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Rainfall-Type Categorization of Radar Echoes Using Polar Coordinate Reflectivity Data

JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY 2016年第3期33卷 523-538页

作者： Powell, Scott W. Houze, Robert A. Brodzik, Stella R. Univ Washington Dept Atmospher Sci Seattle WA 98195 USA

An algorithm used to classify precipitation echoes by rain type without interpolating radar data to a constant height is detailed. The method uses reflectivity data without clutter along the lowest available scan angle so that the classifications yield a more accurate representation of the rain type observed at the surface. The algorithm is based on that of Steiner et al. but is executed within a polar coordinate system. An additional procedure allows for more small, isolated, and/or weak echo objects to be appropriately identified as convective. Echoes in the immediate vicinity of convective cores are included in a new transition category, which consists mostly of echoes for which a convective or stratiform determination cannot be confidently made. The new algorithm more effectively identifies shallow convection embedded within large stratiform regions, correctly identifies isolated shallow and weak convection as such, and more often appropriately identifies periods during which no stratiform precipitation is present.

关键词： Physical Meteorology and Climatology Observational techniques and algorithms Radars/Radar observations Convection Mesoscale processes circulation/dynamics

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High-Resolution Seafloor Absolute Pressure Gauge Measurements Using a Better Counting Method

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JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY 2016年第9期33卷 1859-1874页

作者： Webb, Spahr C. Nooner, Scott L. Columbia Univ Lamont Doherty Earth Observ 61 Route 9W Palisades NY 10964 USA Univ North Carolina Wilmington Dept Geog & Geol Wilmington NC USA

Vibrating quartz force transducers are the critical component of most deep-sea pressure and depth gauges in use in oceanography, producing a frequency output that varies with pressure. Accurate and low drift pressure measurements can be obtained by precisely measuring this frequency. In most implementations, the frequency is determined by counting the number of cycles of a high-frequency standard oscillator occurring during a large number of cycles of the lower-frequency quartz force oscillator. Resolution is limited by the sampling interval (length of counting) and the frequency of the frequency standard. Alternative counting methods can provide significant (20-40 dB) improvements in resolution at sampling rates above 1Hz. Each counting method can be described as a different filter applied to the output of a counter of the frequency standard gated at each transition of the transducer quartz oscillator. Improvements in resolution can be understood as the result of minimizing the aliasing of higher-frequency counting noise into the spectrum below the Nyquist frequency. A simple multipole infinite impulse response (IIR) filter designed to limit spectral leakage of high-frequency noise minimizes the noise spectrum and thereby optimizes the resolution of the pressure output. The resultant noise spectrum rises as frequency squared above 1Hz, independent of the sampling rate. At frequencies below 1 Hz, it is limited by noise in the electronics driving the force transducer quartz oscillator. Resolution increases with frequency of the frequency standard up to about 200 MHz, plateauing for higher frequencies due to other noise sources (likely electronic).

关键词： circulation/dynamics Waves, oceanic Atm/Ocean Structure/Phenomena Tides Observational techniques and algorithms In situ oceanic observations Instrumentation/sensors

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Highly Significant Responses to Anthropogenic Forcings of the Midlatitude Jet in the Southern Hemisphere

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JOURNAL OF CLIMATE 2016年第9期29卷 3463-3470页

作者： Solomon, Abraham Polvani, L. M. Columbia Univ Dept Appl Phys & Appl Math New York NY 10027 USA

It has been suggested that changes in the atmospheric circulation caused by anthropogenic forcings are highly uncertain, owing to the large natural variability intrinsic to the system. Here, to assess the statistical significance of such changes for the midlatitude, large-scale atmospheric circulation of the Southern Hemisphere, a new 40-member ensemble of integrations, from 1920 to 2080, of the Community Earth System Model, version 5, is analyzed together with a companion 1800-yr-long preindustrial control integration of the same fully coupled model. For simplicity, only the latitudinal position and the strength of the zonal-mean eddy-driven jet are considered. Given the large year-to-year variability of these jet properties, this paper focuses on their decadal averages, which reflect the more slowly varying climate state. The analysis herein reveals that the forced response in such decadal averages easily emerges from the natural variability, with only a few model integrations typically needed to establish statistical significance. In particular, a forced summertime poleward shift of the jet in the latter part of the twentieth century and a strengthening of the jet during the twenty-first century in all seasons of the year are found. Contrasting these with changes in the southern annular mode, this confirms earlier studies demonstrating that such a mode is unable to distinguish different structural changes in the midlatitude jet.

