Search Results

You are looking at 1 - 10 of 10 items for

  • Author or Editor: Julie A. Winkler x
  • All content x
Clear All Modify Search
Gregory D. Bierly and Julie A. Winkler

Abstract

Relative wind isentropic analysis was employed to investigate the evolution of airstreams and airstream boundaries within midlatitude cyclones that formed in the Colorado cyclogenesis region of the United States. This study attempts to verify and expand upon existing conceptual models of three-dimensional airflow, while describing how such models vary at different times during cyclone development and when the intensification history of the storm is considered. Forty-nine cyclone events were first divided into three categories: early-developing cyclones (those that intensify with 24 h of cyclogenesis), late-developing cyclones (those that intensify 24–48 h after cyclogenesis), and nondeveloping cyclones (those that either display little change in intensity or weaken with time). Composite isentropic surfaces for multiple levels (315–290 K, separated by 5 K) were constructed by cyclone category for six 12-h time periods within the cyclone life cycle.

Three distinct airstreams and four types of airstream boundaries were identified on the composite isentropic surfaces. Two of the airstreams closely resemble the “drystream” and “warm conveyor belt (WCB)” described in previous studies. The third airstream is referred to here as the cyclonically turning moist airstream (CMA). Until approximately 24 h after cyclogenesis, the CMA and WCB originate at similar latitudes although the CMA occurs at a lower elevation. Later in the storm life cycle, the CMA originates at a more northerly latitude than the WCB and in comparison is a relatively cold airstream. Airstream boundaries separating the WCB and the drystream are seen at almost all time periods. This feature acquires a forward-leaning orientation with time with only the lowermost boundaries being accompanied by a modest to strong temperature gradient. Two airstream boundaries involve the CMA. The first separates the CMA and the drystream and is a lower-tropospheric feature, particularly late in the storm life cycle. The second boundary is located north or northwest of the cyclone center and separates the CMA from northerly descending air. This midtropospheric feature occurs along a relatively weak temperature gradient. The fourth type of airstream boundary is referred to as a southwest confluence zone and separates northerly, descending airflow southwest of the cyclone center from easterly, rising airflow to the southeast. At the middle and later stages of the cyclone life cycle, this boundary is a lower-tropospheric feature. It is often associated with a relatively strong temperature gradient. The composites indicate that the evolution of the airstreams and airstream boundaries is remarkably similar for the three cyclone types, except that they are out of synchrony by one or more 12-h time steps. In particular, all three airstreams are evident on the precyclogenesis (time t − 12) composite surfaces for the nondeveloping cyclones, whereas the full suite of airstreams does not appear until 12 h later for the developing cyclones.

Full access
Claudia K. Walters and Julie A. Winkler

Abstract

A synoptic climatological approach was used to investigate 1) the airflow characteristics of southerly low-level wind maxima in the Great Plains, 2) the typical thermodynamic environments in which low-level wind maxima form, 3) the convergence fields accompanying the wind maxima, and 4) the associated patterns of cloud-to-ground lightning activity. A total of 260 low-level jet events that occurred at either 0000 or 1200 UTC during the warm seasons (Apr–Sep) of 1991–92 were employed in the analysis.

The spatial configuration of southerly low-level jets was shown to be considerably more complex than previously portrayed. A subjective classification of the jet events—based on streamline curvature, latitudinal extent, and the orientation of confluence and deformation zones—resulted in 12 distinct configuration types. Only 37% of the events were broadly classified as anticyclonically curving jets, even though the majority of previous case studies focused on this type of wind maximum. Another unanticipated finding was the substantial variation in wind direction with height, usually to more anticyclonic flow, observed for over 50% of the low-level wind maxima. These events were termed multilevel jets, although it is not clear whether they are characterized by distinct, multiple airstreams separated in the vertical or a more gradual change in airflow with height. Composite temperature and humidity fields indicated that the thermodynamic conditions are fairly similar for the different jet types. All types were associated with a collocated humidity gradient and thermal ridge southwest of the jet axis. In contrast, the composite lower-tropospheric convergence fields varied considerably between types and reflect the complex airflow configurations. The spatial distribution of cloud-to-ground lightning activity corresponded well with local maxima in the composite convergence fields.

This study contributes to a more complete understanding of the climatology of low-level jets. It provides a unique perspective in that no previous climatological study systematically investigated the variability in the airflow configurations of jet events or explicitly related the spatial pattern of convection to the airflow configuration and resulting convergence fields. In Part II of this study, the typical synoptic and subsynoptic-scale environments in which wind maxima with different configurations occur are described.

