Search Results

You are looking at 1 - 10 of 27 items for

  • Author or Editor: Eugene S. Takle x
  • All content x
Clear All Modify Search
Eugene S. Takle

A 1987 survey of the atmospheric and oceanic science community on university instruction in instrumentation and observations revealed that an apparent imbalance existed between observational and theoretical/numerical components of the atmospheric sciences. The committee analyzing the survey results identified several factors contributing to the perceived imbalance. From a follow-up survey conducted of the same community in 1997 to assess the level of change over 10 years, it is found that some progress has been achieved over this period, but that more can be done. Rather than try to implement all of the recommendations from analysis of the initial survey, it is suggested that attention be focused on seven items that build on partial successes over the last 10 years and that offer likelihood of success in the near future.

Full access
Eugene S. Takle

Abstract

A survey of the characteristics of frost occurrences on bridges and roadways derived from questionnaires completed by highway maintenance personnel and analysis of more than 4000 frost observations in Iowa reveal that bridge frost occurs up to 58 times annually in certain parts of the state and, roadway frost, as many as 35 times. Certain bridges or stretches of road seem frost-prone because of their location or because of adjacent features. An expert system designed to provide 20-b forecasts of roadway and bridge frost has been constructed from analysis of meteorological conditions and has been evaluated against human forecasters. The expert system, when supplied with perfect forecasts of commonly forecast meteorological variables, produced accuracy com- parable to or higher than human forecasters. Human forecasters were observed to provide relatively unbiased forecasts for bridge frost but were highly biased toward reducing false alarms for roadway frost. The expert system, by contrast, is configured so that the decision threshold can be adjusted to give unbiased forecasts or forecasts that are biased in either direction without significant degradation of accuracy. A suggestion is made for the use of such a system as a management tool for separating forecast accuracy from (possibly nonmeteorological) decision-threshold criteria.

Full access
Hao Wang and Eugene S. Takle

Abstract

A neutral boundary layer nonhydrostatic numerical model is used to determine the characteristics of shelterbelt effects on mean wind direction and to study the processing causing wind rotation when air passes through a shelterbelt. The model uses a turbulence scheme that includes prognostic equations for turbulence kinetic energy and a master length scale proposed by Mellor and Yamada. The simulated results are in quantitative agreement with Nord's field measurements. The spatial variation of wind rotation and its dependence on incident angle and shelterbelt porosity is analysed. Dynamic processes of the wind rotation and its interactions with drag force and pressure perturbation are also discussed. It is concluded that shear of wind direction should be considered, along with shear of speed, in determining turbulent fluxes in the vicinity of a shelterbelt.

Full access
Shiyuan Zhong and Eugene S. Takle

Abstract

A three-dimensional mesoscale numerical model has been used to examine the effects of large-scale background winds on the characteristics of the sea–land-breeze circulations over an area with an irregular coastline and complex surface-heating patterns at Kennedy Space Center/Cape Canaveral in Florida. A series of numerical experiments was performed in which the large-scale winds were varied in both speed and direction. The surface heating was based on measured surface-temperature variation from the Kennedy Space Center Atmospheric Boundary Layer Experiment (KABLE) during the spring season when the land–sea temperature gradient reaches its maximum. The results from the simulations compared reasonably well with data available from KABLE.

The results show that an onshore large-scale flow produces weaker sea-breeze perturbations compared to those generated by an offshore flow. However, the coastal rivers and lagoons create intense surface convergence with strong vertical motion on the seaward side of the river by the merging of the onshore flow with the offshore river breezes, and such strong vertical motion can last for several hours. The disturbances caused by the inland water bodies are significant in the sea-breeze phase but are very minor in the land-breeze phase. An onshore synoptic wind causes an earlier onset of the sea breeze, but delays the onset of the land breeze, and a strong onshore flow of more than 5 m s−1 does not allow the land breeze to develop at all. The maximum offshore wind speed and vertical motion at night are less sensitive to the magnitude of surface cooling than to the large-scale flow and daytime surface heating, which together determine the initial flow at the beginning of the land–breeze phase. The results also show that the magnitude, the sense of rotation, and the diurnal variation of the dominant forces governing the wind-vector rotation change as the orientation of the synoptic wind direction changes. The rate of rotation in the sea-breeze phase is dominated mainly by the balance between the mesoscale pressure gradient and friction; at night, the Coriolis effect also contributes significantly to the balance of forces in the land-breeze phase.