关键词： Annular mode Atm/Ocean Structure/Phenomena Atmospheric circulation circulation/dynamics Climate variability Jets Variability

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Evaluation of Thermosalinograph and VIIRS Data for the Characterization of Near-Surface Temperature Fields

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JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY 2016年第9期33卷 1843-1858页

作者： Schloesser, Fabian Cornillon, Peter Donohue, Kathleen Boussidi, Brahim Iskin, Emily Univ Rhode Isl Grad Sch Oceanog 215 S Ferry Rd Narragansett RI 02882 USA Inst Mines Telecom Telecom Bretagne UMR Lab STICC Brest France Univ Calif Davis Davis CA 95616 USA

Detailed understanding of submesoscale processes and their role in global ocean circulation is constrained, in part, by the lack of global observational datasets of sufficiently high resolution. Here, the potential of thermosalinograph (TSG) and Visible Infrared Imager Radiometer Suite (VIIRS) data is evaluated, to characterize the submesoscale structure of the near-surface temperature fields in the Gulf Stream and Sargasso Sea. In addition to spectral density, the structure function is considered, a statistical measure less susceptible to data gaps, which are common in the satellite-derived fields. The structure function is found to be an unreliable estimator, especially for steep spectral slopes, nominally between 2 and 3, typical of the Gulf Stream and Sargasso regions. A quality-control threshold is developed based on the number and size of gaps to ensure reliable spectral density estimates. Analysis of the impact of gaps in the VIIRS data on the spectra shows that both the number of missing values and the size of gaps affect the results, and that the steeper the spectral slope the more significant the impact. Furthermore, the TSG, with a nominal resolution of 75m, captures the spectral characteristics of the fields in both regions down to scales substantially smaller than 1 km, while the VIIRS fields, with a nominal resolution of 750m, reproduce the spectra well down to scales of about 20 kmin the Sargasso Sea and 5 km in the Gulf Stream. The scales at which the VIIRS and TSG spectra diverge are thought to be determined by sensor and retrieval noise.

关键词： circulation/dynamics Error analysis Fourier analysis In situ oceanic observations Mathematical and statistical techniques Observational techniques and algorithms Satellite observations Ship observations Subgrid-scale processes

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The Icelandic Low as a Predictor of the Gulf Stream North Wall Position

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JOURNAL OF PHYSICAL OCEANOGRAPHY 2016年第3期46卷 817-826页

作者： Sanchez-Franks, Alejandra Hameed, Sultan Wilson, Robert E. Natl Oceanog Ctr Southampton Hants England SUNY Stony Brook Sch Marine & Atmospher Sci 125 Endeavour Hall Stony Brook NY 11794 USA

The Gulf Stream's north wall east of Cape Hatteras marks the abrupt change in velocity and water properties between the slope sea to the north and the Gulf Stream itself. An index of the north wall position constructed by Taylor and Stephens, called Gulf Stream north wall (GSNW), is analyzed in terms of interannual changes in the Icelandic low (IL) pressure anomaly and longitudinal displacement. Sea surface temperature (SST) composites suggest that when IL pressure is anomalously low, there are lower temperatures in the Labrador Sea and south of the Grand Banks. Two years later, warm SST anomalies are seen over the Northern Recirculation Gyre and a northward shift in the GSNW occurs. Similar changes in SSTs occur during winters in which the IL is anomalously west, resulting in a northward displacement of the GSNW 3 years later. Although time lags of 2 and 3 years between the IL and the GSNW are used in the calculations, it is shown that lags with respect to each atmospheric variable are statistically significant at the 5% level over a range of years. Utilizing the appropriate time lags between the GSNW index and the IL pressure and longitude, as well as the Southern Oscillation index, a regression prediction scheme is developed for forecasting the GSNW with a lead time of 1 year. This scheme, which uses only prior information, was used to forecast the GSNW from 1994 to 2015. The correlation between the observed and forecasted values for 1994-2014 was 0.60, significant at the 1% level. The predicted value for 2015 indicates a small northward shift of the GSNW from its 2014 position.

关键词： Boundary currents Atmosphere-ocean interaction Ocean circulation Atm/Ocean Structure/Phenomena North Atlantic Oscillation circulation/dynamics

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On the Consumption of Antarctic Bottom Water in the Abyssal Ocean

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JOURNAL OF PHYSICAL OCEANOGRAPHY 2016年第2期46卷 635-661页

作者： de Lavergne, Casimir Madec, Gurvan Le Sommer, Julien Nurser, A. J. George Garabato, Alberto C. Naveira Univ Paris 06 Sorbonne Univ CNRS IRD MNHN LOCEAN Lab Paris France Natl Oceanog Ctr Southampton Hants England Univ Grenoble Alpes CNRS Lab Glaciol & Geophys Environm Grenoble France Univ Southampton Natl Oceanog Ctr Southampton Hants England

