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W. S. Wilson and J. P. Dugan


Two multi-ship XBT surveys have acquired data on the mesoscale temperature structure in the region of the Kuroshio Extension. Thermal sections and maps illustrate cold features in the thermocline near 31°N and warm ones near 37°N. Isotherm displacement spectra exhibit a plateau of order 103 m2 per cycle per kilometer for scales longer than 500 km, and they roll off between the second and third inverse power of the wavenumber for shorter scales. There is a significant decrease in spectral amplitude south of about 31°N.

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Robert J. Serafin and James W. Wilson

The NEXRAD operational system consisting of a network of WSR-88D radars is now operational within the 50 states, as well as Puerto Rico and Guam. This technology has been enthusiastically received by weather forecasters in all regions and climatic regimes of the country. Improvements in short-term weather forecasting and nowcasting have resulted, but the potential for further improvement is also great. Many of the advantages of the system are associated with its quantitative and precise digital data, but problems related to accuracy of precipitation estimation, contamination of Doppler radar products by ground clutter, and the range folding of velocity data all deserve attention. These problems and others are being addressed by the Operational Support Facility of the triagencies: the National Weather Service, the Federal Aviation Administration, and the Department of Defense. Further improvements to the system, in both hardware and software, will greatly enhance its capabilities for the future. These improvements are likely to include new open-system signal and data processing architectures that will greatly expand the ability of the system to produce a wide range of better and more sophisticated weather products. In addition, new capabilities such as polarization diversity may also be added. At the same time, it is appropriate to look forward into the future and, within a decade, to begin planning for the successor to NEXRAD.

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F. I. Shimabukuro, P. L. Smith, and W. J. Wilson


The daytime and nighttirne distribution of the ozone density in the atmosphere has been determined from ground-based measurements of the emission spectra of the strong 40,4 = 41,3 rotational line of ozone at 101.737 GHz (λ = 2.9 mm), using a least-squares parameter estimation technique. The inversion procedure is described, and a linearized model is used to obtain approximate error bounds on the ozone parameter estimates.

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Nicholas J. Lutsko, Jane Wilson Baldwin, and Timothy W. Cronin


The impact of large-scale orography on wintertime near-surface (850 hPa) temperature variability on daily and synoptic time scales (from days to weeks) in the Northern Hemisphere is investigated. Using a combination of theory, idealized modeling work, and simulations with a comprehensive climate model, it is shown that large-scale orography reduces upstream temperature gradients, in turn reducing upstream temperature variability, and enhances downstream temperature gradients, enhancing downstream temperature variability. Hence, the presence of the Rockies on the western edge of the North American continent increases temperature gradients over North America and, consequently, increases North American temperature variability. By contrast, the presence of the Tibetan Plateau and the Himalayas on the eastern edge of the Eurasian continent damps temperature variability over most of Eurasia. However, Tibet and the Himalayas also interfere with the downstream development of storms in the North Pacific storm track, and thus damp temperature variability over North America, by approximately as much as the Rockies enhance it. Large-scale orography is also shown to impact the skewness of downstream temperature distributions, as temperatures to the north of the enhanced temperature gradients are more positively skewed while temperatures to the south are more negatively skewed. This effect is most clearly seen in the northwest Pacific, off the east coast of Japan.

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Cathy J. Kessinger, David B. Parsons, and James W. Wilson


On 30 June 1982, a multicellular storm in Colorado produced four downbursts, three misocyclones, a miso-anticyclone, and horizontal vortex circulations within a relatively small area of the storm. Weather events associated with this storm included hail, heavy rain, and strong winds. A sounding taken two hours before storm formation showed the mixed layer was characterized by a nearly dry adiabatic lapse rate to ∼2 km and was relatively moist for eastern Colorado. A hodograph showed the environment had weak to moderate vertical shear of the horizontal wind, a condition conducive to the formation of downdraft misocyclones. The four-dimensional structure of this storm is documented below cloud base using winds, reflectivity, and thermodynamic data derived front multiple Doppler analysis.

One misocyclone (<4 km scale) is particularly intense with a peak vorticity of ≈100 × 10−3 s−1 near cloud base. Despite the intense rotation, no tornadoes or funnels were observed and no damage was reported. Radar characteristics of this misocyclone are similar to those of mesocyclones that produce tornadoes or funnels except that vorticity is a maximum near cloud base and the low-level divergence created by the downbursts weakens the low-level, positive vorticity. While the misocyclone is initially separated from the downdraft, the two features evolve to become collocated. Each misocyclone becomes associated with a local downdraft maximum, suggesting that the misocyclones are important to downdraft development.

Pressure perturbation analysis does not show any evidence for strong, downward-directed pressure gradient forces below cloud base that would act to accelerate a downdraft. Since the downdraft is observed to accelerate below cloud base, other forces must be important. Observations and buoyancy estimates calculated from radar reflectivity show negative buoyancy is playing a role in downdraft intensification. Despite the lack of dynamical forcing of the downdraft by the misocyclone below cloud base, dynamical forces may be playing a role in accelerating the downdraft above cloud base.

