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George S. Young and Todd D. Sikora

Abstract

Examination of visible and infrared imagery from geosynchronous and polar orbiter satellites reveals the occasional existence of mesoscale cloud bands of unusual width and area, originating over the open northwest Atlantic Ocean during cold-air outbreaks. This phenomenon is of both dynamic and synoptic interest. As a dynamic phenomenon, it represents a mesoscale flow that is driven by transient surface features, which are meanders in the Gulf Stream. The forcing geometry and the resulting cloud pattern are similar in many respects to the anomalous cloud lines observed downwind of Chesapeake and Delaware Bays in similar conditions. These open ocean cloud bands are often of a larger scale, however, because the Gulf Stream meanders represent the largest-scale high-amplitude “coastal features” in the western North Atlantic. These cloud bands are of synoptic interest because, when present, they play a major role in determining the cloud pattern over much of this oceanic region.

Examination of surface and 850-hPa analyses demonstrates that these open ocean cloud bands occur during cold-air outbreaks and that they align approximately with the boundary layer wind. Comparison of visible and infrared satellite imagery with contemporaneous sea surface temperature analyses derived from infrared polar orbiter satellite imagery reveals that the open ocean cloud bands originate at the upwind end of Gulf Stream meanders. Climatological data and synoptic observations from land and sea indicate that these events occur only during that part of the spring season in which coastal temperature differences are small but cold-air outbreaks continue to reach the Gulf Stream. Examination of this observational evidence suggests that these open ocean cloud bands result from mesoscale solenoidal circulations driven by the horizontal gradients in sea surface temperature caused by Gulf Stream meanders.

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Alex J. DeCaria and Todd D. Sikora
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Todd D. Sikora and David M. Halverson

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Satellite and corresponding near-surface in situ observations have been made of single- and dual-band cloud events [dubbed anomalous cloud lines (ACLs)] associated with the Chesapeake and Delaware Bays. A previous study developed the basis for two hypotheses concerning the mechanism responsible for ACLs. One explanation is that ACLs are forced in the same manner as Great Lakes lake-effect midlake cloud lines. An alternate explanation is that at least some ACLs are a special type of ship track that forms in statically unstable marine boundary layers. The time period examined in the current research is January 1997–December 2000. National Oceanic and Atmospheric Administration (NOAA) Advanced Very High Resolution Radiometer imagery served as the satellite dataset, and NOAA buoy 44009 and Coastal-Marine Automated Network station CHLV2 provided the in situ data. The findings from the satellite portion of this research show that ACLs associated with both the Chesapeake and Delaware Bays were observed on roughly 3% of the days examined and were more frequent during the onset of the cold season. The data also show that single-band ACLs were, in general, more frequent than dual-band ACLs. For the near-surface in situ portion of this research, the average ACL for both bays was associated with a negative air–sea temperature difference and a larger downbay wind component than cross-bay wind component. On a month-by-month basis, ACLs for both bays tended to be associated with abnormally large downbay wind speeds and negative air–sea temperature differences in comparison with the corresponding weighted monthly norms.

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Alex J. DeCaria and Todd D. Sikora

Abstract

The confusion and ambiguity in the literature regarding the notation (V · )V versus V · ∇V is discussed, and the equivalence of the two expressions is demonstrated. The invariance of this notation in any coordinate system is also shown.

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Thomas R. Parish, Richard D. Clark, and Todd D. Sikora

Abstract

The Great Plains low-level jet (LLJ) has long been associated with summertime nocturnal convection over the central Great Plains of the United States. Destabilization effects of the LLJ are examined using composite fields assembled from the North American Mesoscale Forecast System for June and July 2008–12. Of critical importance are the large isobaric temperature gradients that become established throughout the lowest 3 km of the atmosphere in response to the seasonal heating of the sloping Great Plains. Such temperature gradients provide thermal wind forcing throughout the lower atmosphere, resulting in the establishment of a background horizontal pressure gradient force at the level of the LLJ. The attendant background geostrophic wind is an essential ingredient for the development of a pronounced summertime LLJ. Inertial turning of the ageostrophic wind associated with LLJ provides a westerly wind component directed normal to the terrain-induced orientation of the isotherms. Hence, significant nocturnal low-level warm-air advection occurs, which promotes differential temperature advection within a vertical column of atmosphere between the level just above the LLJ and 500 hPa. Such differential temperature advection destabilizes the nighttime troposphere above the radiatively cooled near-surface layer on a recurring basis during warm weather months over much of the Great Plains and adjacent states to the east. This destabilization process reduces the convective inhibition of air parcels near the level of the LLJ and may be of significance in the development of elevated nocturnal convection. The 5 July 2015 case from the Plains Elevated Convection at Night field program is used to demonstrate this destabilization process.

