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Joseph B. Olson and Brian A. Colle

Abstract

Three-dimensional idealized simulations using the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) down to 6-km grid spacing were performed in order to understand how different ambient conditions (wind speed and direction, stability, and inland cold pool) and terrain characteristics impact barrier jets along the southeastern Alaskan coast. The broad inland terrain of western North America is important in Alaskan jet development, since it rotates the impinging flow cyclonically (more coast parallel) well upstream of the coast, thus favoring more low-level flow blocking while also adding momentum and width to the barrier jet. Near the steep coastal terrain, the largest wind speed enhancement factor (1.9–2.0) in the terrain-parallel direction relative to the ambient onshore-directed wind speed occurs at relatively low Froude numbers (Fr ∼ 0.3–0.4). These low Froude numbers are associated with (10–15 m s−1) ambient wind speeds and wind directions orientated 30°–45° from terrain-parallel. For simulations with an inland cold pool and nearly coast-parallel flow, strong gap outflows develop through the coastal mountain gaps, shifting the largest wind speed enhancement to Fr < 0.2. The widest barrier jets occur with ambient winds oriented nearly terrain-parallel with strong static stability. The gap outflows shift the position of the jet maximum farther offshore from the coast and increase the jet width. The height of the jet maxima is typically located at the top of the shallow gap outflow (∼500 m MSL), but without strong gap outflows, the jet heights are located at the top of the boundary layer, which is higher (lower) for large (small) frictionally induced vertical wind shear and weak (strong) static stability.

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Joseph B. Olson and Brian A. Colle

Abstract

A technique for initializing realistic idealized extratropical cyclones for short-term (0–72 h) numerical simulations is described. The approach modifies select methods from two previous studies to provide more control over the initial cyclone structure. Additional features added to the technique include 1) deformation functions to initialize more realistic low-level fronts, tropopause structure, and enhanced jet maximum at upper levels; 2) a barotropic shear function to help develop different cyclone and frontal geometries; and 3) damping functions to create an isolated baroclinic wave in the horizontal; therefore, the initialized cyclone is not influenced by the domain boundaries for relatively short simulations. Since this procedure allows for control of the initialized cyclone structures, it may be useful for studies of frontal and cyclone interaction with topography and mesoscale predictability. The initialization system produces a variety of basic states and synoptic disturbances, ranging from weak to explosively developing cyclones. Examples are shown to provide some insight on how to adjust selected parameters. The output is compatible with the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model and the Weather Research and Forecasting model. This note describes the procedure as well as presents an example of a landfalling cyclone along the U.S. west coast with and without terrain.

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Brian A. Colle, Joseph B. Olson, and Jeffrey S. Tongue

Abstract

This paper describes the multiseason verification of the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) and the National Centers for Environmental Prediction (NCEP) Eta Model over the eastern two-thirds of the United States and surrounding coastal waters during the cool (1 November–31 March) and warm (1 May–30 September) seasons from the autumn of 1999 through the summer of 2001. Verification statistics are calculated by interpolating model forecasts to the observation sites. The horizontal and vertical distributions of model errors are presented as are the diurnal and intraseasonal trends. During the cool season, both the MM5 and Eta have a low-level cool and moist bias over land, a significant surface warm bias over water, and surface winds that are too strong over land to the east of the Rockies and too weak over water. The low-level cool and moist bias is maximized during the day, and the cool bias is largest during late winter. During the warm season, the MM5 and Eta have little temperature bias over water and a negative wind speed bias over the Rockies. Both the MM5 and Eta have a surface dry and warm bias over land during the warm season; however, the Eta warm-season biases were reduced between 2000 and 2001 because of recent improvements to the land surface model and soil moisture initialization. Using the NCEP Aviation Model rather than the Eta to initialize the MM5 during the 2000/01 cool season resulted in slightly better sea level pressure forecasts over the Northeast on average, but not for wind and temperature. In order to quantify the impact of increased resolution, the MM5 was verified down to 4-km grid spacing around coastal southern New England. For 32 objectively identified sea-breeze events, the 12-km MM5 has significantly greater wind and temperature skill along the coast than the 36-km version, but there is little improvement from 12 to 4 km. The sea breezes in the MM5 are too early on average and are associated with a late afternoon cool bias. Many of the MM5 and Eta errors have slowly evolving intraseasonal trends. In particular, the cool bias amplifies during the winter and the summer dry bias in the MM5 increases during prolonged wet periods. The sea level pressure errors are episodic, with clusters of negative and positive mean errors lasting approximately 3–6 weeks, thereby suggesting a dependence on the large-scale flow.

