Showing papers by "Christopher A. Davis published in 2010"

Global Distribution and Characteristics of Diurnally Varying Low-Level Jets

Abstract: This study documents the global distribution and characteristics of diurnally varying low-level jets (LLJs), including their horizontal, vertical, and temporal structure, with a special emphasis on highlighting the underlying commonalities and unique qualities of the various nocturnal jets Two tools are developed to accomplish this goal The first is a 21-yr global reanalysis performed with the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) using a horizontal grid spacing of 40 km A unique characteristic of the reanalysis is the availability of hourly three-dimensional output, which permits the full diurnal cycle to be analyzed Furthermore, the horizontal grid spacing of 40 km better resolves many physiographic features that host LLJs than other widely used global reanalyses This makes possible a detailed examination of the systematic onset and cessation of the jets, including time–height representations of the diurnal cycle The second tool is an index of nocturnal

Does Increased Horizontal Resolution Improve Hurricane Wind Forecasts

Abstract: The representation of tropical cyclone track, intensity, and structure in a set of 69 parallel forecasts performed at each of two horizontal grid increments with the Advanced Research Hurricane (AHW) component of the Weather and Research and Forecasting Model (WRF) is evaluated. These forecasts covered 10 Atlantic tropical cyclones: 6 from the 2005 season and 4 from 2007. The forecasts were integrated from identical initial conditions produced by a cycling ensemble Kalman filter. The high-resolution forecasts used moving, storm-centered nests of 4- and 1.33-km grid spacing. The coarse-resolution forecasts consisted of a single 12-km domain (which was identical to the outer domain in the forecasts with nests). Forecasts were evaluated out to 120 h. Novel verification techniques were developed to evaluate forecasts of wind radii and the degree of storm asymmetry. Intensity (maximum wind) and rapid intensification, as well as wind radii, were all predicted more accurately with increased horizontal r...

Environmental Controls on the Simulated Diurnal Cycle of Warm-Season Precipitation in the Continental United States

Abstract: The diurnal cycle of warm-season precipitation in the Rocky Mountains and adjacent Great Plains of the United States is examined using a numerical modeling framework designed to isolate the role of terrain-influenced diurnally varying flows within a quasi-stationary longwave pattern common to active periods of midsummer convection. Simulations are initialized using monthly averaged conditions and contain lateral boundary conditions that vary only with the diurnal cycle. Together these attributes mitigate effects of transient weather disturbances originating upstream of the model domain. After a spinup period, the final 7 days of the 10-day model integration are analyzed and compared with observations. Results indicate that many salient features of the monthly precipitation climatology are reproduced by the model. These include a stationary afternoon precipitation frequency maximum over the Rocky Mountains followed overnight by an eastward-progressing zone of maximum precipitation frequencies conf...

Global Precipitation Extremes Associated with Diurnally Varying Low-Level Jets

Abstract: Extreme rainfall events have important societal impacts: for example, by causing flooding, replenishing reservoirs, and affecting agricultural yields. Previous literature has documented linkages between rainfall extremes and nocturnal low-level jets (NLLJs) over the Great Plains of North America and the La Plata River basin of South America. In this study, the authors utilize a 21-yr, hourly global 40-km reanalysis based on the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) to examine whether NLLJ–rainfall linkages are common elsewhere on the earth. The reanalysis is uniquely suited for the task because of its comparatively high spatial and temporal resolution and because a companion paper demonstrated that it realistically simulates the vertical, horizontal, and diurnal structure of the winds in well-known NLLJ regions. The companion paper employed the reanalysis to identify and describe numerous NLLJs across the planet, including several previously unknown NLLJs. The ...