Full access
Julie A. Winkler, Donna Tucker, and Anne K. Smith

Zevin and Seitter's analyses of the 1993 American Meteorological Society membership survey indicated that university/college employees had the largest difference in salary by gender when controlling for experience and age. Further analyses of the membership survey presented here indicate that a large salary discrepancy exists for female full professors in atmospheric science. In addition, the small number of women at the associate professor rank suggests a “leaky pipeline” for female atmospheric science faculty. A comparison of tenure-stream faculty to Ph.D.-level atmospheric scientists outside of academia suggests that female Ph.D.'s have fared better in nonuniversity positions in terms of senior-level salaries and advancement from entry- to midlevel positions. Possible explanations for the salary differential at the full professor level and for the small number of female associate professors in atmospheric science are explored, although no conclusive explanation can be given at this time. Possible actions to remediate the salary differential and poor advancement of faculty are proposed. These remediative actions are directed to heads and chairs of atmospheric science departments who are often in a position to initiate change within their departments and universities.

Full access
Julie A. Winkler, Brent R. Skeeter, and Paul D. Yamamoto

Abstract

Hourly precipitation data from 1967 to 1983 for the coterminous, United States were harmonically analyzed in order to document the diurnal variability of several categories of heavy hourly precipitation during winter, spring, summer, and autumn. The analysis revealed that the diurnal characteristics of hourly precipitation vary considerably with season, geographic region, and precipitation intensity. During winter and spring, a weak, late-morning frequency maximum prevails for the lightest (2.5–6.2 mm h−1) precipitation category. As intensity increases (to 6.3–12.6, 12.7–25.3, and ⩾25.4 mm h−1), the amplitude of the diurnal cycle also increases, and a nocturnal maximum becomes apparent across much of the eastern and central United States. In summer, the diurnal cycle is strongly modulated for all categories. The nocturnal region decreases in areal extent at this time of year, as an afternoon maximum becomes established across the southern and eastern states. In autumn, the nocturnal region again increases in size, although the area it encompasses is smaller than that during winter and spring. Seasonal variations in the semidiurnal cycle are more ambiguous due to the dominance of the diurnal cycle at most locations, although secondary maxima and minima are most likely south of the Great Lakes and in eastern and central Texas. Comparison with the results of previous studies indicates that different definitions of “winter,” “spring,” “summer,” and “autumn” can lead to divergent descriptions of the diurnal cycle of hourly precipitation.

Full access
Donna Tucker, Donna K. Ginther, and Julie A. Winkler

Abstract

No Abstract available.

Full access
Julie A. Winkler, Richard H. Skaggs, and Donald G. Baker

Abstract

In order to better estimate urban influence on local climate, mean temperature series were corrected for biases and heterogeneities. Annual, January and July mean temperatures for 21 stations in and around Minneapolis-St. Paul, Minnesota were adjusted for background climate, differences in observation time, and changes in station location. The effect of the adjustments on the spatial structure of the urban heat island was judged by comparing isopleth analyses of the adjusted and unadjusted temperatures for each of the three time periods. Adjusted temperature data depict a larger heat island that conforms more closely to the urban structure. The influence of the adjustments on the strength of the heat island was estimated by comparing urban-rural temperature differences calculated from both data sets for the three time periods. The mean urban minus mean rural temperature differences calculated from the adjusted data are as much as 5017o larger than the differences calculated from the unadjusted data.

Full access
Julie A. Winkler, Jean P. Palutikof, Jeffrey A. Andresen, and Clare M. Goodess

Abstract

Empirical transfer functions have been proposed as a means for “downscaling” simulations from general circulation models (GCMs) to the local scale. However, subjective decisions made during the development of these functions may influence the ensuing climate scenarios. This research evaluated the sensitivity of a selected empirical transfer function methodology to 1) the definition of the seasons for which separate specification equations are derived, 2) adjustments for known departures of the GCM simulations of the predictor variables from observations, 3) the length of the calibration period, 4) the choice of function form, and 5) the choice of predictor variables. A modified version of the Climatological Projection by Model Statistics method was employed to generate control (1 × CO2) and perturbed (2 × CO2) scenarios of daily maximum and minimum temperature for two locations with diverse climates (Alcantarilla, Spain, and Eau Claire, Michigan). The GCM simulations used in the scenario development were from the Canadian Climate Centre second-generation model (CCC GCMII).

Variations in the downscaling methodology were found to have a statistically significant impact on the 2 × CO2 climate scenarios, even though the 1 × CO2 scenarios for the different transfer function approaches were often similar. The daily temperature scenarios for Alcantarilla and Eau Claire were most sensitive to the decision to adjust for deficiencies in the GCM simulations, the choice of predictor variables, and the seasonal definitions used to derive the functions (i.e., fixed seasons, floating seasons, or no seasons). The scenarios were less sensitive to the choice of function form (i.e., linear versus nonlinear) and to an increase in the length of the calibration period.

