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Azimuthal and Radial Variation of Wind-Generated Surface Waves inside Tropical Cyclones

JOURNAL OF PHYSICAL OCEANOGRAPHY 2016年第9期46卷 2605-2621页

作者： Hwang, Paul A. Walsh, Edward J. Naval Res Lab Remote Sensing Div 4555 Overlook Ave SW Washington DC 20375 USA NOAA Earth Syst Res Lab Div Phys Sci Boulder CO USA

For wind-generated waves, the wind-wave triplets (reference wind speed, significant wave height, and spectral peak wave period) are intimately connected through the fetch-or duration-limited wave growth functions. The full set of the triplets can be obtained knowing only one of the three, together with the input of fetch (duration) information using the pair of fetch-limited (duration limited) wave growth functions. The air-sea energy and momentum exchanges are functions of the wind-wave triplets, and they can be quantified with the wind-wave growth functions. Previous studies have shown that the wave development inside hurricanes follows essentially the same growth functions established for steady wind forcing conditions. This paper presents the analysis of wind-wave triplets collected inside Hurricane Bonnie 1998 at category 2 stage along 10 transects radiating from the hurricane center. A fetch model is formulated for any location inside the hurricane. Applying the fetch model to the 2D hurricane wind field, the detailed spatial distribution of the wave field and the associated energy and momentum exchanges inside the hurricane are investigated. For the case studied, the energy and momentum exchanges display two local maxima resulting from different weightings of wave age and wind speed. Referenced to the hurricane heading, the exchanges on the right half plane of the hurricane are much stronger than those on the left half plane. Integrated over the hurricane coverage area, the right-to-left ratio is about 3:1 for both energy and momentum exchanges. Computed exchange rates with and without considering wave properties differ significantly.

关键词： Circulation/ Dynamics Waves oceanic Wind waves Physical Meteorology and Climatology Atmosphere-ocean interaction Hurricanes/typhoons observational techniques and algorithms Aircraft observations

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Separation of Short Time Series of Currents into "Fluctuations," "Tides," and "Mean" Flow

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JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY 2016年第5期33卷 1089-1095页

作者： Gargett, Ann E. Savidge, Dana K. Inst Ocean Sci POB 6000 Sidney BC V8L 4B2 Canada Old Dominion Univ Norfolk VA USA Skidaway Inst Oceanog Savannah GA USA

With standard low-frequency velocity data from current meter moorings, pressure gradient-driven mean flow is determined by low-pass filtering, while tides are estimated by fitting tidal constituents, with accuracy and numbers of constituents determined by record length. With the advent of higher-frequency measurements from cabled coastal ocean observatories, current data also include supertidal variability (fluctuation) associated with a variety of turbulent and internal wave processes. To examine the relationships of such fluctuations to variability of the tides and/or potentially time-variable mean flows within which they are embedded, it is highly desirable to find a method whereby these flow components can be separated over relatively short periods of intensified event-scale forcing. A method is presented that first isolates "fluctuations" and then separates the remaining longer time-scale variability into "tides" and a remaining "mean," without recourse to extraction of a fitted tide with the error inherent in such a fit over short data records.

关键词： Data processing In situ oceanic observations observational techniques and algorithms Oceanic variability Variability

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Simulated Frequency Dependence of Radar Observations of Tornadoes

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JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY 2016年第9期33卷 1825-1842页

作者： Bodine, David J. Palmer, Robert D. Maruyama, Takashi Fulton, Caleb J. Zhu, Ye Cheong, Boon Leng Natl Ctr Atmospher Res Adv Study Program 3450 Mitchell Lane Boulder CO 80302 USA Univ Oklahoma Sch Meteorol Norman OK 73019 USA Univ Oklahoma Adv Radar Res Ctr Norman OK 73019 USA Kyoto Univ Disaster Prevent Res Inst Kyoto Japan Univ Oklahoma Sch Elect & Comp Engn Norman OK 73019 USA