The abyssal ocean is primarily filled by cold, dense waters formed around Antarctica and collectively referred to as Antarctic Bottom Water (AABW). At steady state, AABW must be consumed in the ocean interior at the same rate it is produced, but how and where this consumption is achieved remains poorly understood. Here, estimates of abyssal water mass transformation by geothermal heating and parameterized internal wave-driven mixing are presented. This study uses maps of the energy input to internal waves by tidal and geostrophic motions interacting with topography combined with assumptions about the distribution of energy dissipation to evaluate dianeutral transports induced by breaking internal tides and lee waves. Geothermal transformation is assessed based on a map of geothermal heat fluxes. Under the hypotheses underlying the constructed climatologies of buoyancy fluxes, the authors calculate that locally dissipating internal tides and geothermal heating contribute, respectively, about 8 and 5 Sverdrups (Sv;1 Sv equivalent to 10(6) m(3) s(-1)) of AABW consumption (upwelling), mostly north of 30 degrees S. In contrast, parameterized lee wave-driven mixing causes significant transformation only in the Southern Ocean, where it forms about 3 Sv of AABW, decreasing the mean density but enhancing the northward flow of abyssal waters. The possible role of remotely dissipating internal tides in complementing AABW consumption is explored based on idealized distributions of mixing energy. Depending mostly on the chosen vertical structure, such mixing could drive 1 to 28 Sv of additional AABW upwelling, highlighting the need to better constrain the spatial distribution of remote dissipation. Though they carry large uncertainties, these climatological transformation estimates shed light on the qualitative functioning and key unknowns of the diabatic overturning.

关键词： Upwelling/downwelling Internal waves Abyssal circulation Diapycnal mixing circulation/dynamics

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The Relationship between Equatorial Mixed Rossby-Gravity and Eastward Inertio-Gravity Waves. Part I

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JOURNAL OF THE ATMOSPHERIC SCIENCES 2016年第5期73卷 2123-2145页

作者： Kiladis, George N. Dias, Juliana Gehne, Maria NOAA Div Phys Sci Earth Syst Res Lab Boulder CO USA Univ Colorado CIRES Boulder CO 80309 USA NOAA Earth Syst Res Lab Boulder CO USA

The relationship between n = 0 mixed Rossby-gravity waves (MRGs) and eastward inertio-gravity waves (EIGs) from Matsuno's shallow-water theory on an equatorial beta plane is studied using statistics of satellite brightness temperature T-b and dynamical fields from ERA-Interim data. Unlike other observed convectively coupled equatorial waves, which have spectral signals well separated into eastward and westward modes, there is a continuum of MRG-EIG power standing above the background that peaks near wavenumber 0. This continuum is also present in the signals of dry stratospheric MRGs. While hundreds of papers have been written on MRGs, very little work on EIGs has appeared in the literature to date. The authors attribute this to the fact that EIG circulations are much weaker than those of MRGs for a given amount of divergence, making them more difficult to observe even though they strongly modulate convection. Empirical orthogonal function (EOF) and cross-spectral analysis of 2-6-day-filtered T-b isolate zonally standing modes of synoptic-scale convection originally identified by Wallace in 1971. These display anti-symmetric T-b signals about the equator that propagate poleward with a period of around 4 days, along with westward-propagating MRG-like circulations that move through the T-b patterns. Further analysis here and in Part II shows that these signatures are not artifacts of the EOF approach but result from a mixture of MRG or EIG modes occurring either in isolation or at the same time.

关键词： Atm/Ocean Structure/Phenomena circulation/dynamics Convection dynamics Fourier analysis Inertia-gravity waves Mathematical and statistical techniques Stratosphere Waves, atmospheric

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On the Nature of Temporal Variability of the Gulf Stream Path from 75° to 55°W

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EARTH INTERACTIONS 2016年第9期20卷 1-17页

作者： Gangopadhyay, Avijit Chaudhuri, Ayan H. Taylor, Arnold H. Univ Massachusetts Dartmouth Sch Marine Sci & Technol Fairhaven MA USA Atmospher & Environm Res Inc 131 Hartwell Ave Lexington MA 02421 USA Univ Plymouth Plymouth Marine Lab Plymouth PL4 8AA Devon England Univ Plymouth Sch Math & Stat Plymouth PL4 8AA Devon England

The response of the Gulf Stream (GS) system to atmospheric forcing is generally linked either to the basin-scale winds on the subtropical gyre or to the buoyancy forcing from the Labrador Sea. This study presents a multiscale synergistic perspective to describe the low-frequency response of the GS system. The authors identify dominant temporal variability in the North Atlantic Oscillation (NAO), in known indices of the GS path, and in the observed GS latitudes along its path derived from sea surface height (SSH) contours over the period 1993-2013. The analysis suggests that the signature of interannual variability changes along the stream's path from 75 degrees to 55 degrees W. From its separation at Cape Hatteras to the west of 65 degrees W, the variability of the GS is mainly in the near-decadal (7-10 years) band, which is missing to the east of 60 degrees W, where a new interannual (4-5 years) band peaks. The latter peak (4-5 years) was missing to the west of 65 degrees W. The region between 65 degrees and 60 degrees W seems to be a transition region. A 2-3-yr secondary peak was pervasive in all time series, including that for the NAO. This multiscale response of the GS system is supported by results from a basin-scale North Atlantic model. The near-decadal response can be attributed to similar forcing periods in the NAO signal;however, the interannual variability of 4-5 years in the eastern segment of the GS path is as yet unexplained. More numerical and observational studies are warranted to understand such causality.