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Anna C. Fitch, Julie K. Lundquist, and Joseph B. Olson

Abstract

Large wind farms are expected to influence local and regional atmospheric circulations. Using a mesoscale parameterization of the effects of wind farms that includes a momentum sink and a wind speed–dependent source of turbulent kinetic energy, simulations were carried out to quantify the impact of a wind farm on an atmospheric boundary layer throughout a diurnal cycle. The presence of a wind farm covering 10 km × 10 km is found to have a significant impact on the local atmospheric flow and on regions up to 60 km downwind at night. Daytime convective conditions show little impact of the wind farm on wind speeds, as the momentum deficits generated by the wind farm rapidly mix through the depth of the boundary layer. At night, the stable layer within the rotor area inhibits turbulent mixing of the momentum deficit, leading to a shallower wake and a greater reduction in the wind speed within the wake. Although a low-level jet forms at altitudes within the rotor area in the hours before dawn, it is completely eliminated within the wind farm. At night, a maximum warming of 1 K is seen at the bottom of the rotor area. Near the surface, there is less warming (0.5 K). Downwind, the surface temperature perturbation is small, with a cooling of up to 0.3 K. Over the simulation period, the mean temperature change over the wind farm area at 2 m is a very slight warming (0.2 K). Mean temperature changes downwind are negligible. Other influences on turbulent kinetic energy, surface heat fluxes, and boundary layer height, are discussed.

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Anna C. Fitch, Joseph B. Olson, and Julie K. Lundquist

Abstract

For assessing the impacts of wind farms on regional climate, wind farms may be represented in climate models by an increase in aerodynamic roughness length. Studies employing this method have found near-surface temperature changes of 1–2 K over wind farm areas. By contrast, mesoscale and large-eddy simulations (LES), which represent wind farms as elevated sinks of momentum, generally showed temperature changes of less than 0.5 K. This study directly compares the two methods of representing wind farms in simulations of a strong diurnal cycle. Nearly the opposite wake structure is seen between the two methods, both during the day and at night. The sensible heat fluxes are generally exaggerated in the enhanced roughness approach, leading to much greater changes in temperature. Frequently, the two methods display the opposite sign in temperature change. Coarse resolution moderates the sensible heat fluxes but does not significantly improve the near-surface temperatures or low-level wind speed deficit. Since wind farm impacts modeled by the elevated momentum sink approach are similar to those seen in observations and from LES, the authors conclude that the increased surface roughness approach is not an appropriate option to represent wind farms or explore their impacts.

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Robert S. Arthur, Katherine A. Lundquist, and Joseph B. Olson

Abstract

The terrain-following vertical coordinate system used by many atmospheric models, including the Weather Research and Forecasting (WRF) Model, is prone to errors in regions of complex terrain. These errors stem, in part, from the calculation of horizontal gradients within the diffusion term of the momentum or scalar evolution equations. In WRF, such gradients can be calculated along coordinate surfaces, or using metric terms that help account for grid skewness. However, neither of these options ensures a truly horizontal gradient calculation, especially if a grid cell is skewed enough that the heights of the neighboring grid points used in the calculation fall outside the vertical range of the cell. In this work, an improved scheme that uses Taylor series approximations to vertically interpolate variables to the level necessary for a truly horizontal gradient calculation is implemented in WRF for the diffusion of potential temperature. The scheme is validated using an atmosphere-at-rest configuration, in which spurious flows develop only as a result of numerical errors and can thus be used as a proxy for model performance. Following validation, the method is applied to the simulation of cold-air pools (CAPs), which occur in regions of complex terrain and are characterized by strong near-surface temperature gradients. Using the truly horizontal scheme, idealized simulations demonstrate reduced numerical mixing in a quiescent CAP, and a realistic case study in the Columbia River basin shows a reduction in positive wind speed bias by up to roughly 20% compared to observations from the Second Wind Forecast Improvement Project.

