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W. James Steenburgh, Scott F. Halvorson, and Daryl J. Onton

Abstract

Characteristics of lake-effect snowstorms associated with the Great Salt Lake are described. Using WSR-88D radar imagery, 16 well-defined and 18 marginal lake-effect events were identified from September 1994 through May 1998 (excluding June–August), with the former used for more detailed analysis. Precipitation during the well-defined events was frequently characterized by the irregular development of radar echoes over and downstream of the Great Salt Lake. The most commonly observed precipitation structures were solitary wind-parallel bands that developed along or near the major axis of the GSL and broad-area precipitation shields with embedded convective elements that formed near the southern shoreline.

Regional-scale composite analyses and rawinsonde-derived statistics showed that the lake-effect events occurred in post frontal westerly to northerly 700-hPa flow following the passage of an upper-level trough and associated low-level cold front. The lake-effect environment was characterized by limited steering layer (800–600 hPa) directional shear (generally 60° or less), moist- to dry-adiabatic low-level lapse rates, and small convective available potential energy (CAPE), although the CAPE may be locally greater over the Great Salt Lake. In all events, the lake–700-hPa temperature difference exceeded 16°C, which roughly corresponds to a dry-adiabatic lapse rate. The lake–land temperature difference was always positive and usually exceeded 6°C, indicating significant potential for the development of land-breeze circulations and associated low-level convergence over the lake. Radar-derived statistics suggest that lake enhancement is strongest during periods of northwesterly to northerly flow and large lake–land temperature differences. These characteristics are compared with those associated with lake-effect snowstorms of the Great Lakes and implications for operational forecasting are discussed.

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Miguel F. Piñeros, Elizabeth A. Ritchie, and J. Scott Tyo

Abstract

This paper describes results from a near-real-time objective technique for estimating the intensity of tropical cyclones from satellite infrared imagery in the North Atlantic Ocean basin. The technique quantifies the level of organization or axisymmetry of the infrared cloud signature of a tropical cyclone as an indirect measurement of its maximum wind speed. The final maximum wind speed calculated by the technique is an independent estimate of tropical cyclone intensity. Seventy-eight tropical cyclones from the 2004–09 seasons are used both to train and to test independently the intensity estimation technique. Two independent tests are performed to test the ability of the technique to estimate tropical cyclone intensity accurately. The best results from these tests have a root-mean-square intensity error of between 13 and 15 kt (where 1 kt ≈ 0.5 m s−1) for the two test sets.

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Guojun Gu, Robert F. Adler, George J. Huffman, and Scott Curtis

Abstract

Global and large regional rainfall variations and possible long-term changes are examined using the 27-yr (1979–2005) Global Precipitation Climatology Project (GPCP) monthly dataset. Emphasis is placed on discriminating among variations due to ENSO, volcanic events, and possible long-term climate changes in the Tropics. Although the global linear change of precipitation in the dataset is near zero during the time period, an increase in tropical rainfall is noted in the dataset, with a weaker decrease over Northern Hemisphere middle latitudes. Focusing on the Tropics (25°S–25°N), the dataset indicates an upward linear change (0.06 mm day−1 decade−1) and a downward linear change (−0.01 mm day−1 decade−1) over tropical ocean and land, respectively. This corresponds to an about 5.5% increase (ocean) and 1% decrease (land) during the entire 27-yr time period. The year 2005 has the largest annual tropical total precipitation (land plus ocean) for the GPCP record. The five highest years are (in descending order) 2005, 2004, 1998, 2003, and 2002. For tropical ocean the five highest years are 1998, 2004, 2005, 2002, and 2003.

Techniques are applied to isolate and quantify variations due to ENSO and two major volcanic eruptions during the time period (El Chichón, March 1982; Mount Pinatubo, June 1991) in order to examine longer-time-scale changes. The ENSO events generally do not impact the tropical total rainfall, but rather induce significant anomalies with opposite signs over tropical land and ocean. The impact of the two volcanic eruptions is estimated to be about a 5% reduction in tropical rainfall over both land and ocean. A modified dataset (with ENSO and volcano effects removed) retains the same approximate linear change slopes, but with reduced variances, thereby increasing the statistical significance levels associated with the long-term rainfall changes in the Tropics. However, although care has been taken to ensure that this dataset is as homogeneous as possible, firm establishment of the existence of the discussed changes as long-term trends may require continued analysis of the input datasets and a lengthening of the observation period.