关键词： circulation/dynamics Ocean circulation Ocean dynamics Atmosphere/ocean structure/phenomena Atmosphere-ocean interaction North Atlantic Oscillation

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Climate Signals in the Mid- to High-Latitude North Atlantic from Altimeter Observations

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JOURNAL OF CLIMATE 2016年第13期29卷 4905-4925页

作者： Li, Feili Jo, Young-Heon Yan, Xiao-Hai Liu, W. Timothy Univ Delaware Coll Earth Ocean & Environm 261 S Coll Ave Newark DE 19716 USA Univ Delaware Xiamen Univ Joint Inst Coastal Res Newark DE USA CALTECH Jet Prop Lab Pasadena CA USA Duke Univ Nicholas Sch Environm Div Earth & Ocean Sci Durham NC 27708 USA Pusan Natl Univ Dept Oceanog Busan South Korea

The variability of the sea surface height anomaly (SSHA) in the mid-to high-latitude North Atlantic for the period of 1993-2010 was investigated using the ensemble empirical mode decomposition to identify the dominant time scales. Sea level variations in the North Atlantic subpolar gyre (SPG) are dominated by the annual cycle and the long-term increasing trend. In comparison, the SSHA along the Gulf Stream (GS) is dominated by variability at intraseasonal and annual time scales. Moreover, the sea level rise in the SPG developed at a reduced rate in the 2000s compared to rates in the 1990s, which was accompanied by a rebound in SSHA variability following a period of lower variability in the system. These changes in both apparent trend and low-frequency SSHA oscillations reveal the importance of low-frequency variability in the SPG. To identify the possible contributing factors for these changes, the heat content balance (equivalent variations in the sea level) in the subpolar region was examined. The results indicate that horizontal circulations may primarily contribute to the interannual to decadal variations, while the air-sea heat flux is not negligible at annual time scale. Furthermore, the low-frequency variability in the SPG relates to the propagation of Atlantic meridional overturning circulation (AMOC) variations from the deep-water formation region to midlatitudes in the North Atlantic, which might have the implications for recent global surface warming hiatus.

关键词： Geographic location/entity North Atlantic Ocean circulation/dynamics Ocean circulation Atm/Ocean Structure/Phenomena Sea level Variability Climate variability Oceanic variability

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The Relationship between Equatorial Mixed Rossby-Gravity and Eastward Inertio-Gravity Waves. Part II

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JOURNAL OF THE ATMOSPHERIC SCIENCES 2016年第5期73卷 2147-2163页

作者： Dias, Juliana Kiladis, George N. Univ Colorado CIRES Boulder CO 80309 USA NOAA Earth Syst Res Lab Boulder CO 80305 USA NOAA Div Phys Sci Earth Syst Res Lab Boulder CO 80305 USA

Space-time spectral analysis of tropical cloudiness data shows strong evidence that convectively coupled n = 0 mixed Rossby-gravity waves (MRGs) and eastward inertio-gravity waves (EIGs) occur primarily within the western/central Pacific Ocean. Spectral filtering also shows that MRG and EIG cloudiness patterns are anti-symmetric with respect to the equator, and they propagate coherently toward the west and east, respectively, with periods between 3 and 5 days, in agreement with Matsuno's linear shallow-water theory. In contrast to the spectral approach, in a companion paper it has been shown that empirical orthogonal functions (EOFs) of 2-6-day-filtered cloudiness data within the tropical Pacific Ocean also suggest an antisymmetric pattern, but with the leading EOFs implying a zonally standing but poleward-propagating oscillation, along with the associated tropospheric flow moving to the west. In the present paper, these two views are reconciled by applying an independent approach based on a tracking method to assess tropical convection organization. It is shown that, on average, two-thirds of MRG and EIG events develop independently of one another, and one-third of the events overlap in space and time. This analysis also verifies that MRG and EIG cloudiness fields tend to propagate meridionally away from the equator. It is demonstrated that the lack of zonal propagation implied from the EOF analysis is likely due to the interference between eastward- and westward-propagating disturbances. In addition, it is shown that the westward-propagating circulation associated with the leading EOF is consistent with the expected theoretical behavior of an interference between MRGs and EIGs.

关键词： circulation/dynamics Convection dynamics Empirical orthogonal functions Inertia-gravity waves Mathematical and statistical techniques Spectral analysis/models/distribution Waves, atmospheric

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