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W. C. CONNER, R. H. KRAFT, and D. LEE HARRIS

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R. M. Clancy, J. D. Thompson, H. E. Hurlburt, and J. D. Lee

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A time-dependent, two-dimensional numerical model is constructed by coupling a four-layer atmosphere to a two-layer ocean through fluxes of heat and momentum. Idealized experiments are performed to investigate the oceanic response to sea breeze forcing, changes induced in the sea breeze by coastal upwelling, and air-sea feedback during periods of active coastal upwelling. This problem is motivated by the fact that the time scale of the coastal upwelling response is short compared to most oceanic response times and is comparable to the sea breeze time scale.

When forced with a longshore wind stress, the model ocean reproduces several features commonly observed in coastal upwelling regimes, including an equatorward surface jet, a poleward undercurrent, and a region of low sea surface temperatures near the coast. For the cases considered here, the sea breeze contributes significantly to the mean longshore wind stress and, consequently, plays a role in driving the coastal upwelling circulation. It also substantially increases the kinetic energy of the nearshore ocean by forcing inertial oscillations and internal gravity waves with a diurnal period.

When the sea surface temperature is held constant and the land temperature is varied diurnally, the model atmosphere cyclically reproduces a realistic simulation of the sea breeze-land breeze circulation which includes such features as the sea breeze forerunner and the sea breeze front. However, a rapid decrease in sea surface temperature near the coast characteristic of coastal upwelling produces important alterations of the sea breeze-land breeze circulation. Low-level cooling of the atmosphere over the cold water leads ultimately to the formation of a shallower, sharper, faster and longer lasting sea breeze front that penetrates more than twice as far inland than it would without the upwelling. In general, the cold water causes an increase in the low-level sea breeze intensity landward of ∼6 km inland but a decrease seaward of this point. The cold water decreases the land-sea thermal contrast at night and weakens the low-level land breeze everywhere.

Since the cold water in the upwelling zone perturbs the atmosphere on a horizontal scale that is small compared to the internal radius of deformation for the atmosphere, the increase in the longshore geostrophic wind it induces near the coast is small. Furthermore, the reduction in low-level sea breeze amplitude over the cold water compensates the effect of slightly increased mean longshore wind such that the change in mean longshore wind stress is negligible. Thus, although the sea breeze affects the upwelling and the upwelling affects the sea breeze, the air-sea feedback loop to the coastal upwelling process is exceedingly weak.

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A. K. Wåhlin, R. D. Muench, L. Arneborg, G. Björk, H. K. Ha, S. H. Lee, and H. Alsén

Abstract

The exchange of warm, salty seawater across the continental shelves off West Antarctica leads to subsurface glacial melting at the interface between the ocean and the West Antarctic Ice Sheet. One mechanism that contributes to the cross-shelf transport is Ekman transport induced by along-slope currents over the slope and shelf break. An investigation of this process is applied to the Amundsen Sea shelfbreak region, using recently acquired and historical field data to guide the analyses. Along-slope currents were observed at transects across the eastern and western reaches of the Amundsen slope. Currents in the east flowed eastward, and currents farther west flowed westward. Under the eastward-flowing currents, hydrographic isolines sloped upward paralleling the seabed. In this layer, declining buoyancy forces rather than friction were bringing the velocity to zero at the seabed. The basin water in the eastern part of the shelf was dominated by water originating from 800–1000-m depth off shelf, suggesting that transport of such water across the shelf frequently occurs. The authors show that arrested Ekman layers mechanism can supply deep water to the shelf break in the eastern section, where it has access to the shelf. Because no unmodified off-shelf water was found on the shelf in the western part, bottom layer Ekman transport does not appear a likely mechanism for delivery of warm deep water to the western shelf area. Warming of the warm bottom water was most pronounced on the western shelf, where the deep-water temperature increased by 0.6°C during the past decade.