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F. Chevallier, F. Chéruy, R. Armante, C. J. Stubenrauch, and N. A. Scott

Abstract

At a time when a new generation of satellite vertical sounders is going to be launched (including the Infrared Atmospheric Sounder Interferometer and Advanced Infrared Radiometric Sounder instruments), this paper assesses the possibilities of retrieving the vertical profiles of longwave clear-sky fluxes and cooling rates from the Television and Infrared Observation Satellite (TIROS) Operational Vertical Sounder (TOVS) radiometers aboard the polar-orbiting National Oceanic and Atmospheric Administration satellites since 1979. It focuses on two different methodologies that have been developed at Laboratoire de Météorologie Dynamique (France). The first one uses a neural network approach for the parameterization of the links between the TOVS radiances and the longwave fluxes. The second one combines the geophysical variables retrieved by the Improved Initialization Inversion method and a forward radiative transfer model used in atmospheric general circulation models. The accuracy of these two methods is evaluated using both theoretical studies and comparisons with global observations.

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E. J. Walsh, D. W. Hancock III, D. E. Hines, R. N. Swift, and J. F. Scott

Abstract

The Surface Contour Radar is a 36-GHz computer-controlled airborne radar which generates a false-color coded elevation map of the sea surface below the aircraft in real-time, and can routinely produce ocean directional wave spectra with post-flight data processing which has much higher angular resolution than pitch-and-roll buoys. When compared with waveriders and the XERB and EDECO pitch-and-roll buoys, there is good agreement among the nondirectional spectra. There is also good agreement among the angles associated with a 1, b 1, and a 2, b 2 Fourier coefficients of the spreading function for XERB, ENDECO, and the Surface Contour Radar. There are indications that the pitch-and-roll buoys in this study may have calibration problems with the magnitudes of the Fourier coefficients of the spreading function, and that the radar system determines the Fourier coefficients with significantly less noise and bias. The high spatial resolution and rapid mapping capability over extensive areas make the Surface Contour Contour Radar ideal for the study of fetch-limited wave spectra, diffraction and refraction wave patterns is coastal areas, and wave phenomena associated with hurricanes and other highly mobile events.

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Scott Curtis, Robert F. Adler, George J. Huffman, Guojun Gu, David T. Bolvin, and Eric J. Nelkin
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J-F. Flobert, E. Andersson, A. Chédin, A. Hollingsworth, G. Kelly, J. Pailleux, and N. A. Scott

Abstract

This is an evaluation of a retrieval, data assimilation, and forecast experiment on satellite sounding data from the NOAA-9 and NOAA-10 polar orbiting satellites during a 15.5 day period in January–February 1987. Retrievals were produced globally through the 3I (improved initialization inversion) scheme developed at Laboratoire de Météorologie Dynamique, Paris. In this paper we use the European Centre for Medium Range Weather Forecasts (ECMWF) global data assimilation system to evaluate the global performance of the 3I retrieval scheme. The 3I-retrieved temperature profiles have also been compared with the operational retrievals produced by NESDIS (National Environmental Satellite, Data, and Information Service), Washington.

It is shown that the large airmass-dependent biases, characteristic of the NESDIS retrievals, also occur with 3I, but with smaller amplitude. Some large local 3I retrieval errors are identified and explained, and their impact on analysis and forecasts demonstrated. The forecast scores with the 3I retrievals are worse than the scores with the NESDIS retrievals. This may be due to the much larger volume of soundings used in the 3I experiment. The use of a global assimilation system has identified aspects of the 3I system which can be improved.

