Todd D. Sikora

Satellite Imagery of Sea Surface Temperature Cooling in the Wake of Hurricane Edouard (1996)

It is well documented that in the wake of a hurricane there is significant cooling of sea surface temperature (SST) (Hazelworth 1968; Brand 1971; Federov 1972; Stramma et al. 1986; Shay et al. 1992; Pudov et al. 1978; Pudov 1980; Black and Holland 1995). Figure 1 represents a dramatic recent example of such cooling observed from satellite radiometer data after the passage of Hurricane Edouard (1996) off the east coast of the United States. The figure is a composite of SSTs derived from Advanced Very High Resolution Radiometer (AVHRR) data broadcast over 3 days (0531 UTC 31 August 1996–1001 UTC 3 September 1996) by the polar-orbiting NOAA-12 and NOAA-14 satellites. This sea surface temperature image clearly shows a swath of water approximately 48C less than the surrounding water centered slightly east of the track of Hurricane Edouard’s eye. In this paper, we describe the process used to construct Fig. 1 from AVHRR data. We also review the various mechanisms by which hurricanes induce SST cooling.

Synthetic Aperture Radar as a Tool for Investigating Polar Mesoscale Cyclones

Abstract Polar mesoscale cyclones are intense vortices that form in cold, marine air masses poleward of major jet streams and frontal zones. Synthetic aperture radar (SAR) should be considered as a potential tool for the study of polar mesoscale cyclones because of its ability to remotely sense, at least qualitatively, the high-resolution near-surface wind field independent of daylight and atmospheric conditions. Four case studies demonstrating this ability are presented. SAR imagery from the Canadian Space Agency’s RADARSAT are compared to corresponding infrared imagery, surface analyses, and upper-air analyses. In three of the four case studies, it is argued that the addition of SAR imagery to the process of generating a manual surface analysis would have led to a better product. Moreover, it is demonstrated that the SAR imagery reveals a host of marine-meteorological phenomena in the vicinity of the polar mesoscale cyclones including atmospheric gravity waves, roll vortices, and cellular convection. Be...

Estimating Convective Atmospheric Boundary Layer Depth from Microwave Radar Imagery of the Sea Surface

Abstract Kilometer-scale mottling seen on real and synthetic aperture radar imagery of the sea surface can be linked to the presence of microscale cellular convection (thermals) spanning the marine atmospheric boundary layer. In the current study, it is hypothesized that the typical scale of the mottling, found via standard Fourier spectral analysis, can be used to estimate the depth of the convective marine atmospheric boundary layer (zi) using a modified form of traditional mixed-layer similarity theory for these thermals’ aspect ratio. The hypothesis linking the typical scale of mottling to zi is substantiated using previously published boundary layer results and supporting meteorological and oceanographic data from a number of case studies.

Mesoscale Stratocumulus Bands Caused by Gulf Stream Meanders

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 ...

Direct Influence of Gravity Waves on Surface-Layer Stress during a Cold Air Outbreak, as shown by Synthetic Aperture Radar

Abstract 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.

Using Spaceborne Synthetic Aperture Radar to Improve Marine Surface Analyses

Abstract The ever-changing weather and lack of in situ data in the Bering Sea warrants experimentation with new meteorological observing systems for this region. Spaceborne synthetic aperture radar (SAR) is well suited for observing the sea surface footprints of marine meteorological phenomena because its radiation is sensitive to centimeter-scale sea surface roughness, regardless of the time of day or cloud conditions. The near-surface wind field generates this sea surface roughness. Therefore, the sea surface footprints of meteorological phenomena are often revealed by SAR imagery when the main modulator of sea surface roughness is the wind. These attributes, in addition to the relatively high resolution of SAR products, make this instrument an excellent candidate for filling the meteorological observing needs over the Bering Sea. This study demonstrates the potential usefulness of SAR for observing Bering Sea meteorology by focusing on its ability to image the sea surface footprints of polar mesoscale ...

A Case Study of Satellite Synthetic Aperture Radar Signatures of Spatially Evolving Atmospheric Convection over the Western Atlantic Ocean

A case study of a particularly intense cold air outbreak over the northAtlantic Ocean extending from the northeast coast of the UnitedStates to the Gulf Stream is described. A RADARSAT satellite synthetic apertureradar (SAR) image of this outbreak dramatically illustrates the spatialevolution of convection. Nearly coincident images from the National Oceanic and Atmospheric Administrations Advanced Very HighResolution Radiometer are used to compare many interesting features.In addition, National Weather Service rawinsonde data, National Data Buoy Center buoy data, and Woods Hole Oceanographic Institute Coastal Mixing and Optics mooring data arepresented. We use these data to help describe the spatial evolution of the atmospheric boundary-layer processes involved in this outbreak.Rows of cellular convective clouds begin to appear some distance offshore and then slowly increase in horizontal diameter and wavelength in the downwind direction, with a subsequent jump in cloud diameter downwind of the Gulf Stream North Wall (GSNW). The SAR image shows a similar evolution of sea-surface footprints of these boundary-layer features. This change in boundary-layer structure is attributed to corresponding changes in static stability. About 300 km south of the GSNW in the SAR image, an even larger jump in cell diameter appears and the cells becomenon-uniform with bright crescents and filled semi-circles on thedownwind sides of the cells. These are believed to be surface effectsof gust fronts induced by the mesoscale cellular convection and enhanced by the overall northwesterly flow.

The Ontario Winter Lake-Effect Systems Field Campaign: Scientific and Educational Adventures to Further Our Knowledge and Prediction of Lake-Effect Storms

AbstractIntense lake-effect snowstorms regularly develop over the eastern Great Lakes, resulting in extreme winter weather conditions with snowfalls sometimes exceeding 1 m. The Ontario Winter Lake-effect Systems (OWLeS) field campaign sought to obtain unprecedented observations of these highly complex winter storms.OWLeS employed an extensive and diverse array of instrumentation, including the University of Wyoming King Air research aircraft, five university-owned upper-air sounding systems, three Center for Severe Weather Research Doppler on Wheels radars, a wind profiler, profiling cloud and precipitation radars, an airborne lidar, mobile mesonets, deployable weather Pods, and snowfall and particle measuring systems. Close collaborations with National Weather Service Forecast Offices during and following OWLeS have provided a direct pathway for results of observational and numerical modeling analyses to improve the prediction of severe lake-effect snowstorm evolution. The roles of atmospheric boundary ...

A Novel Approach to Marine Wind Speed Assessment Using Synthetic Aperture Radar

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.

Inferring Marine Atmospheric Boundary Layer Properties from Spectral Characteristics of Satellite-Borne SAR Imagery

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...