Full access
Shiyuan Zhong and Eugene S. Takle

Abstract

The diurnal evolution of the three-dimensional structure of a mesoscale circulation system frequently occurring in the area of Kennedy Span Center-Cape Canaveral has been studied using the data from the Kennedy Space Center Atmospheric Boundary Layer Experiment (KABLE). The case was chosen from the spring intensive data-collection period when the greatest daytime temperature difference between land and water (sea and inland rivers) occurs and the local circulations are most intense. The daytime flow structure was determined primarily by the mesoscale pressure-gradient form created by the temperature contrast between land and water. A strong sea-breeze circulation, the dominant feature of the daytime flow field, was modified by a local inland river breeze known as the Indian River breeze, in that divergent flow over the river enhanced the sea-breeze convergence on the seaward side and generated additional convergence on the landward side of the river. The rivers near the coastline also modified the initial flow field by enhancing convergence in the surrounding areas and speeding up the movement of the sea-breeze front. The nighttime flow structure was dominated by a large-scale land breeze that was relatively uniform over the area and became quasi-stationary after midnight. The nonuniformity of the wind-vector rotation rate suggests that mesoscale forcing significantly modifies the Coriolis-induced oscillation. No clear convergence patterns associated with the rivers were observed at night. Detailed characteristics over a diurnal cycle of the sea-land breeze and of the river breeze onset time, strength, depth, propagation speed and both landward and seaward extension, are documented in this study. Some boundary-layer characteristics needed for predicting diffusion of pollutants released from coastal launch pads, including atmospheric stability, depth of the thermal internal boundary layer, and turbulent mixing are also discussed.

Full access
Er Lu, Eugene S. Takle, and Jha Manoj

Abstract

Changes in major climatic and hydrological quantities in the upper Mississippi River basin and their interrelationships are studied with the Soil and Water Assessment Tool being driven by the contemporary climate and future scenario simulations of 10 global models in the Intergovernmental Panel on Climate Change (IPCC) Data Archive. Although the seasonal cycles of climate and hydrological quantities simulated by the 10 models have differences, the ensemble is very close to the observation. Ensemble predictions show that with warming in all months, precipitation decreases in summer but increases in all other seasons. Correspondingly, streamflow decreases in all seasons except winter, evapotranspiration decreases in July–September and increases in all other months, and snowmelt increases in winter but decreases in spring and fall. To understand the linkages between the cross-century changes of climate and hydrological quantities and the relative importance of the changes of temperature and precipitation to the changes of hydrological quantities, relationships between interannual variations of these quantities are investigated. It is shown that the change rates of the hydrological quantities with respect to temperature and precipitation obtained from regressions of interannual variations can vary greatly from month to month; however, on a monthly basis, they do not change much from the current to the future periods. Evaluations with these change rates indicate that for interannual variations of hydrological quantities, both variations of temperature and precipitation are important, and their relative importance depends on the month of the year. However, the changes of hydrological quantities from the means of the current years to the means of the future are dominated by warming in all months, and the influence from change of precipitation is much smaller. The changes of the hydrological quantities can be well predicted with the change rates from the warming alone.

Full access
Tina M. Greenfield and Eugene S. Takle

Abstract

Frost on roadways and bridges can present hazardous conditions to motorists, particularly when it occurs in patches or on bridges when adjacent roadways are clear of frost. To minimize materials costs, vehicle corrosion, and negative environmental impacts, frost-suppression chemicals should be applied only when, where, and in the appropriate amounts needed to maintain roadways in a safe condition for motorists. Accurate forecasts of frost onset times, frost intensity, and frost disappearance (e.g., melting or sublimation) are needed to help roadway maintenance personnel decide when, where, and how much frost-suppression chemical to use. A finite-difference algorithm (BridgeT) has been developed that simulates vertical heat transfer in a bridge based on evolving meteorological conditions at its top and bottom as supplied by a weather forecast model. BridgeT simulates bridge temperatures at numerous points within the bridge (including its upper and lower surface) at each time step of the weather forecast model and calculates volume per unit area (i.e., depth) of deposited, melted, or sublimed frost. This model produces forecasts of bridge surface temperature, frost depth, and bridge condition (i.e., dry, wet, icy/snowy). Bridge frost predictions and bridge surface temperature are compared with observed and measured values to assess BridgeT's skill in forecasting bridge frost and associated conditions.

