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David H. Levinson and Robert M. Banta

Abstract

Observations taken during the February 1991 Atmospheric Studies in Complex Terrain (ASCOT) Winter Validation Study are used to describe the wind field associated with a terrain-forced mesoscale vortex and thermally forced canyon drainage flows along the Front Range of northeastern Colorado. A case study is presented of the night of 6/7 February 1991 when a weak vortex formed and propagated through the ASCOT domain.

The NOAA/ERL Environmental Technology Laboratory Doppler lidar, one of an ensemble of instruments participating in the ASCOT field experiment, obtained high-resolution measurements of the structure of both the vortex and the canyon drainage flows. The lidar observations documented the kinematic and structural changes in the cyclone and their relationship to a drainage jet exiting a nearby canyon. Lidar analyses clearly show the layering and stratification present during this case, specifically the drainage jet flowing under the cyclone. A period of strong intensification of the drainage flows occurred, following the apparent inhibition of the exit jet by southeasterly flow and the subsequent release of the exit jet, as north-northwesterly flow developed along the foothills.

Additional analyses of the mesoscale surface wind field reveal the movement and spatial variations of the cyclone from initiation to dissipation. The ambient flow remained weak and the cyclone propagated from north to south, which is opposite to previous modeled and observational studies, and on several occasions the cyclone split into two separate vortices. A tracer diffusion test performed during this case shows that the vortex changed the trajectories of the test release cloud from northerly to southerly due both to the movement of the cyclone and to the presence of northerly flow associated with the vortex. Estimates of Froude number are consistent with previous studies that showed Denver cyclones are associated with periods of low-Froude number flow.

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Michael C. Kruk, Kenneth R. Knapp, and David H. Levinson

Abstract

Best track data generally consist of the positions and intensities during the life cycle of a tropical cyclone. Despite the widespread interest in the distribution, frequency, and intensity of tropical cyclones worldwide, no publicly available central repository of global best track data from international agencies has been in existence. While there are numerous international centers that forecast tropical cyclones and archive best track data for their defined regions, most researchers traditionally use best track data from a very small subset of centers to construct global datasets and climatologies. This practice results in tropical cyclones that are either missed and/or misrepresented. While the process of combining positions and intensities from disparate data sources can be arduous, it is worthwhile and necessary in light of their importance. The nature of historical best track data is that they are prone to issues with intensity (maximum surface wind and minimum central pressure), especially in the presatellite era. This study is not a reanalysis effort and makes no attempt to correct any longstanding debates about the accuracy of the historical data. Rather, it simply and objectively combines all of the best track data from each of the regional forecast centers that provided best tracks into one single point for distribution, and the methods used to construct the dataset are the focus of this work. Processes are therefore described herein that detail the combining of tropical cyclone best track data with the techniques used to assess the quality of the minimum central pressure and maximum sustained wind speed of each reported tropical cyclone. The result is a comprehensive global best track compilation dataset that contains information on all documented tropical cyclones: the International Best Track Archive for Climate Stewardship (IBTrACS).

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Robert M. Banta, Lisa D. Olivier, and David H. Levinson

Abstract

As part of the Land/Sea Breeze Experiment (LASBEX) to study the sea breeze at Monterey Bay, the pulsed Doppler lidar of the NOAA/ERL Wave Propagation Laboratory performed vertical and nearly horizontal scans of the developing sea breeze on 12 days. Analyses of Doppler velocity data from these scans revealed details on the growth of the sea-breeze layer and on the horizontal variability of the sea breeze resulting from inland topography. Two days were selected for study when the ambient flow was offshore, because the onshore flow of the sea breeze was easy to discern from the background flow. Sequences of vertical cross sections taken perpendicular to the coast showed the beginnings of the sea breeze beneath the land breeze at the coast and the subsequent growth of the sea-breeze layer horizontally and vertically. On one of the days a transient precursor—a “minor sea breeze”—appeared and disappeared before the main sea breeze began in midmorning. Other issues that the lidar was well suited to study were the compensating return flow, the Coriolis effect, the effects of topography, and the growth of the dimensions of the sea-breeze layer. No return flow above the sea breeze and no Coriolis turning of the sea-breeze flow were found even through the late afternoon hours. Terrain effects included an asymmetry in the development of the sea breeze over water as opposed to over land and the persistence into the late morning hours of southeasterly flow from the Salinas River valley toward the vicinity of the lidar. Vertical and horizontal dimensions of the sea-breeze layer were determined from lidar vertical cross sections. From these, length-to-width aspect ratios were calculated, which were then compared with aspect ratios derived from recent analytical models. The theoretical values compared poorly with the observed values, most likely because the complicating effects of topography and stability were not accounted for in the theoretical models.