Open access
Brian A. Colle, Joseph B. Olson, and Jeffrey S. Tongue

Abstract

This paper evaluates the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) precipitation forecasts over the northeastern United States to show the effects of increasing resolution, the spatial variations in model skill, and the impact of convective parameterizations on the MM5 precipitation forecasts. The MM5 is verified during the cool seasons (November–March) of 1999–2001 and the warm season (May–September) of 2000 using approximately 500 cooperative observer and National Weather Service precipitation sites. During the cool season, the 12-km MM5 produces excessive precipitation immediately downwind of the Great Lakes and along the windward slopes of the Appalachians and too little precipitation in the lee of the barrier. The 36-km MM5 has slightly more skill than at 12-km grid spacing for the light to moderate thresholds, while the 12-km precipitation forecasts are slightly better on average for the heavy precipitation events. During the 2000/01 cool season, two separate MM5 runs were completed twice daily using the National Centers for Environmental Prediction Eta (Eta–MM5) and Aviation (AVN–MM5) Models for initial and boundary conditions. The Eta–MM5 had slightly lower (better) rms errors than the AVN–MM5 for the weak to moderate thresholds (2.54–50.8 mm in 24 h), while for the heavier thresholds the AVN–MM5 had significantly lower rms errors than the Eta–MM5. As a result, the 36-km AVN–MM5 was as skillful as the 12-km Eta–MM5 for these higher thresholds. During the warm season, both the 36- and 12-km grid spacings overpredict precipitation just inland of the coast and significantly underpredict farther inland over the Appalachians. This coastal overprediction originated from an overactive Kain–Fritsch (KF) convective parameterization, while the inland underprediction is associated with a low-level dry bias during the warm season. A representative case study shows that both the Betts–Miller and Grell parameterizations produce less precipitation near the coast than the overactive KF scheme. A new and alternate version of KF (KF2) in the MM5 may also help to reduce this coastal overprediction. The 4-km MM5 explicit precipitation during the summer is sensitive to which convective parameterization is applied in the outer domains. Using KF or KF2 in the 36- and 12-km domains suppresses the explicit precipitation in the 4-km nest, especially for weak to moderate events over western sections of the 4-km domain. For a representative event, the Betts–Miller and Grell convective schemes allowed for a more realistic 4-km precipitation distribution, while a simulation using no convective parameterization in the 36- and 12-km domains produced excessive rain rates in the 4-km forecasts.

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Stephanie Redfern, Joseph B. Olson, Julie K. Lundquist, and Christopher T. M. Clack

Abstract

Wind power installations have been increasing in recent years. Because wind turbines can influence local wind speeds, temperatures, and surface fluxes, weather forecasting models should consider their effects. Wind farm parameterizations do currently exist for numerical weather prediction models. They generally consider two turbine impacts: elevated drag in the region of the wind turbine rotor disk and increased turbulent kinetic energy production. The wind farm parameterization available in the Weather Research and Forecasting (WRF) Model calculates this drag and TKE as a function of hub-height wind speed. However, recent work has suggested that integrating momentum over the entire rotor disk [via a rotor-equivalent wind speed (REWS)] is more appropriate, especially for cases with high wind shear. In this study, we implement the REWS in the WRF wind farm parameterization and evaluate its impacts in an idealized environment, with varying amounts of wind speed shear and wind directional veer. Specifically, we evaluate three separate cases: neutral stability with low wind shear, high stability with high wind shear, and high stability with nonlinear wind shear. For most situations, use of the REWS with the wind farm parameterization has marginal impacts on model forecasts. However, for scenarios with highly nonlinear wind shear, the REWS can significantly affect results.