To obtain accurate radar-measured wind measurements in tornadoes, differences between air and Doppler velocities must be corrected. These differences can cause large errors in radar estimates of maximum tangential wind speeds, and large errors in single-Doppler retrievals of radial and vertical velocities. Since larger scatterers (e.g., debris) exhibit larger differences from air velocities compared to small scatterers (e.g., raindrops), the dominant scatterer type affecting radar measurements is examined. In this study, radar variables are simulated for common weather radar frequencies using debris and raindrop trajectories computed with a large-eddy simulation model and two electromagnetic scattering models. These simulations include a large range of raindrop and wood board sizes and concentrations, and reveal the significant frequency dependence of the equivalent reflectivity factor and Doppler velocity. At S band, dominant scatterers are wood boards, except when wood board concentrations are very low. In contrast, raindrops are the dominant scatterers at Ka and W bands even when large concentrations of wood boards are present, except for low raindrop concentrations. Dual-wavelength velocity differences exhibit high correlation with air and Doppler velocity differences for most cases, which may enable direct measurements of scatterer-induced Doppler velocity bias in tornadoes. Moreover, dual-wavelength ratios are shown to exhibit strong correlations with dominant scatterer size, except when Rayleigh scatterers are dominant. Finally, vertical velocity retrievals are shown to exhibit lower errors at high frequencies, and large errors remain at centimeter wavelengths even after debris centrifuging corrections are applied in cases with high debris concentration.

关键词： Atm/Ocean Structure/ Phenomena Large eddy simulations Models and modeling observational techniques and algorithms Radars/Radar observations Severe storms Tornadoes

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On the Use of X-Band Weather Radar for Wind Field Retrieval in Coastal Zone

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JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY 2016年第5期33卷 899-917页

作者： Forget, P. Saillard, M. Guerin, C. -A. Testud, J. Le Bouar, E. Univ Aix Marseille Univ Toulon Mediterranean Inst Oceanog CNRS INSUIRDUM110 CS60584 Toulon France NOVIMET Guyancourt France

This paper documents the study of the radar signature of the sea surface on the images collected by an X-band weather radar and its application to remote sensing of the coastal zone. The main radar parameters considered here are the reflectivity factor [which was converted into a normalized radar cross section (NRCS)], the differential reflectivity, and the mean radial velocity. Measurements of the NRCS in the vicinity of an instrumented offshore buoy allowed for identifying its variations with wind speed and relative direction, which are found to be consistent with the Georgia Institute of Technology (GIT) model and other measurements. A more accurate empirical model for the NRCS is developed. For moderate to strong winds, the mean radial velocity is linearly related to the radial component of the wind speed, showing the potentiality of weather radars to map the radial surface wind speed at the scale of a basin. The influence of surface currents is weak here but should be likely taken into account in other places with stronger currents. Measurements of the differential reflectivity are also consistent with other measurements made elsewhere for the polarization ratio. The analysis of radar data over the entire coverage could also be performed using surface wind estimates from a meteorological model. Similar but less accurate results are obtained for the NRCS and the mean radial velocity. The extra spatial variability not due to wind speed variations is identified. An inversion procedure is proposed to recover the wind vector field from radar measurements after calibration of the NRCS values. Results are promising and pave the way to the concept of weather radar for ocean remote sensing.

关键词： Microwave observations observational techniques and algorithms Radars/Radar observations

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Aerosol Impacts on Thermally Driven Orographic Convection

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JOURNAL OF THE ATMOSPHERIC SCIENCES 2016年第8期73卷 3115-3132页

作者： Nugent, Alison D. Watson, Campbell D. Thompson, Gregory Smith, Ronald B. Natl Ctr Atmospher Res 3450 Mitchell Ln Boulder CO 80301 USA IBM TJ Watson Res Ctr Yorktown Hts NY USA Yale Univ Dept Geol & Geophys New Haven CT USA

Observations from the Dominica Experiment (DOMEX) field campaign clearly show aerosols having an impact on cloud microphysical properties in thermally driven orographic clouds. It is hypothesized that when convection is forced by island surface heating, aerosols from the mostly forested island surface are lofted into the clouds, resulting in the observed high concentration of aerosols and the high concentration of small cloud droplets. When trying to understand the impact of these surface-based aerosols on precipitation, however, observed differences in cloud-layer moisture add to the complexity. The WRF Model with the aerosol-aware Thompson microphysics scheme is used to study six idealized scenarios of thermally driven island convection: with and without a surface aerosol source, with a relatively dry cloud layer and with a moist cloud layer, and with no wind and with a weak background wind. It is found that at least a weak background wind is needed to ensure Dominica-relevant results and that the effect of cloud-layer moisture on cloud and precipitation formation dominates over the effect of aerosol. The aerosol impact is limited by the dominance of precipitation formation through accretion. Nevertheless, in order to match observed cloud microphysical properties and precipitation, both a relatively dry cloud layer and a surface aerosol source are needed. The impact of a surface aerosol source on precipitation is strongest when the environment is not conducive to cloud growth.