Horizontal vortex circulations, or rotors, form along the edge of the misocyclone and downdraft and propagate away from their source region. Strongest surface winds are associated with the rotors. Pressure perturbation analysis shows that a low forms at the center of the circulation that may cause an acceleration of the low-level outflow into the rotor and may explain the strong winds. Rotors may be an integral part of downburst outflows and perhaps multiple rotors are created by pulsating downdrafts. An explanation of these circulations is important since they seem to have been involved in the Dallas-Fort Worth Regional Airport crash of an L-1011 jet.

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Edward A. Brandes, J. Vivekanandan, and James W. Wilson


Radar reflectivity–based rainfall estimates from collocated radars are examined. The usual large storm-to-storm variations in radar bias and high correlation between radar estimates and rain gauge observations are found. For three storms in Colorado, the radar bias factor (the ratio between gauge observations and radar estimates) with the National Center for Atmospheric Research’s S-band, dual-polarization radar (S-Pol) varied from 0.78 (an overestimate with radar) to 1.88. The correlation coefficient between gauge and radar amounts varied from 0.78 to 0.90. For a collocated Weather Surveillance Radar-1988 Doppler (WSR-88D), the bias factor varied from 0.56 to 1.49, and the correlation between gauge and radar amounts ranged from 0.77 to 0.87. In Kansas, bias factors varied from 0.86 to 1.41 for S-Pol (10 storms) and 0.82 to 1.71 for a paired WSR-88D (9 storms). The spread in correlation coefficients was 0.82–0.95 for S-Pol and 0.87–0.95 for the WSR-88D.

Correspondence between the radar-derived rainfall estimates for the paired radars was very high; correlation coefficients were 0.88 to 0.98. Moreover, the ratio between rainfall estimates (S-Pol/paired WSR-88D) varied only from 0.72 to 0.85 in Colorado and 0.82 to 1.05 in Kansas. The total variation in radar-to-radar rainfall estimates, roughly a factor of 1.2, is attributed primarily to nonmeteorological factors relating to radar hardware and processing. The radar-to-radar variation is small compared to the spread in storm-to-storm biases, which varied from a low of 1.64 with the S-Pol radar in Kansas to a high of 2.66 with the WSR-88D in Colorado. For this investigation, the storm-to-storm bias must have a large meteorological component—probably due to temporal and spatial changes in drop size distributions and consequently variations in the relationship between radar reflectivity and rainfall rate.

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C. Anderson, J. Figa, H. Bonekamp, J. J. W. Wilson, J. Verspeek, A. Stoffelen, and M. Portabella


The Advanced Scatterometer (ASCAT) on the Meteorological Operational (MetOp) series of satellites is designed to provide data for the retrieval of ocean wind fields. Three transponders were used to give an absolute calibration and the worst-case calibration error is estimated to be 0.15–0.25 dB.

In this paper the calibrated data are validated by comparing the backscatter from a range of naturally distributed targets against models developed from European Remote Sensing Satellite (ERS) scatterometer data.

For the Amazon rainforest it is found that the isotropic backscatter decreases from −6.2 to −6.8 dB over the incidence angle range. The ERS value is around −6.5 dB. All ASCAT beams are within 0.1 dB of each other. Rainforest backscatter over a 3-yr period is found to be very stable with annual changes of approximately 0.02 dB.

ASCAT ocean backscatter is compared against values from the C-band geophysical model function (CMOD-5) using ECMWF wind fields. A difference of approximately 0.2 dB below 55° incidence is found. Differences of over 1 dB above 55° are likely due to inaccuracies in CMOD-5, which has not been fully validated at large incidence angles. All beams are within 0.1 dB of each other.

Backscatter from regions of stable Antarctic sea ice is found to be consistent with model backscatter except at large incidence angles where the model has not been validated. The noise in the ice backscatter indicates that the normalized standard deviation of the backscatter values Kp is around 4.5%, which is consistent with the expected value.

These results agree well with the expected calibration accuracy and give confidence that the calibration has been successful and that ASCAT products are of high quality.

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J. W. Wilson, S. B. Trier, D. W. Reif, R. D. Roberts, and T. M. Weckwerth


During the Plains Elevated Convection at Night (PECAN) experiment, an isolated hailstorm developed on the western side of the PECAN study area on the night of 3–4 July 2015. One of the objectives of PECAN was to advance knowledge of the processes and conditions leading to pristine nocturnal convection initiation (CI). This nocturnal hailstorm developed more than 160 km from any other convective storms and in the absence of any surface fronts or bores. The storm initiated within 110 km of the S-Pol radar; directly over a vertically pointing Doppler lidar; within 25 km of the University of Wyoming King Air flight track; within a network of nine sounding sites taking 2-hourly soundings; and near a mobile mesonet track. Importantly, even beyond 100 km in range, S-Pol observed the preconvection initiation cloud that was collocated with the satellite infrared cloud image and provided information on the evolution of cloud growth. The multiple observations of cloud base, thermodynamic stability, and direct updraft observations were used to determine that the updraft roots were elevated. Diagnostic analysis presented in the paper suggests that CI was aided by lower-tropospheric gravity waves occurring in an environment of weak but persistent mesoscale lifting.