Ensemble-Based Forecast Uncertainty Analysis of Diverse Heavy Rainfall Events

Abstract: This study examines widespread heavy rainfall over 5-day periods in the central and eastern United States. First, a climatology is presented that identifies events in which more than 100 mm of precipitation fell over more than 800 000 km2 in 5 days. This climatology shows that such events are most common in the cool season near the Gulf of Mexico coast and are rare in the warm season. Then, the focus turns to the years 2007 and 2008, when nine such events occurred in the United States, all of them leading to flooding. Three of these were associated with warm-season convection, three took place in the cool season, and three were caused by landfalling tropical cyclones. Global ensemble forecasts from the European Centre for Medium-Range Weather Forecasts Ensemble Prediction System are used to assess forecast skill and uncertainty for these nine events, and to identify the types of weather systems associated with their relative levels of skill and uncertainty. Objective verification metrics and subj...

Simulations of Subtropical Cyclones in a Baroclinic Channel Model

Abstract: The present study considers a variety of cyclone developments that occur in an idealized, baroclinic channel model featuring full condensation heating effects over an ocean with prescribed sea surface temperature variation. The geostrophic basic-state jet is specified by the tropopause shape, and horizontal shear is included by specifying the meridional variation of zonal wind on the lower boundary. The horizontal shear induces anticyclonic wave breaking of baroclinic waves. Normal mode perturbations are computed using a “fake-dry” version of the model but integrated forward using full physics. Low-latitude moist convection is particularly strong in simulations with strong surface easterlies that destabilize the troposphere through water vapor fluxes from the ocean surface. Deep convection produces a locally elevated dynamic tropopause and an associated anticyclone. This modified zonal flow supports moist baroclinic instability. The resulting cyclones, identified as subtropical cyclones, occur in...

H13-219 corridors of enhanced transport and dispersion: global distribution and characteristics

Abstract: This paper documents the global distribution and characteristics of corridors of enhanced transport and dispersion (T&D), particularly their diurnal and vertical structure. These corridors, generically termed low-level jets (LLJs), are ubiquitous features within the world's land covered areas, and regularly occur dur ing the warm season. To accomplish this goal, a ne w 21-year global reanalysis was performed with an MM5-based global climate downscaling system using a grid spacing of 40 km. A unique characteristic of the reanalysis is the availability of hourly three-dimensional output , which permits the full diurnal cycle to be analyz ed. The first available objectively constructed global maps of recurring NLLJs are crea ted, where the various NLLJs can be simultaneously viewed at or near their peak time. These maps not only highlight all the locations whe re NLLJs are known to recur, but also reveal a numb er of new jets. The authors examine the basic mechanisms that give rise to the NLLJs identified in two disparate locat ions, each having a profound influence on the regional T&D, and illustrate the v ariety of physiographic and thermal forcing mechani sms that can produce NLLJs. The shallowness of the jets, their dependence on turbul ence, their ubiquity and intensity underscore a fun damental challenge to global weather and T&D modeling of the distribution of atmospheric constituents originating from the Earth's surface and human activity. The study focuses on jets that have an along-jet sc ale of several hundred kilometers, and therefore ex ert a profound influence upon the regional weather and T&D. Our goal is to emphasize the underlying commonalities of the jets, using a 21-year global reanalysis performed with the MM5 model using a horizontal grid spacing of 40 km. Low-level je ts with substantial diurnal variability have traditionally been difficu lt to study from a global perspective because of th e lack of spatial and temporal resolution of available global reanalyses. A unique characteristic of the reanalysis used in this study is the availability of hourly output. This allows the ful l diurnal cycle to be analyzed. Furthermore, with a horizontal grid spacing of 40 km, many topographic features are better reso lved than in widely used global datasets such as th e NCEP-NCAR Reanalysis (NNRP). Thus, the diurnal variation of NL LJs, as well as the local forcing, is well represen ted in our analysis. This makes possible a detailed examination of the s ystematic onset and cessation of the jets, includin g time-height representations of the diurnal cycle. Global maps of NLLJ activity are developed, based on the temporal variations in the wind's vertical pr ofile, defined in local time. This allows us to simultaneously view the ob jectively defined NLLJs index and thus have a quant itative analogue of Fig. 1 given in S96. This index is defined and app lied to document the global occurrences of NLLJs. A representative case for one NLLJ is presented to better understand the mechanisms that produce these jets.