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Rainer Bleck, Renate Brummer, and Melvyn A. Shapiro

Abstract

Ground-based remote sensing devices have recently been developed which provide high-resolution tropospheric wind measurements and coarse-resolution radiometric temperature measurements under most weather conditions. A variational analysis scheme for inferring missing details in the three-dimensional temperature field from concurrent wind observations is proposed. The scheme is based on the solution of a three-dimensional boundary value problem and thus requires input from a network of profilers, rather than one individual instrument. Since observing networks of this kind do not presently exist, the scheme is tested on objectively analyzed, thermally degraded radiosonde data. The ultimate purpose of the thermal enhancement procedure is to improve the dynamic balance between mass and wind fields observed by future ground- or space-based profiler networks and to lessen the initialization shock if these data are used in numerical prediction models.

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Thomas J. Galarneau Jr., Christopher A. Davis, and Melvyn A. Shapiro

Abstract

Hurricane Sandy's landfall along the New Jersey shoreline at 2330 UTC 29 October 2012 produced a catastrophic storm surge stretching from New Jersey to Rhode Island that contributed to damage in excess of $50 billion—the sixth costliest U.S. tropical cyclone on record since 1900—and directly caused 72 fatalities. Hurricane Sandy's life cycle was marked by two upper-level trough interactions while it moved northward over the western North Atlantic on 26–29 October. During the second trough interaction on 29 October, Sandy turned northwestward and intensified as cold continental air encircled the warm core vortex and Sandy acquired characteristics of a warm seclusion. The aim of this study is to determine the dynamical processes that contributed to Sandy's secondary peak in intensity during its warm seclusion phase using high-resolution numerical simulations. The modeling results show that intensification occurred in response to shallow low-level convergence below 850 hPa that was consistent with the Sawyer–Eliassen solution for the secondary circulation that accompanied the increased baroclinicity in the radial direction. Additionally, cyclonic vertical vorticity generated by tilting of horizontal vorticity along an axis of frontogenesis northwest of Sandy was axisymmetrized. The axis of frontogenesis was anchored to the Gulf Stream in a region of near-surface differential diabatic heating. The unusual northwestward track of Sandy allowed the cyclonic vorticity over the Gulf Stream to form ahead of the main vortex and be readily axisymmetrized. The underlying dynamics driving intensification were nontropical in origin, and supported the reclassification of Sandy as extratropical prior to landfall.

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Daniel Keyser, Melvyn A. Shapiro, and Donald J. Perkey

Abstract

The structure of upper level and surface frontal zones associated with a cyclone developing over the central United States on 21–22 February 1971, as predicted by a limited-area, moist, primitive equation model with horizontal and vertical grid spacing on the order of 100 and 1.5 km, respectively, Is qualitatively examined and discussed. A comparison of crow-section analyses of the frontal zones, constructed from rawinsondo observations and from model output data, reveals that the horizontal and vertical scales of the observed fronts are ∼100 and ∼1 km, while those for the model-predicted fronts are ∼200–400 and ∼1–2 km. The discrepancy in scale can be explained by the coarse model resolution, which essentially renders be frontal zones subgrid-scale phenomena. Despite the model's lack of fidelity in reproduce the observed details in frontal structure, point calculations with Miller' equation appear reasonable in view of those results obtained in previous synoptic investigations. Vertical tilting dominates the frontolysis predicted in the upper level frontal exit region, and the stretching deformation term provides a strong frontogenetical contribution in the surface frontal zone.

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Rolf H. Langland, Melvyn A. Shapiro, and Ronald Gelaro

Abstract

Short- and medium-range (24–96-h) forecasts of the January 2000 U.S. east coast cyclone and associated snowstorm are examined using the U.S. Navy global forecast model and adjoint system. Attention is given to errors on the synoptic scale, including forecast position and central pressure of the cyclone at the verification time of 1200 UTC 25 January 2000. There is a substantial loss of predictive skill in the 72- and 96-h forecasts, while the 24- and 48-h forecasts capture the synoptic-scale features of the cyclone development with moderate errors. Sensitivity information from the adjoint model suggests that the initial conditions for the 72-h forecast starting at 1200 UTC 22 January 2000 contained relatively small, but critical, errors in upper-air wind and temperature over a large upstream area, including part of the eastern Pacific and “well observed” areas of western and central North America. The rapid growth of these initial errors in a highly unstable flow regime (large singular-vector growth factors) is the most likely cause of the large errors that developed in operational short- and medium-range forecasts of the snowstorm. The large extent of the upstream sensitive area in this case would appear to make “targeting” a small set of new observations an impractical method to improve forecast skill. A diagnostic correction (derived from adjoint sensitivity information) of a part of the initial condition error in the 72-h forecast reduces the forecast error norm by 75% and improves a 1860-km error in cyclone position to a 105-km error. This demonstrates that the model is capable of making a skillful forecast starting from an initial state that is plausible and not far from the original initial conditions. It is also shown that forecast errors in this case propagate at speeds that are greater than those of the synoptic-scale trough and ridge features of the cyclone.

