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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.

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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.

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Adam J. Deppe
,
William A. Gallus Jr.
, and
Eugene S. Takle

Abstract

The Weather Research and Forecasting Model (WRF) with 10-km horizontal grid spacing was used to explore improvements in wind speed forecasts at a typical wind turbine hub height (80 m). An ensemble consisting of WRF model simulations with different planetary boundary layer (PBL) schemes showed little spread among the individual ensemble members for forecasting wind speed. A second configuration using three random perturbations of the Global Forecast System model produced more spread in the wind speed forecasts, but the ensemble mean possessed a higher mean absolute error (MAE). A third ensemble of different initialization times showed larger model spread, but model MAE was not compromised. In addition, postprocessing techniques such as training of the model for the day 2 forecast based on day 1 results and bias correction based on observed wind direction are examined. Ramp event forecasting was also explored. An event was considered to be a ramp event if the change in wind power was 50% or more of total capacity in either 4 or 2 h or less. This was approximated using a typical wind turbine power curve such that any wind speed increase or decrease of more than 3 m s−1 within the 6–12 m s−1 window (where power production varies greatly) in 4 h or less would be considered a ramp. Model MAE, climatology of ramp events, and causes were examined. All PBL schemes examined predicted fewer ramp events compared to the observations, and model forecasts for ramps in general were poor.

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Brian J. Vanderwende
,
Julie K. Lundquist
,
Michael E. Rhodes
,
Eugene S. Takle
, and
Samantha L. Irvin

Abstract

In the U.S. state of Iowa, the increase in wind power production has motivated interest into the impacts of low-level jets on turbine performance. In this study, two commercial lidar systems were used to sample wind profiles in August 2013. Jets were systematically detected and assigned an intensity rating from 0 (weak) to 3 (strong). Many similarities were found between observed jets and the well-studied Great Plains low-level jet in summer, including average jet heights between 300 and 500 m above ground level, a preference for southerly wind directions, and a nighttime bias for stronger jets. Strong vertical wind shear and veer were observed, as well as veering over time associated with the LLJs. Speed, shear, and veer increases extended into the turbine-rotor layer during intense jets. Ramp events, in which winds rapidly increase or decrease in the rotor layer, were also commonly observed during jet formation periods. The lidar data were also used to evaluate various configurations of the Weather Research and Forecasting Model. Jet occurrence exhibited a stronger dependence on the choice of initial and boundary condition data, while reproduction of the strongest jets was influenced more strongly by the choice of planetary boundary layer scheme. A decomposition of mean model winds suggested that the main forcing mechanism for observed jets was the inertial oscillation. These results have implications for wind energy forecasting and site assessment in the Midwest.

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David A. R. Kristovich
,
Eugene Takle
,
George S. Young
, and
Ashish Sharma

Abstract

This chapter outlines the development of our understanding of several examples of mesoscale atmospheric circulations that are tied directly to surface forcings, starting from thermally driven variations over the ocean and progressing inland to man-made variations in temperature and roughness, and ending with forced boundary layer circulations. Examples include atmospheric responses to 1) overocean temperature variations, 2) coastlines (sea breezes), 3) mesoscale regions of inland water (lake-effect storms), and 4) variations in land-based surface usage (urban land cover). This chapter provides brief summaries of the historical evolution of, and tools for, understanding such mesoscale atmospheric circulations and their importance to the field, as well as physical processes responsible for initiating and determining their evolution. Some avenues of future research we see as critical are provided. The American Meteorological Society (AMS) has played a direct and important role in fostering the development of understanding mesoscale surface-forced circulations. The significance of AMS journal publications and conferences on this and interrelated atmospheric, oceanic, and hydrological fields, as well as those by sister scientific organizations, are demonstrated through extensive relevant citations.

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Tsing-Chang Chen
,
Ming-Cheng Yen
,
Jenq-Dar Tsay
,
Chi-Chang Liao
, and
Eugene S. Takle

Abstract

Environmental conditions for the roughly three million people living in the Taipei basin of Taiwan are greatly affected by the land–sea breeze and afternoon thunderstorm activities. A new perspective on the land–sea breeze life cycle and how it is affected by afternoon thunderstorm activity in the Taipei basin during the dry season is provided. During the summer monsoon break–revival phase, about 75% of rainfall in the Taipei basin is produced by afternoon thunderstorms triggered by sea-breeze interactions with the mountains to the south of this basin. Because the basic characteristics of the land–sea breeze and the changes it undergoes through the influence of afternoon thunderstorms have not been comprehensively analyzed/documented, a mini–field experiment was conducted during the summers of 2004 and 2005 to explore these aspects of the land–sea breeze in this basin. Thunderstorm rainfall is found to change not only the basin’s land–sea-breeze life cycle, but also its ventilation mechanism. On the nonthunderstorm day, the sea breeze supplies the open-sea fresh air for about 8 h during the daytime, but the land breeze persists on the thunderstorm day from afternoon to the next morning, acting to sweep polluted urban air out of the basin.