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James W. Wilson, Tammy M. Weckwerth, J. Vivekanandan, Roger M. Wakimoto, and Robert W. Russell


Boundary layer clear-air echoes are routinely observed with sensitive, microwave, Doppler radars similar to the WSR-88D. Operational and research meteorologists are using these Doppler velocities to derive winds. The accuracy of the winds derived from clear-air Doppler velocities depends on the nature of the scatterers. This paper uses dual-wavelength and dual-polarization radars to examine the cause of these echoes and the use of Doppler velocities from the clear-air return to estimate winds. The origin of these echoes has been an ongoing controversy in radar meteorology. These echoes have been attributed to refractive-index gradient (Bragg scattering) and insects and birds (particulate scattering). These echoes are most commonly observed over land from spring through autumn. Seldom do they occur over large bodies of water. Widespread clear-air echoes have also been observed in winter when temperatures are above 10°C.

Radar reflectivity comparisons of clear-air echoes in Florida and Colorado were made at radar wavelengths of 3, 5, and 10 cm. These comparisons, when analyzed along with a theoretical backscattering model, indicate that the echoes result from both particulate and Bragg scattering with particulate scattering dominating in the well-mixed boundary layer. The return signal in this layer is highly horizontally polarized with differential reflectivity ZDR values of 5–10 dB. This asymmetry causes the backscattering cross section to be considerably larger than one for a spherical water droplet of equal mass. At X band and possibly even at C and S hand the scattering enters the Mie region. It is concluded that insects are primarily responsible for the clear-air echo in the mixed boundary layer. At and above the top of the well-mixed boundary layer, Bragg scattering dominates and is frequently observed at S band.

When insects and birds are not migrating, the Doppler velocities can be used to estimate horizontal winds in the boundary layer. Viewing angle comparisons of ZDR values were made to determine if migrations were occurring. Migrations were not observed in Florida and Colorado during summer daylight hours. Limited comparison of winds derived from Doppler radar with balloon-sounding winds showed good agreement. However, a more extensive study is recommended to determine the generality of this conclusion.

Dual-Doppler analyses show that thin-line echoes are updraft regions. Comparison of these radar-derived vertical velocities with aircraft-measured vertical velocities showed a correlation coefficient of 0.79. In addition, the position of small-scale updraft maxima (1–2 km in diameter) along the sea-breeze front correspond to individual cumulus clouds. The good agreement between dual-Doppler-derived vertical motion fields and these other independent vertical velocity measurements provides evidence that the dual-Doppler-derived wind fields in the clear-air boundary layer are accurate and capable of providing details of the wind circulations associated with horizontal convective rolls and the sea breeze.

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R. E. Carbone, J. W. Wilson, T. D. Keenan, and J. M. Hacker


Diurnally forced convection was observed over the Tiwi Islands, north of the Australian continent, as part of the Maritime Continent Thunderstorm Experiment. Immature peninsula-scale (5–15 km) sea breezes were observed to initiate moist convection early each day, principally through convergence that results from the confluence or collision of peninsula breeze fronts. Convection initiated by peninsula-scale breezes usually fails to organize beyond a small cluster of cells and dissipates as a local event. Mature island-scale (∼100 km) breezes develop by late morning and subsequently play a pivotal role in the forcing and evolution of organized convection.

The initiation of mesoscale convective systems (MCSs) is observed to be a direct consequence of breeze front collisions for only ∼20% of the days on which organized convection develops. This is referred to as “type A” forcing and it occurs when normal convective development is delayed or otherwise suppressed. Type A forcing is nature’s backup mechanism and it is less likely to produce large or strong mesoscale convective systems when compared to the general population of events.

On approximately 80% of days during which organized convection develops, a multiple-stage forcing process evolves through complex interactions between preferred sea breezes and convectively generated cold pools. So-called type B forcing emerges 1–3 h before penetration of the sea-breeze fronts to the interior island. Type B evolution has at least four stages: 1) leeward- or other preferred-coast sea-breeze showers that develop small cold pools, 2) showers that travel inland when their cold pools become denser than the marine boundary layer, 3) westward propagation of squalls that result from a merge or maturation of small cold pools, and 4) interaction between a gust front and a zonally oriented sea-breeze front of island scale (∼100 km). A collision of gust fronts, emanating from separate convective areas over Bathurst and Melville Islands, can excite a fifth stage of development associated with many of the strongest systems.

A principal finding of this study is that all MCSs over the Tiwi Islands can be traced backward in time to the initiation of convection by island-scale sea breezes, usually of type B near leeward coasts. Subsequent convective evolution is characteristic of traveling free convection elsewhere in that it organizes according to cold pool, shear balance, and mean flow factors. The presence of a critical level in the lower troposphere is a unique aspect of the theoretical “optimal condition” associated with island convection in a low-level jet regime; however, the data presented here suggest that the effects of surface layer stagnation may be of greater practical importance.

Since the aforestated conclusions are based on time series of rather limited duration, the reader is cautioned as to uncertainty associated with the climatological frequency of events as described herein. Furthermore, the authors have not examined external forcings, which may be associated with large-scale circulations.

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