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A. Hannachi, D. Mitchell, L. Gray, and A. Charlton-Perez

Abstract

The polar winter stratospheric vortex is a coherent structure that undergoes different types of deformation that can be revealed by the geometric invariant moments. Three moments are used—the aspect ratio, the centroid latitude, and the area of the vortex based on stratospheric data from the 40-yr ECMWF Re-Analysis (ERA-40) project—to study sudden stratospheric warmings. Hierarchical clustering combined with data image visualization techniques is used as well. Using the gap statistic, three optimal clusters are obtained based on the three geometric moments considered here. The 850-K potential vorticity field, as well as the vertical profiles of polar temperature and zonal wind, provides evidence that the clusters represent, respectively, the undisturbed (U), displaced (D), and split (S) states of the polar vortex. This systematic method for identifying and characterizing the state of the polar vortex using objective methods is useful as a tool for analyzing observations and as a test for climate models to simulate the observations. The method correctly identifies all previously identified major warmings and also identifies significant minor warmings where the atmosphere is substantially disturbed but does not quite meet the criteria to qualify as a major stratospheric warming.

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K. D. Williams, C. A. Senior, and J. F. B. Mitchell

Abstract

A comparison of the response to increasing greenhouse gas concentrations of two versions of the Met Office's (Hadley Centre) coupled atmosphere–ocean model reveals differences that result in large local variations in the modeled impact of climate change. With the aim of understanding the important processes and feedbacks associated with climate change, and ultimately reducing uncertainty in predictions, a series of sensitivity experiments were performed using a coupled atmosphere–mixed layer ocean model. The primary differences in the atmospheric response of the coupled models studied are found to be due to changes made to the physical representation of the atmosphere rather than to the ocean. In particular, many of the different patterns of response can be explained through changes made to the boundary layer scheme combining in a nonlinear way with changes to the cloud scheme to alter the tropical temperature and precipitation response in the model. A new land surface exchange scheme largely accounts for the different Northern Hemisphere continental surface temperature response.

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Owen E. Thompson, Mitchell D. Goldberg, and Donald A. Dazlich

Abstract

Two Pattern recognition procedures are developed to provide improvements to first-guess fields for satellite temperature retrievals. The first is a technique whereby a radiometer measurement may be used to select one or more historical radiosonde temperature profiles as analog estimates of ambient thermal structure. The vertical scales of the analogs are those of radiosondes—the vertical resolving power of the satellite radiometer being relevant only to a decision process. The analog selection process is shown to be much more effective if implemented in an orthogonalized space of measurement information. The second procedure is one which partitions a priori dependent data into shape-coherent pattern libraries using structure information inherent in the data itself. This is an alternative to traditional partitioning schemes whereby proxy classifiers such as season, location and surface type are used.

These pattern recognition techniques are shown to be capable of reducing first-guess profile errors by nearly 50%, in an independent test of about 800 diverse retrievals. The impact of pattern recognition on temperature retrieval error is assessed using regression and physical-iterative retrieval algorithms. The influence of improved first-guess fields is markedly different on these two types of algorithms. Pattern recognition is shown to have a strong, positive impact on the physical-iterative method but little significant impact on regression when evaluated in an overall batch sense. A case study suggests that a small number of very poor retrievals may particularly mask the potential benefits of pattern recognition on both methods.

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D. A. Mitchell, M. Wimbush, D. R. Watts, and W. J. Teague

Abstract

The standard gravest empirical mode (GEM) technique for utilizing hydrography in concert with integral ocean measurements performs poorly in the southwestern Japan/East Sea (JES) because of a spatially variable seasonal signal and a shallow thermocline. This paper presents a new method that combines the U.S. Navy's Modular Ocean Data Assimilation System (MODAS) static climatology (which implicitly contains the mean seasonal signal) with historical hydrography to construct a “residual GEM” from which profiles of such parameters as temperature (T) and specific volume anomaly (δ) can be estimated from measurements of an integral quantity such as geopotential height or acoustic echo time (τ). This is called the residual GEM technique. In a further refinement, sea surface temperature (SST) measurements are included in the profile determinations. In the southwestern JES, profiles determined by the standard GEM technique capture 70% of the T variance and 64% of the δ variance, while the residual GEM technique using SST captures 89% of the T variance and 84% of the δ variance. The residual GEM technique was applied to optimally interpolated τ measurements from a two-dimensional array of pressure-gauge-equipped inverted echo sounders moored from June 1999 to July 2001 in the southwestern JES, resulting in daily 3D estimated fields of T and δ throughout the region. These estimates are compared with those from direct measurements and good agreement is found between them.