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David R. Easterling, Grant Goodge, Matthew J. Menne, Claude N. Williams Jr., and David Levinson

Abstract

Temperature time series for stations in western North Carolina are used to evaluate the potential for an urban signal in the local temperature trend, and to compare a homogeneous temperature record from a mountain-top station to two versions of the lower-tropospheric, satellite-derived temperatures from the Microwave Sounding Unit (MSU). Results regarding the urban signal are in agreement with the conclusion from previous investigations that after a location is urbanized, the local temperature trend is consistent with trends derived from surrounding, more rural stations. With respect to the mountain top and lower-tropospheric temperature comparison, the magnitudes of the two MSU-derived trends for the western North Carolina area are closer to the average annual minimum temperature trend than to the annual average maximum temperature trend.

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Michael C. Kruk, John J. Marra, Peter Ruggiero, David Atkinson, Mark Merrifield, David Levinson, and Mark Lander
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Karin L. Gleason, Jay H. Lawrimore, David H. Levinson, Thomas R. Karl, and David J. Karoly

Abstract

A revised framework is presented that quantifies observed changes in the climate of the contiguous United States through analysis of a revised version of the U.S. Climate Extremes Index (CEI). The CEI is based on a set of climate extremes indicators that measure the fraction of the area of the United States experiencing extremes in monthly mean surface temperature, daily precipitation, and drought (or moisture surplus). In the revised CEI, auxiliary station data, including recently digitized pre-1948 data, are incorporated to extend it further back in time and to improve spatial coverage. The revised CEI is updated for the period from 1910 to the present in near–real time and is calculated for eight separate seasons, or periods.

Results for the annual revised CEI are similar to those from the original CEI. Observations over the past decade continue to support the finding that the area experiencing much above-normal maximum and minimum temperatures in recent years has been on the rise, with infrequent occurrence of much below- normal mean maximum and minimum temperatures. Conversely, extremes in much below-normal mean maximum and minimum temperatures indicate a decline from about 1910 to 1930. An increasing trend in the area experiencing much above-normal proportion of heavy daily precipitation is observed from about 1950 to the present. A period with a much greater-than-normal number of days without precipitation is also noted from about 1910 to the mid-1930s. Warm extremes in mean maximum and minimum temperature observed during the summer and warm seasons show a more pronounced increasing trend since the mid-1970s. Results from the winter season show large variability in extremes and little evidence of a trend. The cold season CEI indicates an increase in extremes since the early 1970s yet has large multidecadal variability.

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F. Martin Ralph, Paul J. Neiman, Teddie L. Keller, David Levinson, and Len Fedor

Abstract

Although considerable understanding of mountain waves has been gained with the aid of the steady-state assumption, it is clear that mountain waves evolve over time. Group velocity arguments indicate that this evolution can occur in less than 1 h. This study uses observations of trapped lee waves to measure the rate at which their horizontal wavelengths change, including a detailed analysis of two events in which such changes are clearly documented. In one case, Doppler lidar observations show a steady increase in horizontal wavelength of 6% h−1 over 4 h and clearly illustrates the relationship between the wave clouds and wave motions. In a second case, visible satellite imagery reveals an increase in wavelength of 14% h−1, which is related to temporal changes in vertical air motions measured by wind profilers within the wave field. Hourly vertical profiles of wind and virtual temperature measured by radio acoustic sounding systems (RASS) and wind profilers reveal important changes in the wave environment. These data were used to initialize a two-dimensional nonlinear nonhydrostatic numerical model with soundings representing five times over 8 h. Each simulation produced trapped lee waves. The simulations support the conclusion that the observed increase in wavelength resulted from changes in the wave environment. Uncertainty in the predicted wavelength due to using measurements within the trapped lee waves as initial conditions is shown to be small in this case. The wind profiler and RASS measurement accuracies are adequate to measure changes leading to trapped lee wave nonstationarity.