The results of Part I, which identified significant departures of the CCC GCMII simulations of two candidate predictor variables from observations, together with those presented here in Part II, 1) illustrate the importance of detailed comparisons of observed and GCM 1 × CO2 series of candidate predictor variables as an initial step in impact analysis, 2) demonstrate that decisions made when developing the transfer functions can have a substantial influence on the 2 × CO2 scenarios and their interpretation, 3) highlight the uncertainty in the appropriate criteria for evaluating transfer function approaches, and 4) suggest that automation of empirical transfer function methodologies is inappropriate because of differences in the performance of transfer functions between sites and because of spatial differences in the GCM’s ability to adequately simulate the predictor variables used in the functions.

Full access
Dana L. Doubler, Julie A. Winkler, Xindi Bian, Claudia K. Walters, and Shiyuan Zhong

Abstract

The North American Regional Reanalysis (NARR) was used to develop an expanded, long-term (1979–2009) climatology of meridional (southerly and northerly) low-level jets over North America and surrounding coastal environs. NARR has greater spatial coverage and finer temporal (3 hourly) and horizontal (32 km) resolutions than do routine rawinsonde wind measurements. The NARR climatology focuses on jet frequency and average speed and elevation by month and 3-hourly time step. To evaluate the plausibility of the climatology, jet characteristics were compared with those obtained from prior climatological analyses, case studies, field campaigns, and numerical simulations. Strong agreement was found with many of the previously documented characteristics of well-known jets, including the northerly Pacific coast jet and southerly Great Plains jet. The NARR climatology provides additional insights into the spatial extent and seasonal shifts of large jet frequencies and into diurnal fluctuations in frequency, speed, and elevation. Weaker and/or less spatially extensive jets are also well depicted in the NARR climatology, including the southerly Gulf of California jet, summertime southerly jets and autumn northerly jets off the mid-Atlantic coast, and northerly jets in the high plains. Furthermore, several new areas of relatively frequent jet occurrence, most of which align with shallow thermal gradients, are seen in the NARR climatology. The NARR climatology supplements and enhances our understanding of North American low-level jets and points to the need for additional research on both the climatological characteristics of these jets and on the processes contributing to their formation.

Full access
Claudia K. Walters, Julie A. Winkler, Sara Husseini, Ryan Keeling, Jovanka Nikolic, and Shiyuan Zhong

Abstract

Climatological analyses of low-level jets (LLJs) can be negatively influenced by the coarse spatial and temporal resolution and frequent changes in observing and archiving protocols of rawinsonde observations (raobs). The introduction of reanalysis datasets, such as the North American Regional Reanalysis (NARR), provides new resources for climatological research with finer spatial and temporal resolution and potentially fewer inhomogeneities. To assess the compatibility of LLJ characteristics identified from NARR wind profiles with those obtained from raob profiles, LLJs were extracted using standard jet definitions from NARR and raobs at 12 locations in the central United States for four representative years that reflect different rawinsonde protocols. LLJ characteristics (e.g., between-station differences in relative frequency, diurnal fluctuations, and mean speed and elevation) are generally consistent, although absolute frequencies are smaller for NARR relative to raobs at most stations. LLJs are concurrently identified in the NARR and raob wind profiles on less than 60% of the observation times with LLJ activity. Variations are seen between analysis years and locations. Of particular note is the substantial increase in LLJ frequency seen in raobs since the introduction of the Radiosonde Replacement System, which has led to a greater discrepancy in jet frequency between the NARR and raob datasets. The analyses suggest that NARR is a viable additional resource for climatological analyses of LLJs. Many of the findings are likely applicable for other fine-resolution reanalysis datasets, although differences between reanalyses require that each be carefully evaluated before its use in climatological analyses of wind maxima.

Full access
Brian E. Potter, Julie A. Winkler, Dwight F. Wilhelm, Ryan P. Shadbolt, and Xindi Bian

Abstract

The Haines index is used in wildfire forecasting and monitoring to evaluate the potential contributions of atmospheric stability and humidity to the behavior of plume-dominated wildfires. The index has three variants (“low,” “mid,” and “high”) that accommodate differences in surface elevation. As originally formulated, the low variant is calculated from temperature observations at the 950- and 850-hPa levels and humidity observations at 850 hPa. In the early 1990s the National Weather Service implemented a new mandatory level for radiosonde observations at 925 hPa. Following this change, measurements at 950 hPa became less frequent. An informal survey of several forecast offices found no formalized adjustment to the calculation of the low Haines index to take into account the nonavailability of 950-hPa measurements. Some sources continue to use 950-hPa temperature, usually interpolated from 925-hPa and surface temperatures, to calculate the low Haines index. Others directly substitute the 925-hPa temperature for the originally specified 950-hPa value. This study employs soundings from the central United States when both 950- and 925-hPa levels were available to investigate the impact of different calculation approaches on the resulting values of the low variant of the Haines index. Results show that direct substitution of 925-hPa temperature for the 950-hPa temperature can dramatically underestimate the potential wildfire severity compared with the original formulation of the Haines index. On the other hand, a low-elevation variant of the Haines index calculated from the interpolated 950-hPa temperature is usually in close agreement with the original formulation of the index.

Full access