. When compared to the anomalously cold temperatures experienced by the Scott polar party in late February and March 1912, the temperature change is in the top 3% based on more than 35 years of reanalysis data. Scott and his companions therefore faced an exceptional decrease in temperature when transiting to the Ross Ice Shelf in February and March 1912, which likely made the persistent cold spell they experienced on the Ross Ice Shelf seem even more intense by comparison.

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