Open access
Joseph B. Olson, Brian A. Colle, Nicholas A. Bond, and Nathaniel Winstead

Abstract

The Southeastern Alaskan Regional Jets experiment investigated the structures and physical processes of barrier jets along the coastal Fairweather Mountains near Juneau, Alaska, from 24 September to 21 October 2004. This paper compares in situ aircraft data and high-resolution simulations from the first intensive observation period (IOP1), which featured a nearly terrain-parallel barrier jet (classical jet) with another coastal jet event (IOP7) that was influenced by offshore-directed gap flows at the coast (hybrid jet). IOP1 featured southerly onshore flow preceding a landfalling trough, which was blocked by the coastal terrain and accelerated down the pressure gradient to produce a 5–10 m s−1 wind enhancement in the alongshore direction in the lowest 1 km MSL. In contrast, IOP7 featured higher surface pressure and colder low-level temperatures to the east (inland) of the study area than did IOP1, which resulted in offshore-directed coastal gap flow exiting Cross Sound below ∼500 m that turned anticyclonically and merged with the ambient flow. Unlike the classical jet (IOP1), IOP7 had a surface warm anomaly adjacent to the steep coastal terrain, while a cold anomaly existed farther offshore within the gap outflow. Above the shallow gap flow (>500 m MSL), there were more classical barrier jet characteristics. High-resolution fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) simulations were performed to compare the structures and underlying dynamics between the two cases. Model trajectories show that coastal winds for IOP1 originated offshore, while much of the coastal flow in IOP7 had gap flow origins near the surface and offshore origins above the gap outflow. A model momentum budget suggests that the vertical mixing of southerly momentum from aloft forced the gap outflow in IOP7 to turn anticyclonically more rapidly than an inertial circle. A simulation of IOP7 with the Cross Sound gap removed (filled in) produced a coastal jet with similar maximum wind speeds to the control but resulted in a reduction in the width of the coastal jet by about 40%.

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Joseph B. Olson, Brian A. Colle, Nicholas A. Bond, and Nathaniel Winstead

Abstract

The Southeastern Alaskan Regional Jets experiment investigated the structures and physical processes of barrier jets along the coastal Fairweather Mountains near Juneau, Alaska, from 24 September to 21 October 2004. This paper compares in situ aircraft data and high-resolution simulations from the first intensive observation period (IOP1), which featured a nearly terrain-parallel barrier jet (classical jet) with another coastal jet event (IOP7) that was influenced by offshore-directed gap flows at the coast (hybrid jet). IOP1 featured southerly onshore flow preceding a landfalling trough, which was blocked by the coastal terrain and accelerated down the pressure gradient to produce a 5–10 m s−1 wind enhancement in the alongshore direction in the lowest 1 km MSL. In contrast, IOP7 featured higher surface pressure and colder low-level temperatures to the east (inland) of the study area than did IOP1, which resulted in offshore-directed coastal gap flow exiting Cross Sound below ∼500 m that turned anticyclonically and merged with the ambient flow. Unlike the classical jet (IOP1), IOP7 had a surface warm anomaly adjacent to the steep coastal terrain, while a cold anomaly existed farther offshore within the gap outflow. Above the shallow gap flow (>500 m MSL), there were more classical barrier jet characteristics. High-resolution fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) simulations were performed to compare the structures and underlying dynamics between the two cases. Model trajectories show that coastal winds for IOP1 originated offshore, while much of the coastal flow in IOP7 had gap flow origins near the surface and offshore origins above the gap outflow. A model momentum budget suggests that the vertical mixing of southerly momentum from aloft forced the gap outflow in IOP7 to turn anticyclonically more rapidly than an inertial circle. A simulation of IOP7 with the Cross Sound gap removed (filled in) produced a coastal jet with similar maximum wind speeds to the control but resulted in a reduction in the width of the coastal jet by about 40%.

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