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Joseph M. Prusa
,
Moti Segal
,
Bradley R. Temeyer
,
William A. Gallus Jr.
, and
Eugene S. Takle

Abstract

The potential thermal effects of traffic on road surface thermal energy balance under frost/snow cover conditions have been largely ignored in meteorological evaluations of road ice deposit conditions. Preliminary exploration of these effects, particularly for heavy traffic scenarios with calm wind conditions and an ambient temperature of 0°C, is provided in this study using a conceptual model. Observational data were used to constrain the model, and parameterizations were employed to estimate the various heat transfer processes involved. The results indicate that, for heavy traffic situations, as well as for stopped traffic at intersections, the traffic thermal flux contribution at the surface is noticeable in a wide range of possible frost/snow-covered road conditions. The sensitivity to variation in traffic density, speed, and the emissivity of vehicle radiative surfaces, among others, is evaluated. Simple quantification of these traffic thermal effects, which might be considered in operational meteorological model forecasting of icy road conditions, is offered.

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Tsing-Chang Chen
,
Jin-ho Yoon
,
Kathryn J. St. Croix
, and
Eugene S. Takle

Analyzing the Global Historical Climatology Network, outgoing longwave radiation, and NCEP–NCAR reanalysis data over the Amazon Basin, the authors find a clear interdecadal increasing trend over the past four decades in both rainfall and intensity of the hydrological cycle. These interdecadal variations are a result of the interdecadal change of the global divergent circulation. On the contrary, the impact of the Amazon deforestation as evaluated by all numerical studies has found a reduction of rainfall and evaporation, and an increase of temperature in the Amazon Basin extending its dry season. Evidently, the interdecadal trend of the basin's hydrological cycle revealed from observations functions in a course opposite to the deforestation scenario. Results of this study suggest that future studies analyzing the impact of the basin-scale deforestation on the regional hydrological cycle and climate should be reassessed with multidecade numerical simulations including both schemes handling the land-surface processes and the mechanism generating proper interdecadal variation of the global divergent circulation.

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William J. Gutowski Jr.
,
Francis O. Otieno
,
Raymond W. Arritt
,
Eugene S. Takle
, and
Zaitao Pan

Abstract

Precipitation from a 10-yr regional climate simulation is evaluated using three complementary analyses: self-organizing maps, bias scores, and arithmetic bias. Collectively, the three reveal a precipitation deficit in the south-central United States that emerges in September and lingers through February. Deficient precipitation for this region and time of year is also evident in other simulations, indicating a generic problem in climate simulation.

Analysis of terrestrial and atmospheric water balances shows that the 10-yr average precipitation error for the region results primarily from a deficit in horizontal water vapor convergence. However, the 10-yr average for fall only suggests that the primary contributor is a deficit in evapotranspiration. Evaluation of simulated temperature and soil moisture suggests the model has insufficient terrestrial water for evaporation during fall. Results for winter are mixed; errors in both evapotranspiration and lateral moisture convergence may contribute substantially to the precipitation deficit. The model reproduces well both the time-average and time-filtered large-scale circulation, implying that the moisture convergence error arises from an error in simulating mesoscale circulation.

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Kenneth E. Kunkel
,
Karen Andsager
,
Xin-Zhong Liang
,
Raymond W. Arritt
,
Eugene S. Takle
,
William J. Gutowski Jr.
, and
Zaitao Pan

Abstract

A regional climate model simulation of the period of 1979–88 over the contiguous United States, driven by lateral boundary conditions from the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis, was analyzed to assess the ability of the model to simulate heavy precipitation events and seasonal precipitation anomalies. Heavy events were defined by precipitation totals that exceed the threshold value for a specified return period and duration. The model magnitudes of the thresholds for 1-day heavy precipitation events were in good agreement with observed thresholds for much of the central United States. Model thresholds were greater than observed for the eastern and intermountain western portions of the region and were smaller than observed for the lower Mississippi River basin. For 7-day events, model thresholds were in good agreement with observed thresholds for the eastern United States and Great Plains, were less than observed for the most of the Mississippi River valley, and were greater than observed for the intermountain western region. The interannual variability in frequency of heavy events in the model simulation exhibited similar behavior to that of the observed variability in the South, Southwest, West, and North-Central study regions. The agreement was poorer for the Midwest and Northeast, although the magnitude of variability was similar for both model and observations. There was good agreement between the model and observational data in the seasonal distribution of extreme events for the West and North-Central study regions; in the Southwest, Midwest, and Northeast, there were general similarities but some differences in the details of the distributions. The most notable differences occurred for the southern Gulf Coast region, for which the model produced a summer peak that is not present in the observational data. There was not a very high correlation in the timing of individual heavy events between the model and observations, reflecting differences between model and observations in the speed and path of many of the synoptic-scale events triggering the precipitation.

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