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Todd D. Sikora, Eric B. Wendoloski, and Robert E. Marter

Abstract

This brief paper addresses the frequency of precipitating open-cell convection over the northeastern Gulf of Alaska during a 5-yr period (2002–06). The research employs 154 previously documented satellite synthetic aperture radar–derived wind speed (SDWS) images that contain open-cell convection signatures. Each SDWS image is paired with a near-in-time, National Weather Service Weather Surveillance Radar-1988 Doppler Level-III 0.5°-elevation-angle short-range base reflectivity image from coastal Alaska for which coverage spatially overlaps open-cell convection signatures. The time difference between any two images of a single pair is typically a few minutes or less. For 65% of the image pairs, at least one SDWS open-cell convection signature in the overlap region is associated with precipitation. That percentage may be conservative given the method used in this research. Thus, the results of this research support a suggestion that has been posed in previous studies that the organization of open-cell convection can be controlled by the interaction of the environmental vertical wind shear and precipitation-driven cold pools.

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Todd D. Sikora, George S. Young, and Nathaniel S. Winstead

Abstract

This paper describes a product that allows one to assess the lower and upper bounds on synthetic aperture radar (SAR)-based marine wind speed. The SAR-based wind speed fields of the current research are generated using scatterometry techniques and, thus, depend on a priori knowledge of the wind direction field. The assessment product described here consists of a pair of wind speed images bounding the wind speed range consistent with the observed SAR data. The minimum wind speed field is generated by setting the wind direction field to be directly opposite to the radar look direction. The maximum wind speed field is generated by setting the wind direction field to be perpendicular to the radar look direction. Although the assessment product could be generated using any marine SAR scene, it is expected to be most useful in coastal regions where the large concentration of maritime operations requires accurate, high-resolution wind speed data and when uncertainty in the a priori knowledge of the wind direction precludes the generation of accurate SAR-based wind speed fields. The assessment product is demonstrated using a case in the northern Gulf of Alaska where synoptic-scale and mesoscale meteorological events coexist. The corresponding range of possible SAR-based wind speed is large enough to have operational significance to mariners and weather forecasters. It is recommended that the product become available to the public through an appropriate government outlet.

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George S. Young, Todd D. Sikora, and Nathaniel S. Winstead

Abstract

For the commonly observed range of air–sea temperature difference and surface wind speed, the static stability of the atmospheric surface layer can have a significant effect on the mean surface stress and its turbulence-scale horizontal variability. While traditional satellite-borne scatterometers have insufficient horizontal resolution to map this turbulence-scale horizontal variability, satellite-borne synthetic aperture radars (SAR) can. This paper explores the potential for applying existing boundary layer similarity theories to these SAR-derived maps of turbulence-scale horizontal variability in air–sea stress.

Two potential approaches for deriving boundary layer turbulence and stability statistics from SAR backscatter imagery are considered. The first approach employs Monin–Obukhov similarity theory, mixed layer similarity theory, and a SAR-based estimate of the atmospheric boundary layer depth to relate the ratio of the mean and standard deviation of the SAR-derived wind speed field to the stability of the atmospheric surface layer and the convective scale velocity of the atmospheric mixed layer. The second approach addresses these same goals by application of mixed layer similarity theory for the inertial subrange of the SAR-derived wind speed spectra. In both approaches, the resulting quantitative estimates of Monin–Obukhov and mixed layer scaling parameters are then used to make a stability correction to the SAR-derived wind speed and to estimate the surface buoyancy flux.

The impact of operational and theoretical constraints on the practical utility of these two approaches is considered in depth. Calibration and resolution issues are found to impose wind speed limits on the approaches’ applicability. The sensitivity of the two approaches’ results to uncertainty in their nondimensional parameters is also discussed.

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George S. Young, Todd D. Sikora, and Nathaniel S. Winstead

Abstract

Previous studies have demonstrated that satellite synthetic aperture radar (SAR) can be used as an accurate scatterometer, yielding wind speed fields with subkilometer resolution. This wind speed generation is only possible, however, if a corresponding accurate wind direction field is available. The potential sources of this wind direction information include satellite scatterometers, numerical weather prediction models, and SAR itself through analysis of the spatial patterns caused by boundary layer wind structures. Each of these wind direction sources has shortcomings that can lead to wind speed errors in the SAR-derived field. Manual and semiautomated methods are presented for identifying and correcting numerical weather prediction model wind direction errors. The utility of this approach is demonstrated for a set of cases in which the first-guess wind direction data did not adequately portray the features seen in the SAR imagery. These situations include poorly resolved mesoscale phenomena and misplaced synoptic-scale fronts and cyclones.

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Nathaniel S. Winstead, Todd D. Sikora, Donald R. Thompson, and Pierre D. Mourad

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Synthetic aperture radar (SAR) images of oceans and the Great Lakes have provided a highly detailed means of observing atmospheric boundary layer phenomena such as convection, land breezes, and internal gravity waves. This is possible because the backscattered radiation detected by SAR can be dominated by scattering from wind-driven capillary waves whose spatial variation is visible as patterns in the SAR images. In this paper, we present two case studies in which SAR images taken over Lake Superior demonstrate spatial variability in the surface wind stress created over the lake by coincident gravity waves and boundary layer convection during cold air outbreaks. Of particular interest is the direct influence of the gravity waves on the lake-surface stress despite the intervening highly convective atmosphere as well as the detailed view of the fetch dependence of that stress.

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