The results from these two case studies are combined with evidence of nonstationarity found in earlier papers and additional events documented here using visible satellite imagery. These 24 observations of nonstationarity indicate that the horizontal wavelength of trapped lee waves can change by as much as 30% h−1. The average of all events, most of which occurred at midday, is a 9% h−1 temporal increase. It is suggested that the deepening of the mixed layer thins the elevated stable layer that is a key part of the waveguide and that this thinning causes the systematic temporal increase of the horizontal wavelength.

While this study focused on trapped lee waves, it can be inferred that vertically propagating waves can also change significantly over a few hours. Because aircraft measurements of vertical momentum flux profiles typically require 2–5 h to complete, and stationarity is required over that time, it is recommended that steadiness should be measured rather than assumed in such studies.

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Michael C. Kruk, Ethan J. Gibney, David H. Levinson, and Michael Squires

Abstract

Tropical cyclones pose a significant threat to life and property along coastal regions of the United States. As these systems move inland and dissipate, they can also pose a threat to life and property, through heavy rains, high winds, and other severe weather such as tornadoes. While many studies have focused on the impacts from tropical cyclones on coastal counties of the United States, this study goes beyond the coast and examines the impacts caused by tropical cyclones on inland locations. Using geographical information system software, historical track data are used in conjunction with the radial maximum extent of the maximum sustained winds at 34-, 50-, and 64-kt (1 kt ≈ 0.5 m s−1) thresholds for all intensities of tropical cyclones and overlaid on a 30-km equal-area grid that covers the eastern half of the United States. The result is a series of maps with frequency distributions and an estimation of return intervals for inland tropical storm– and hurricane-force winds. Knowing where the climatologically favored areas are for tropical cyclones, combined with a climatological expectation of the inland penetration frequency of these storms, can be of tremendous value to forecasters, emergency managers, and the public.

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Robert M. Banta, Lisa S. Darby, Pirmin Kaufmann, David H. Levinson, and Cui-Juan Zhu

Abstract

Many interesting flow patterns were found in the Grand Canyon by a scanning Doppler lidar deployed to the south rim during the 1990 Wintertime Visibility Study. Three are analyzed in this study: 1) flow reversal in the canyon, where the flow in the canyon was in the opposite direction from the flow above the canyon rim; 2) under strong, gusty flow from the southwest, the flow inside and above the canyon was from a similar direction and coupled; and 3) under light large-scale ambient flow, the lidar found evidence of local, thermally forced up- and down-canyon winds in the bottom of the canyon.

On the days with flow reversal in the canyon, the strongest in-canyon flow response was found for days with northwesterly flow and a strong inversion at the canyon rim. The aerosol backscatter profiles were well mixed within the canyon but poorly mixed across the rim because of the inversion. The gusty southwest flow days showed strong evidence of vertical mixing across the rim both in the momentum and in the aerosol backscatter profiles, as one would expect in turbulent flow. The days with light ambient flow showed poor vertical mixing even inside the canyon, where the jet of down-canyon flow in the bottom of the canyon at night was often either cleaner or dirtier than the air in the upper portions of the canyon. In a case study presented, the light ambient flow regime ended with an intrusion of polluted, gusty, southwesterly flow. The polluted, high-backscatter air took several hours to mix into the upper parts of the canyon. An example is also given of high-backscatter air in the upper portions of the canyon being mixed rapidly down into a jet of cleaner air in the bottom of the canyon in just a few minutes.

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Kenneth R. Knapp, Michael C. Kruk, David H. Levinson, Howard J. Diamond, and Charles J. Neumann

The goal of the International Best Track Archive for Climate Stewardship (IBTrACS) project is to collect the historical tropical cyclone best-track data from all available Regional Specialized Meteorological Centers (RSMCs) and other agencies, combine the disparate datasets into one product, and disseminate in formats used by the tropical cyclone community. Each RSMC forecasts and monitors storms for a specific region and annually archives best-track data, which consist of information on a storm's position, intensity, and other related parameters. IBTrACS is a new dataset based on the best-track data from numerous sources. Moreover, rather than preferentially selecting one track and intensity for each storm, the mean position, the original intensities from the agencies, and summary statistics are provided. This article discusses the dataset construction, explores the tropical cyclone climatology from IBTrACS, and concludes with an analysis of uncertainty in the tropical cyclone intensity record.

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