关键词： Physical Meteorology and Climatology Aerosols Orographic effects observational techniques and algorithms Aircraft observations Models and modeling Numerical analysis/modeling

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Urban Sprawl Patterns and Processes in Delhi from 1977 to 2014 Based on Remote Sensing and Spatial Metrics Approaches

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EARTH INTERACTIONS 2016年第14期20卷 1-29页

作者： Jain, Madhavi Dimri, A. P. Niyogi, D. Jawaharlal Nehru Univ Sch Environm Sci New Delhi 110067 India Purdue Univ Dept Earth & Atmospher Sci W Lafayette IN 47907 USA

Recent decades have witnessed rapid urbanization and urban population growth resulting in urban sprawl of cities. This paper analyzes the spatiotemporal dynamics of the urbanization process (using remote sensing and spatial metrics) that has occurred in Delhi, the capital city of India, which is divided into nine districts. The urban patterns and processes within the nine administrative districts of the city based on raw satellite data have been taken into consideration. Area, population, patch, edge, and shape metrics along with Pearson's chi statistics and Shannon's entropy have been calculated. Three types of urban patterns exist in the city: 1) highly sprawled districts, namely, West, North, North East, and East;2) medium sprawled districts, namely, North West, South, and South West;and 3) least sprawled districts-Central and New Delhi. Relative entropy, which scales Shannon's entropy values from 0 to 1, is calculated for the districts and time spans. Its values are 0.80, 0.92, and 0.50 from 1977 to 1993, 1993 to 2006, and 2006 to 2014, respectively, indicating a high degree of urban sprawl. Parametric and nonparametric correlation tests suggest the existence of associations between built-up density and population density, area-weighted mean patch fractal dimension (AWMPFD) and area-weighted mean shape index (AWMSI), compactness index and edge density, normalized compactness index and number of patches, and AWMPFD and built-up density.

关键词： observational techniques and algorithms Data processing Geographic information systems (GIS) Remote sensing Satellite observations

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Towards an Understanding of the Twentieth-Century Cooling Trend in the Southeastern United States: Biogeophysical Impacts of Land-Use Change

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EARTH INTERACTIONS 2016年第18期20卷 1-31页

作者： Ellenburg, W. L. McNider, R. T. Cruise, J. F. Christy, John R. Univ Alabama Ctr Earth Syst Sci 320 Sparkman Dr Huntsville AL 35805 USA Univ Alabama Dept Atmospher Sci Huntsville AL 35899 USA

This paper explores the link between the anomalous warming hole in the southeastern United States and a major land-use/land-cover (LULC) change in the region. Land surface and satellite observations were analyzed to estimate the net radiative forcing due to LULC change. Albedo and latent energy were specifically addressed for the dominant LULC change of agriculture to forests. It was assumed that in the energy-limited environment of the region, the partition of changes in available energy due to albedo will mostly impact the sensible heat. The results show that in the southeastern United States, for the period of 1920 to 1992, the changes in sensible (as a result of albedo) and latent energies are in direct competition with each other. In the spring and early summer months, the croplands are in peak production and the latent energy associated with their evapotranspiration (ET) is comparable to that of the forests so the decrease in radiation due to albedo dominates the signal. However, during the late summer and fall months, most major crops have matured, thus reducing their transpiration rate while forests (particularly evergreens) maintain their foliage and with their deep roots are able to continue to transpire as long as atmospheric conditions are favorable. This later influence of latent energy appears to more than offset the increased radiative forcing from the spring and early summer. Overall, a mean annual net radiative forcing resulting from a LULC change from cropland to forests was estimated to be -1.06 W m(-2) and thus a probable contribution to the "warming hole'' over the Southeast during the majority of the twentieth century.

关键词： Physical meteorology and climatology Climate change Latent heating/cooling Radiative forcing observational techniques and algorithms Satellite observations Surface observations Applications Land use

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Automated Processing of Oceanic Bubble Images for Measuring Bubble Size Distributions underneath Breaking Waves

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JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY 2016年第8期33卷 1701-1714页

作者： Al-Lashi, Raied S. Gunn, Steve R. Czerski, Helen Univ Southampton Fac Phys Sci & Engn Elect & Comp Sci Southampton Hants England UCL Dept Mech Engn London England Univ Leeds Fac Math & Phys Sci Sch Food Sci & Nutr Woodhouse Lane Leeds LS2 9JT W Yorkshire England

