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HARRIS A. JONES

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HARRIS A. JONES

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DOUGLAS M. A. JONES

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The reduction in catch due to the shape of the housing of the U.S. Weather Bureau standard recording gage was explored using data from Weather Bureau stations with both recording and nonrecording gages, a gaging site which included both a standard nonrecording gage and a Stevens recording gage, and gages on the East Central Raingage Network. It was found that, on the average, the standard 8.0-in. diameter orifice recording gage caught 2.5 to 6 percent less rain than the nonrecording gage and 2.5 percent less rain than the recording gage fitted with a 12-in. diameter orifice. The Stevens recording gage caught 5.5 percent less rain than the nonrecording gage. It is concluded that proximity of a sloping portion of the gage housing on the 8-in. diameter orifice recording gages is responsible for the catch reduction.

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Douglas M. A. Jones

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It is shown that the Z-R relationship determined by Cunning and Sax (1977) includes additional useful information of cloud physics. The physical processes by which the tropical rainshafts were formed was simple, probably with a single method of drop formation. Comparable Z-R relationships from the Marshall Islands are given. It is shown that the selection of R as the independent variable usually results in a laser coefficient and a smaller exponent than when Z is taken as the independent variable.

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WALTER A. JONES

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David A. Jones and Ian Simmonds

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This study examines the time-space structure of the standard deviation of daily summer and winter mean sea level pressure over the Southern Hemisphere, as identified in 20 years of analyses generated by the Australian Bureau of Meteorology and two long simulations with a GCM. The unfiltered variability derived from the operational analyses generally display a deal of zonal symmetry, particularly during the summer period, with maxima in the midlatitudes. The percentage of the temporal variance which is explained by the bandpass and low-pass components is calculated; in January and July the percentage of the variance explained by the bandpass data is maximized between 30° and 60°S and assumes values of typically 25%. In general, the low-pass data account for more of the variance and tends to achieve its maxima in low and high latitudes. The greatest contribution to the low-frequency field comes from the planetary-scale waves, particularly at higher latitudes. The synoptic and small-scale waves are generally found to be the dominant contributors to the variance in the higher-frequency bandpass fields, particularly in the region of the hemispheric storm track.

Similar analyses applied to the output of the GCM suggest that, overall, the model performs reasonably well, although the quality of the simulation of the low-frequency variability is inferior to that of the synoptic time scales. The tendency of the model to overpredict the winter daily mean sea level pressure variability in the South Pacific appears to be mostly due to this error in the low-frequency part of the field.

These results reveal a considerable difference between the location of cyclone centers and bandpassed mean sea level pressure variability in the high southern latitudes. The model data imply that the maxima of the bandpassed variability tend to be some 30°–40° of longitude to the west and 5°–7° latitude to the north of those of cyclone centers. This serves to underline the dangers and ambiguity of referring to regions of high variability as “storm tracks.”

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DOUGLAS M. A. JONES and FLOYD A. HUFF

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Thomas A. Jones and David J. Stensrud

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The sensitivity of assimilating satellite retrievals of cloud water path (CWP) to the microphysics scheme used by a convection-allowing numerical model is explored. All experiments use the Advanced Research core of the Weather Research and Forecasting Model (WRF-ARW), with observations assimilated using the Data Assimilation Research Testbed ensemble adjustment Kalman filter and a 40-member ensemble. Three-dimensional idealized supercell simulations are generated from a deterministic WRF nature run started from a homogeneous set of initial conditions. Four cloud microphysics schemes are tested: Lin–Farley–Orville (LFO), Thompson (THOMP), Morrison double-moment (MOR), and Milbrandt–Yau (MY).

For the idealized experiments, assimilating CWP generates a mature supercell after approximately 1 h for all microphysics schemes. Vertical profiles of ensemble covariances show large differences in the relationship between CWP and various hydrometeor mixing ratios. While the differences in overall CWP are small, the experiments generate very different reflectivity analyses of the simulated storm, with MOR and MY underestimating reflectivity by a large margin. Vertical profiles of hydrometeor mixing ratios from each experiment are generally consistent with scheme design, such that the Thompson scheme characterizes the storm top as mostly snow whereas the Milbrandt–Yau scheme characterizes the storm top as mostly ice. The impacts of these differences on 30-min forecasts show that MOR and MY are unable to maintain convection within the model while THOMP and LFO perform somewhat better, though all fail to capture the divergent movement of the storm split in the nature run.

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

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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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Danahé Paquin-Ricard, Colin Jones, and Paul A. Vaillancourt

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The total downwelling shortwave (SWD) and longwave (LWD) radiation and its components are assessed for the limited-area version of the Global Environmental Multiscale Model (GEM-LAM) against Atmospheric Radiation Measurements (ARM) at two sites: the southern Great Plains (SGP) and the North Slope of Alaska (NSA) for the 1998–2005 period. The model and observed SWD and LWD are evaluated as a function of the cloud fraction (CF), that is, for overcast and clear-sky conditions separately, to isolate and analyze different interactions between radiation and 1) atmospheric aerosols and water vapor and 2) cloud liquid water. Through analysis of the mean diurnal cycle and normalized frequency distributions of surface radiation fluxes, the primary radiation error in GEM-LAM is seen to be an excess in SWD in the middle of the day. The SWD bias results from a combination of underestimated CF and clouds, when present, possessing a too-high solar transmissivity, which is particularly the case for optically thin clouds. Concurrent with the SWD bias, a near-surface warm bias develops in GEM-LAM, particularly at the SGP site in the summer. The ultimate cause of this warm bias is difficult to uniquely determine because of the range of complex interactions between the surface, atmospheric, and radiation processes that are involved. Possible feedback loops influencing this warm bias are discussed. The near-surface warm bias is the primary cause of an excess clear-sky LWD. This excess is partially balanced with respect to the all-sky LWD by an underestimated CF, which causes a negative bias in simulated all-sky emissivity. It is shown that there is a strong interaction between all the components influencing the simulated surface radiation fluxes with frequent error compensation, emphasizing the need to evaluate the individual radiation components at high time frequency.

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