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David B. Parsons, Melvyn A. Shapiro, and Erik Miller

Abstract

A horizontal gradient in moisture, termed the dryline, is often detected at the surface over the southern Great Plains of the United States during the spring and early summer. The dryline exhibits distinct diurnal variations in both its movement and structure. Recent research has focused on dryline structure during the afternoon and evening, particularly showing how strong (∼1–5 m s−1) ascent frequently creates an environment favorable to the initiation of convection, quite close (within ∼10 km) to the dryline interface. To date, however, there have been very few detailed analyses of the dryline interface at night, so that the nocturnal behavior of the interface predicted by theory and numerical studies is relatively poorly evaluated. In this study, special observations taken by a Doppler lidar, serial rawinsonde ascents, and a dual-channel microwave radiometer are utilized to describe the behavior of a nocturnal dryline observed on 12–13 May 1985. The analysis presented here reveals that the mesoscale structure of the nocturnal dryline prior to the formation of deep convection is a gently sloping, slow-moving interface. The movement of the dryline at night was related to the evolution of the low-level jet within the moist air. Wavelike structures and evidence for vertical mixing were observed in the moist air as low Richardson numbers occurred below the height of the jet. The previously discussed strong ascent is largely lacking in the present nocturnal case so that the circulations inherent to an undisturbed dryline at night are far less favorable for the initiation of deep convection than in the afternoon and early evening.

In the present case, severe convection developed as a weak cold front approached and merged with the nocturnal dryline and the environment rapidly destabilized. Between soundings taken 2.5 h apart, the convective available potential energy increased from 524 to 3417 J kg−1 and the absolute value of the convective inhibition decreased from 412 to 9 J kg−1. The vertical shear of the horizontal wind also dramatically increased with time, so that the bulk Richardson number was within values normally associated with supercell convection. The timescale of the changes in stability and in the moisture field (∼1–2.5 h) has implications for the type of observing network needed to nowcast severe convection and for assessing the performance of research and operational numerical models.

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Ying-Hwa Kuo, Evelyn G. Donall, and Melvyn A. Shapiro

Abstract

A series of observing system simulation experiments was conducted to investigate the feasibility of shortrange numerical weather prediction using a network of profilers. A mesoscale model was used to generate datasets which mimic observations from a network of profilers and from an array of rawinsondes. The sensitivity of the model forecast to the characteristic measurement errors of a number of hypothetical profiler networks was tested.

Our results demonstrate that profiler wind observations would have a positive impact on short-range numerical weather prediction with a simple static initialization. We also found that forecasts based on retrieved temperatures (calculated from profiler wind data) are significantly better than those based on direct radiometric temperature measurements (using climatology as the first guess for radiometric retrieval). However, the temperature fields from either radiometric measurements or from thermodynamic retrieval need further improvement-before they can be as accurate as the radiosonde temperature observations for model initialization.

Various hypothetical networks, each having a regular array of stations at a separation of 360 km, provided the initial conditions for short-range numerical forecasts. These predictions can be ranked by performance in the following order. (1) profiler wind with radiosonde temperature and moisture; (2) mixed profiler and rawinsonde wind with rawinsonde temperature and moisture; (3) rawinsonde wind; temperature and moisture; (4) profiler wind and moisture with retrieved temperature., and (5) profiles wind, temperature and moisture.

It was found that, with a domain of 4320 × 2880 km centered at 40°N and a grid spacing of 40 km, accuracy in both the wind field and the temperature field is needed to define the initial state of the model properly. Even within the mesoscale range, the wind field and the temperature field adjust to each other during the course of the model integration. This is because temperature and wind errors associated with observing systems are often projected onto several different vertical modes at a wide range of horizontal scales, both larger and smaller than the Rossby radius of deformation, thus forcing the mutual adjustment of wind and mass fields.

These conclusions are considered tentative because only one synoptic situation was tested with a simple static initialization procedure. Further modeling studies should utilize a four-dimensional data assimilation technique to take advantage of the high temporal resolution of the profiler observations. Also, the experimental procedure should be repeated for more synoptic events to obtain statistically significant results.