Accurate in situ measurements of oceanic bubble size distributions beneath breaking waves are needed for a better understanding of air sea gas transfer and aerosol production processes. To achieve this goal, a novel high-resolution optical instrument for imaging oceanic bubbles was designed and built in 2013 for the High Wind Gas Exchange Study (HiWinGS) campaign in the North Atlantic Ocean. The instrument is able to operate autonomously and can continuously capture high-resolution images at 15 frames per second over an 8-h deployment. The large number of images means that it is essential to use an automated processing algorithm to process these images. This paper describes an automated algorithm for processing oceanic images based on a robust feature extraction technique. The main advantages of this robust algorithm are it is significantly less sensitive to the noise and insusceptible to the background changes in illumination, can extract circular bubbles as small as one pixel (approximately 20 mu m) in radius accurately, has low computing time (approximately 5 seconds per image), and is simple to implement. The algorithm was successfully used to analyze a large number of images (850 000 images) from deployment in the North Atlantic Ocean as part of the HiWinGS campaign in 2013.

关键词： algorithms Data processing Instrumentation/sensors observational techniques and algorithms

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Synoptic Classes as a Predictor of Hourly Surface Wind Regimes: The Case of the Central and Southern Israeli Coastal Plains

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JOURNAL OF APPLIED METEOROLOGY AND CLIMATOLOGY 2016年第7期55卷 1533-1547页

作者： Berkovic, Sigalit Israel Inst Biol Res Dept Math Ness Ziona Israel

This work considers whether it is possible to use semiobjective synoptic classes as a year-round predictor of hourly surface winds in Israel. Previous literature has quantitatively characterized the hourly surface flow during the summer under a single synoptic group-the Persian trough-but no quantitative verification has been made for the other seasons. This study undertook a statistical characterization of the surface winds under frequent synoptic classes. The calculated statistical parameters were averages, standard deviations, steadiness, and statistical significance, determined using measurements from 12 weather stations of the Israel Meteorological Service. High steadiness of the surface wind field was found for a significant number of hours. The verification of the characterization at 0000 UTC by two calculations, the first according to the class defined at the previous 1200 UTC and the second according to the next 1200 UTC, showed similar results. Therefore, the synoptic class can be used as a predictor of hourly surface winds during the entire year. These statistical results provide an automatic prediction of the hourly surface wind average and standard deviation according to synoptic class. They may aid in risk assessment or statistical downscaling of seasonal or monthly winds. Such applications can be employed in other parts of Israel. Typical values for hours at which steady flow was obtained are as follows: steadiness > 0.65, wind direction standard deviation, < 60 degrees, wind speed standard deviation of 1-2 ms(-1), and sector direction widths of 50 degrees-150 degrees, within 70% statistical significance.

关键词： observational techniques and algorithms Climate classification/regimes Surface observations Mathematical and statistical techniques Classification Risk assessment Applications Air pollution Wind effects

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A Comparative Study of the 3 June 2015 Great Plains Low-Level Jet

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MONTHLY WEATHER REVIEW 2016年第8期144卷 2963-2979页

作者： Parish, Thomas R. Univ Wyoming Dept Atmospher Sci 1000 E Univ Ave Laramie WY 82071 USA

Detailed ground-based and airborne measurements were conducted of the summertime Great Plains lowlevel jet (LLJ) in central Kansas during the Plains Elevated Convection at Night (PECAN) campaign. Airborne measurements using the University of Wyoming King Air were made to document the vertical wind profile and the forcing of the jet during the nighttime hours on 3 June 2015. Two flights were conducted that document the evolution of the LLJ from sunset to dawn. Each flight included a series of vertical sawtooth and isobaric legs along a fixed track at 38.78N between longitudes 98.98 and 1008W. Comparison of the 3 June 2015 LLJ was made with a composite LLJ case obtained from gridded output from the North American Mesoscale Forecast System for June and July of 2008 and 2009. Forcing of the LLJ was detected using cross sections of D values that allow measurement of the vertical profile of the horizontal pressure gradient force and the thermal wind. Combined with observations of the actual wind, ageostrophic components normal to the flight track can be detected. Observations show that the 3 June 2015 LLJ displayed classic features of the LLJ, including an inertial oscillation of the ageostrophic wind. Oscillations in the geostrophic wind as a result of diurnal heating and cooling of the sloping terrain are not responsible for the nocturnal wind maximum. Net daytime heating of the sloping Great Plains, however, is responsible for the development of a strong background geostrophic wind that is critical to formation of the LLJ.

关键词： Atm/Ocean Structure/ Phenomena Boundary layer Jets observational techniques and algorithms Aircraft observations

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