Full access
Tsing-Chang Chen, Wan-Ru Huang, and Eugene S. Takle

Abstract

Annual variation of midlatitude precipitation and its maintenance through divergent water vapor flux were explored by the use of hydrological variables from three reanalyses [(NCEP–NCAR, ECMWF Re-Analysis (ERA), and Goddard Earth Observing System (GEOS-1)] and two global precipitation datasets [Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) and Global Precipitation Climatology Project (GPCP)]. Two annual variation patterns of midlatitude precipitation were identified:

  1. Tropical–midlatitude precipitation contrast: Midlatitude precipitation along storm tracks over the oceans attains its maximum in winter and its minimum in summer opposite to that over the tropical continents.
  2. Land–ocean precipitation contrast: The annual precipitation variation between the oceans and the continent masses exhibits a pronounced seesaw.
The annual variation of precipitation along storm tracks of both hemispheres follows that of the convergence of transient water vapor flux. On the other hand, the land–ocean precipitation contrast in the Northern Hemisphere midlatitudes is primarily maintained by the annual seesaw between the divergence of stationary water vapor flux over the western oceans and the convergence of this water vapor flux over the eastern oceans during winter. The pattern is reversed during the summer. This divergence–convergence exchange of stationary water vapor flux is coupled with the annual evolution of upper-level ridges over continents and troughs over the oceans.

Full access
Tsing-Chang Chen, Jenq-Dar Tsay, and Eugene S. Takle

Abstract

The Taipei basin, located in northern Taiwan, is formed at the intersection of the Tanshui River valley (~30 km) and the Keelung River valley (~60 km). Summer is the dry season in northern Taiwan, but the maximum rainfall in the Taipei basin occurs during 15 June–31 August. The majority of summer rainfall in this basin is produced by afternoon thunderstorms. Thus, the water supply, air/land traffic, and pollution for this basin can be profoundly affected by interannual variations of thunderstorm days and rainfall. Because the mechanism for these interannual variations is still unknown, a systematic analysis is made of thunderstorm days and rainfall for the past two decades (1993–2013). These two variables are found to correlate opposite interannual variations of sea surface temperature anomalies over the National Oceanic and Atmospheric Administration Niño-3.4 region. Occurrence days for afternoon thunderstorms and rainfall amounts in the Taipei basin double during the cold El Niño–Southern Oscillation (ENSO) phase relative to the warm phase. During the latter phase, a stronger cold/drier monsoon southwesterly flow caused by the Pacific–Japan Oscillation weakens the thunderstorm activity in the Taipei basin through the land–sea breeze. In contrast, the opposite condition occurs during the cold ENSO phase. The water vapor flux over the East/Southeast Asian monsoon region converges more toward Taiwan to maintain rainfall over the Taipei basin during the cold ENSO phase than during the warm ENSO phase.

Full access
Eugene S. Takle, Daniel A. Rajewski, and Samantha L. Purdy

Abstract

The Iowa Atmospheric Observatory was established to better understand the unique microclimate characteristics of a wind farm. The facility consists of a pair of 120-m towers identically instrumented to observe basic landscape–atmosphere interactions in a highly managed agricultural landscape. The towers, one within and one outside of a utility-scale low-density-array wind farm, are equipped to measure vertical profiles of temperature, wind, moisture, and pressure and can host specialized sensors for a wide range of environmental conditions. Tower measurements during the 2016 growing season demonstrate the ability to distinguish microclimate differences created by single or multiple turbines from natural conditions over homogeneous agricultural fields. Microclimate differences between the two towers are reported as contrasts in normalized wind speed, normalized turbulence intensity, potential temperature, and water vapor mixing ratio. Differences are analyzed according to conditions of no wind farm influence (i.e., no wake) versus wind farm influence (i.e., waked flow) with distance downwind from a single wind turbine or a large group of turbines. Differences are also determined for more specific atmospheric conditions according to thermal stratification. Results demonstrate agreement with most, but not all, currently available numerical flow-field simulations of large wind farm arrays and of individual turbines. In particular, the well-documented higher nighttime surface temperature in wind farms is examined in vertical profiles that confirm this effect to be a “suppression of cooling” rather than a warming process. A summary is provided of how the wind farm boundary layer differs from the natural boundary layer derived from concurrent measurements over the summer of 2016.

Full access