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W. J. Teague, E. Jarosz, D. W. Wang, and D. A. Mitchell

Abstract

Hurricane Ivan passed directly over an array of 14 acoustic Doppler current profilers deployed along the outer continental shelf and upper slope in the northeastern Gulf of Mexico. Currents in excess of 200 cm s−1 were generated during this hurricane. Shelf currents followed Ekman dynamics with overlapping surface and bottom layers during Ivan’s approach and transitioned to a dominant surface boundary layer as the wind stress peaked. Slope currents at the onset of Ivan were wind driven near the surface, but deeper in the water column they were dominated during and after the passage of Ivan by subinertial waves with a period of 2–5 days that had several characteristics of topographic Rossby waves. Currents on the slope at 50 m and greater depths commonly exceeded 50 cm s−1. Surprisingly, the strongest currents were present to the left of the storm track on the shelf while more energetic currents were to the right of the hurricane path on the slope during the forced stage. Near-inertial motion lasting for a time period of about 10 days was excited by the storm on the shelf and slope. Record wave heights were measured near the eyewall of Hurricane Ivan and were shown not to be rogue waves. The large surface waves and strong near-bottom currents caused significant bottom scour on the outer shelf at water depths as deep as 90 m.

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D. A. Mitchell, W. J. Teague, M. Wimbush, D. R. Watts, and G. G. Sutyrin

Abstract

Current and temperature patterns in the Ulleung Basin of the Japan/East Sea are examined using acoustic travel-time measurements from an array of pressure-gauge-equipped inverted echo sounders moored between June 1999 and July 2001. The focus here is the formation and behavior of a persistent cold eddy observed south of Dok Island, referred to as the Dok Cold Eddy (DCE), and meandering of the Subpolar Front. The DCE is typically about 60 km in diameter and originates from the pinching off of a Subpolar Front meander between Ulleung and Dok Islands. After formation, the DCE dwells southwest of Dok Island for 1–6 months before propagating westward toward Korea, where it deflects the path of the East Korean Warm Current (EKWC). Four such DCE propagation events between January and June 2000 each deflected the EKWC, and after the fourth deflection the EKWC changed paths and flowed westward along the Japanese shelf as the “Offshore Branch” from June through November 2000. Beginning in March 2001, a deep, persistent meander of the Subpolar Front developed and oscillated with a period near 60 days, resulting in the deformation and northwestward displacement of the Ulleung Eddy. Satellite-altimeter data suggest that the Ulleung Eddy may have entered the northern Japan/East Sea. The evolution of this meander is compared with thin-jet nonlinear dynamics described by the modified Korteweg–deVries equation.

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D. M. O’Brien, R. M. Mitchell, S. A. English, and G. A. Da Costa

Abstract

Airborne experiments to assess the feasibility of remote sensing surface pressure from a space platform are described. The data are high-resolution spectra in the O2 A band (759–771 nm) of sunlight reflected from the sea surface, measured by a grating spectrograph directed toward sunglint from a research aircraft. It is shown that in the first approximation the reflected radiance is a function of just one variable, the adjusted air mass, defined in terms of geometrical factors, surface pressure, pressure altitude of the aircraft, and an estimate of the mean temperature of the lower atmosphere. This result allows the experiments to be used to determine surface pressure if the pressure altitude is given or vice versa. Since surface pressure varies slowly, whereas the pressure altitude of the aircraft is under experimental control, most of the results apply to retrieval of pressure altitude. The principal difficulty in accurate retrievals of pressure lies in modeling the scattered component of the radiance since this quantity is sensitive to aerosol and thin cloud whose properties often are poorly known. It is shown that low-resolution spectra allow the pressure altitude to be tracked with high precision (0.1%), but the absolute accuracy is low (2%) because from low-resolution spectra it is not possible to determine whether scattered radiance is a significant fraction of the total radiance. However, high-resolution spectra contain additional information that allows the reflected and scattered components of the radiance to be distinguished. Experimental data are presented to demonstrate the sensitivity of high-resolution spectra to scattered radiance. Pressure altitude retrievals based on the use of both low- and high-resolution spectra are shown to achieve an accuracy of 0.1% under a wide range of conditions, including moderate haze below and thin cirrus above the airplane. Finally, data are presented to show that variations in reflectance over the footprint of the spectrograph are unlikely to cause pressure errors exceeding 0.1%.