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Robert F. Adler, George J. Huffman, David T. Bolvin, Scott Curtis, and Eric J. Nelkin

Abstract

A technique is described to use Tropical Rainfall Measuring Mission (TRMM) combined radar–radiometer information to adjust geosynchronous infrared satellite data [the TRMM Adjusted Geostationary Operational Environmental Satellite Precipitation Index (AGPI)]. The AGPI is then merged with rain gauge information (mostly over land) to provide finescale (1° latitude × 1° longitude) pentad and monthly analyses, respectively. The TRMM merged estimates are 10% higher than those from the Global Precipitation Climatology Project (GPCP) when integrated over the tropical oceans (37°N–37°S) for 1998, with 20% differences noted in the most heavily raining areas. In the dry subtropics the TRMM values are smaller than the GPCP estimates. The TRMM merged product tropical-mean estimates for 1998 are 3.3 mm day−1 over ocean and 3.1 mm day−1 over land and ocean combined. Regional differences are noted between the western and eastern Pacific Ocean maxima when TRMM and GPCP are compared. In the eastern Pacific rain maximum the TRMM and GPCP mean values are nearly equal, which is very different from the other tropical rainy areas where TRMM merged product estimates are higher. This regional difference may indicate that TRMM is better at taking into account the vertical structure of the rain systems and the difference in structure between the western and eastern (shallower) Pacific convection.

Comparisons of these TRMM merged analysis estimates with surface datasets shows varied results; the bias is near zero when compared with western Pacific Ocean atoll rain gauge data, but is significantly positive as compared with Kwajalein radar estimates (adjusted by rain gauges). Over land the TRMM estimates also show a significant positive bias. The inclusion of gauge information in the final merged product significantly reduces the bias over land, as expected.

The monthly precipitation patterns produced by the TRMM merged data process clearly show the evolution of the El Niño–Southern Oscillation (ENSO) tropical precipitation pattern from early 1998 (El Niño) to early 1999 (La Niña) and beyond. The El Niño-minus-La Niña difference map shows the expected eastern Pacific maximum, the “Maritime Continent” minima, and other tropical and midlatitude features, very similar to those detected by the GPCP analyses. However, summing the El Niño-minus-La Niña differences over the global tropical oceans yields divergent answers for interannual changes from TRMM, GPCP, and other estimates. This emphasizes the need for additional validation and analysis before it is feasible to understand the relations between global precipitation anomalies and Pacific Ocean ENSO temperature changes.

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Laurence S. Kalkstein, Paul F. Jamason, J. Scott Greene, Jerry Libby, and Lawrence Robinson

Last summer, Philadelphia, Pennsylvania, instituted a new Hot Weather–Health Watch/Warning System (PWWS) to alert the city's residents of potentially oppressive weather situations that could negatively affect health. In addition, the system was used by the Philadelphia Department of Public Health for guidance in the implementation of mitigation procedures during dangerous weather. The system is based on a synoptic climatological procedure that identifies “oppressive” air masses historically associated with increased human mortality. Airmass occurrence can be predicted up to 48 h in advance with use of model output statistics guidance forecast data. The development and statistical basis of the system are discussed, and an analysis of the procedure's ability to forecast weather situations associated with elevated mortality counts is presented. The PWWS, through greater public awareness of excessive heat conditions, may have played an important role in reducing Philadelphia's total heat-related deaths during the summer of 1995.

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Daniel Bannister, Michael Herzog, Hans-F. Graf, J. Scott Hosking, and C. Alan Short

Abstract

The Sichuan basin is one of the most densely populated regions of China, making the area particularly vulnerable to the adverse impacts associated with future climate change. As such, climate models are important for understanding regional and local impacts of climate change and variability, like heat stress and drought. In this study, climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) are validated over the Sichuan basin by evaluating how well each model can capture the phase, amplitude, and variability of the regionally observed mean, maximum, and minimum temperature between 1979 and 2005. The results reveal that the majority of the models do not capture the basic spatial pattern and observed means, trends, and probability distribution functions. In particular, mean and minimum temperatures are underestimated, especially during the winter, resulting in biases exceeding −3°C. Models that reasonably represent the complex basin topography are found to generally have lower biases overall. The five most skillful climate models with respect to the regional climate of the Sichuan basin are selected to explore twenty-first-century temperature projections for the region. Under the CMIP5 high-emission future climate change scenario, representative concentration pathway 8.5 (RCP8.5), the temperatures are projected to increase by approximately 4°C (with an average warming rate of +0.72°C decade−1), with the greatest warming located over the central plains of the Sichuan basin, by 2100. Moreover, the frequency of extreme months (where mean temperature exceeds 28°C) is shown to increase in the twenty-first century at a faster rate compared to the twentieth century.

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