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David B. Parsons, Melvyn A. Shapiro, R. Michael Hardesty, Robert J. Zamora, and Janet M. Intrieri

Abstract

During spring and early summer, a surface confluence zone, often referred to as the dryline, forms in the midwestern United States between continental and maritime air masses. The dewpoint temperature across the dryline can vary in excess of 18°C in a distance of less than 10 km. The movement of the dryline varies diurnally with boundary layer growth over sloping terrain leading to an eastward apparent propagation of the dryline during the day and a westward advection or retrogression during the evening. In this study, we examine the finescale structure of a retrogressing, dryline using data taken by a Doppler lidar, a dual-channel radiometer, and serial rawinsonde ascents. While many previous studies were unable to accurately measure the vertical motions in the vicinity of the dryline, our lidar measurements suggest that the convergence at the dryline is intense with maximum vertical motions of ∼5 m s−1. The winds obtained from the Doppler lidar Measurements were combined with the equations of motion to derive perturbation fields of pressure and virtual potential temperature θv. Our observations indicate that the circulations associated with this retrogressing dryline were dominated by hot, dry air riding over a westward moving denser, moist flow in a manner similar to a density current. Gravity waves were observed above the dryline interface. Previous observational and numerical studies have shown that differential heating across the dryline may sometimes enhance regional pressure gradients and thus impact dryline movement. We propose that this regional gradient in surface heating in the presence of a confluent flow results in observed intense wind shifts and large horizontal gradients in θv across the dryline. The local gradient in θv influences the movement and flow characteristics of the dryline interface. This study is one of the most complete and novel uses of Doppler lidar to date.

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Todd P. Lane, James D. Doyle, Robert D. Sharman, Melvyn A. Shapiro, and Campbell D. Watson

Abstract

Historical records of aviation turbulence encounters above Greenland are examined for the period from 2000 to 2006. These data identify an important flow regime that contributes to the occurrence of aircraft turbulence encounters, associated with the passage of surface cyclones that direct easterly or southeasterly flow over Greenland’s imposing terrain. The result of this incident flow is the generation of mountain waves that may become unstable through interactions with the background directional wind shear. It is shown that this regime accounted for approximately 40% of the significant turbulent events identified in the 7-yr database. In addition, two specific cases from the database are examined in more detail using a high-resolution mesoscale model. The model simulations highlight the important role of three-dimensional gravity wave–critical level interactions and demonstrate the utility of high-resolution forecasts in the prediction of such events.

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Todd P. Lane, James D. Doyle, Riwal Plougonven, Melvyn A. Shapiro, and Robert D. Sharman

Abstract

The characteristics and dynamics of inertia–gravity waves generated in the vicinity of an intense jet stream/ upper-level frontal system on 18 February 2001 are investigated using observations from the NOAA Gulfstream-IV research aircraft and numerical simulations. Aircraft dropsonde observations and numerical simulations elucidate the detailed mesoscale structure of this system, including its associated inertia–gravity waves and clear-air turbulence. Results from a multiply nested numerical model show inertia–gravity wave development above the developing jet/front system. These inertia–gravity waves propagate through the highly sheared flow above the jet stream, perturb the background wind shear and stability, and create bands of reduced and increased Richardson numbers. These bands of reduced Richardson numbers are regions of likely Kelvin–Helmholtz instability and a possible source of the clear-air turbulence that was observed.

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James D. Doyle, Melvyn A. Shapiro, Qingfang Jiang, and Diana L. Bartels

Abstract

A large-amplitude mountain wave generated by strong southwesterly flow over southern Greenland was observed during the Fronts and Atlantic Storm-Track Experiment (FASTEX) on 29 January 1997 by the NOAA G-IV research aircraft. Dropwindsondes deployed every 50 km and flight level data depict a vertically propagating large-amplitude wave with deep convectively unstable layers, potential temperature perturbations of 25 K that deformed the tropopause and lower stratosphere, and a vertical velocity maximum of nearly 10 m s−1 in the stratosphere. The wave breaking was associated with a large vertical flux of horizontal momentum and dominated by quasi-isotropic turbulence. The Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) nonhydrostatic model with four-nested grid meshes with a minimum resolution of 1.7 km accurately simulates the amplitude, location, and timing of the mountain wave and turbulent breakdown. Finescale low-velocity plumes that resemble wakelike structures emanate from highly dissipative turbulent regions of wave breaking in the lower stratosphere. Idealized adiabatic three-dimensional simulations suggest that steep terrain slopes increase the effective Rossby number of the relatively wide Greenland plateau, decrease the sensitivity of the wave characteristics to rotation, and ultimately increase the tendency for wave breaking. Linear theory and idealized simulations indicate that diabatic cooling within the boundary layer above the Greenland ice sheet augments the effective mountain height and increases the wave amplitude and potential for wave breaking for relatively wide obstacles such as Greenland.

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