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R. J. Trapp, E. D. Mitchell, G. A. Tipton, D. W. Effertz, A. I. Watson, D. L. Andra Jr., and M. A. Magsig

Abstract

Tornadic vortex signatures (TVSs) of 52 tornadoes were identified and analyzed, then characterized as either descending or nondescending. This characterization refers to a known tendency of radar-observed tornadic vortices, namely, that of their initial detection aloft and then of their subsequent descent leading to tornadogenesis. Only 52% of the sampled TVSs descended according to this archetypal model. The remaining 48% were detected first near the ground and grew upward or appeared nearly simultaneously over a several kilometer depth; these represent primary modes of tornado development that have been explained theoretically. The descending–nondescending TVSs were stratified according to attributes of the tornado and TVS. Significantly, tornadoes within quasi-linear convective systems tended to be associated with nondescending TVSs, identification of which provided a mean tornado lead time of 5 min.

Two case studies are presented for illustrative purposes. On 1 July 1997 in southern Minnesota, nondescending TVSs and associated tornadogenesis were revealed in the leading edge of a squall line, with a squall line–supercell merger, and later during that day, with the cyclonic bookend vortex of a bow echo. On 22 June 1995 in southern Colorado, a low-topped supercell storm produced a tornado that was associated with a descending TVS.

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John A. Knaff, Raymond M. Zehr, Mitchell D. Goldberg, and Stanley Q. Kidder

Abstract

The Advanced Microwave Sounding Unit (AMSU) has better horizontal resolution and vertical temperature sounding abilities than its predecessor, the Microwave Sounding Unit (MSU). Those improved capabilities are demonstrated with observations of two cyclonic weather systems located in the South Pacific Ocean on 1 March 1999. These weather systems appear quite similar in conventional infrared satellite imagery, suggesting that they are comparable in structure and intensity. However, an analysis using temperature retrievals created from the AMSU shows that their vertical thermal structure is quite different.

This is just one example of an application highlighting the improved sounding capabilities available with the AMSU instrument suite. A preliminary look at what the AMSU can provide in data-void regions and a discussion of future plans to create AMSU-based products to better diagnose synoptic-scale weather systems are presented.

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Randal D. Koster, Zhichang Guo, Rongqian Yang, Paul A. Dirmeyer, Kenneth Mitchell, and Michael J. Puma

Abstract

The soil moisture state simulated by a land surface model is a highly model-dependent quantity, meaning that the direct transfer of one model’s soil moisture into another can lead to a fundamental, and potentially detrimental, inconsistency. This is first illustrated with two recent examples, one from the National Centers for Environmental Prediction (NCEP) involving seasonal precipitation forecasting and another from the realm of ecological modeling. The issue is then further addressed through a quantitative analysis of soil moisture contents produced as part of a global offline simulation experiment in which a number of land surface models were driven with the same atmospheric forcing fields. These latter comparisons clearly demonstrate, on a global scale, the degree to which model-simulated soil moisture variables differ from each other and that these differences extend beyond those associated with model-specific layer thicknesses or soil texture. The offline comparisons also show, however, that once the climatological statistics of each model’s soil moisture variable are accounted for (here, through a simple scaling using the first two moments), the different land models tend to produce very similar information on temporal soil moisture variability in most parts of the world. This common information can perhaps be used as the basis for successful mappings between the soil moisture variables in different land models.

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