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Daniel P. Stern, George H. Bryan, Chia-Ying Lee, and James D. Doyle

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Recent studies have shown that extreme wind gusts are ubiquitous within the eyewall of intense tropical cyclones (TCs). These gusts pose a substantial hazard to human life and property, but both the short-term (i.e., during the passage of a single TC) and long-term (over many years) risk of encountering such a gust at a given location is poorly understood. Here, simulated tower data from large-eddy simulations of idealized TCs in a quiescent (i.e., no mean flow or vertical wind shear) environment are used to estimate these risks for the offshore region of the United States. For both a category 5 TC and a category 3 TC, there is a radial region where nearly all simulated towers experience near-surface (the lowest 200 m) 3-s gusts exceeding 70 m s−1 within a 10-min period; on average, these towers respectively sample peak 3-s gusts of 110 and 80 m s−1. Analysis of an observational dropsonde database supports the idealized simulations, and indicates that offshore structures (such as wind turbines) in the eyewall of a major hurricane are likely to encounter damaging wind speeds. This result is then incorporated into an estimate of the long-term risk, using analyses of the return period for major hurricanes from both a best-track database and a statistical–dynamical model forced by reanalysis. For much of the nearshore region of the Gulf of Mexico and southeastern U.S. coasts, this analysis yields an estimate of a 30%–60% probability of any given point experiencing at least one 70 m s−1 gust within a 30-yr period.

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Wen-Chau Lee, Peter Dodge, Frank D. Marks Jr., and Peter H. Hildebrand

Abstract

Two sets of equations are derived to 1) map airborne Doppler radar data from an aircraft-relative coordinate system to an earth-relative coordinate system, and 2) remove the platform motion from the observed Doppler velocities. These equations can be applied to data collected by the National Oceanic and Atmospheric Administration WP-3D system, the National Center for Atmospheric Research ELDORA system, and other airborne radar systems.

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K. Miyakoda, L. Umscheid, D. H. Lee, J. Sirutis, R. Lusen, and F. Pratte

Abstract

Global upper air and surface data for the entire GATE period from 15 June to 24 September 1974, were collected by the Data Assimilation Branch of NMC and mailed to GFDL. After processing these data, a four-dimensional analysis technique was applied for the entire GATE period, using a global numerical model. For a selected period, several different versions of the data processing scheme were tested. The resulting analyses were compared with each other and with the objective analysis of NMC in Washington D.C., and ANMRC in Melbourne. Overall, the analyses for the extratropics were satisfactory for the Northern Hemisphere, and to a lesser extent, for the Southern Hemisphere, though flow patterns are somewhat excessively smoothed. The analyses for the tropics were not of the same quality as those for the extratropics, and yet they were much improved compared with those of several years ago. A noteworthy point is that tropical cyclones were successfully represented in several cases.

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E. C. Massoud, H. Lee, P. B. Gibson, P. Loikith, and D. E. Waliser

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This study utilizes Bayesian model averaging (BMA) as a framework to constrain the spread of uncertainty in climate projections of precipitation over the contiguous United States (CONUS). We use a subset of historical model simulations and future model projections (RCP8.5) from the Coupled Model Intercomparison Project phase 5 (CMIP5). We evaluate the representation of five precipitation summary metrics in the historical simulations using observations from the NASA Tropical Rainfall Measuring Mission (TRMM) satellites. The summary metrics include mean, annual and interannual variability, and maximum and minimum extremes of precipitation. The estimated model average produced with BMA is shown to have higher accuracy in simulating mean rainfall than the ensemble mean (RMSE of 0.49 for BMA versus 0.65 for ensemble mean), and a more constrained spread of uncertainty with roughly a third of the total uncertainty than is produced with the multimodel ensemble. The results show that, by the end of the century, the mean daily rainfall is projected to increase for most of the East Coast and the Northwest, may decrease in the southern United States, and with little change expected for the Southwest. For extremes, the wettest year on record is projected to become wetter for the majority of CONUS and the driest year to become drier. We show that BMA offers a framework to more accurately estimate and to constrain the spread of uncertainties of future climate, such as precipitation changes over CONUS.

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Shakti P. C.,, M. Maki, S. Shimizu, T. Maesaka, D.-S. Kim, D.-I. Lee, and H. Iida

Abstract

Two approaches to correcting the partial beam blockage of radar reflectivity in mountainous areas were evaluated using X-band dual polarization radar data from the Hakone mountain region, Kanto, Japan. The comparatively simple digital elevation model (DEM) method calculates the power loss in the received signal based on the geometrical relationship between radar beams and a DEM. The second approach, the modified DEM method, attempts to account for unknown power losses related to ground clutter, hardware calibration errors, etc. Comparison between ground data and reflectivity data corrected by both methods suggests that the DEM method alone was insufficient to correct beam blockage problems but that the modified DEM data were in generally good agreement with the ground data. The authors also estimated 10-min rainfall amounts using reflectivity corrected by the modified DEM method and compared these with data from a network of rain gauges in the mountainous region. In general, the results show good agreement between radar estimates and rain gauge measurements. On the basis of their results, the authors conclude that the modified DEM method offers a suitable solution to the problem of beam blockage in mountainous regions, provided that the beam blockage rate is less than 80%.

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A. D. Collard, S. A. Ackerman, W. L. Smith, X. Ma, H. E. Revercomb, R. O. Knuteson, and S-C. Lee

Abstract

During FIRE II, cirrus clouds were observed in the wavelength range 3–19, µm with two High Resolution Interferometer Sounders as described in the Part I companion paper. One, known as AC-HIS, was mounted on the NASA ER-2 aircraft in order to look down on the clouds; these results are described in the Part II companion paper. The other, GB-HIS, also known as the Atmospheric Emitted Radiance Interferometer (AERI), was ground based. The AERI observations have been simulated, assuming scattering from spherical ice particles, using a single-layer doubling model for the cloud, for two atmospheric windows at 700–1250 and 2650–3000 cm−1. The second of these windows is affected by scattered sunlight, which has been included in the calculations. The sensitivity of the cloud signal to quantities such as the ice water path (IWP) and effective radius (r eff) have been determined. Using the cloud model, best fits have been derived for IWP and r eff, for both windows individually and together. Possible errors in these derivations have been investigated.

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Peter Black, Lee Harrison, Mark Beaubien, Robert Bluth, Roy Woods, Andrew Penny, Robert W. Smith, and James D. Doyle

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The High-Definition Sounding System (HDSS) is an automated system deploying the expendable digital dropsonde (XDD) designed to measure wind and pressure–temperature–humidity (PTH) profiles, and skin sea surface temperature (SST) within and around tropical cyclones (TCs) and other high-impact weather events needing high sampling density. Three experiments were conducted to validate the XDD.

On two successive days off the California coast, 10 XDDs and 14 Vaisala RD-94s were deployed from the navy’s Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft over offshore buoys. The Twin Otter made spiral descents from 4 km to 60 m at the same descent rate as the sondes. Differences between successive XDD and RD-94 profiles due to true meteorological variability were on the same order as the profile differences between the spirals, XDDs, and RD-94s. XDD SST measured via infrared microradiometer, referred to as infrared skin SST (SSTir), and surface wind measurements were within 0.5°C and 1.5 m s−1, respectively, of buoy and Twin Otter values.

A NASA DC-8 flight launched six XDDs from 12 km between ex-TC Cosme and the Baja California coast. Repeatability was shown with good agreement between features in successive profiles. XDD SSTir measurements from 18° to 28°C and surface winds agreed well with drifting buoy- and satellite-derived estimates.

Excellent agreement was found between PTH and wind profiles measured by XDDs deployed from a NASA WB-57 at 18-km altitude offshore from the Texas coast and NWS radiosonde profiles from Brownsville and Corpus Christi, Texas. Successful XDD profiles were obtained in the clear and within precipitation over an offshore squall line.

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