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Chibuike Onwukwe
and
Peter L. Jackson

Abstract

Evaluation of downscaled meteorological information is crucial to identifying model behaviors that may propagate to end applications such as the simulation of local air quality. This study conducted and assessed yearlong simulations of hourly meteorological conditions over the Terrace–Kitimat Valley of northwestern British Columbia, Canada, at 1-km horizontal gridding for six PBL schemes in the Weather and Forecasting (WRF) Model, version 4.0. In terms of key surface meteorological variables that affect air quality, simulations over land demonstrated better skill for specific humidity and wind direction than for air temperature and wind speed. Spatial differences in modeled atmospheric properties and vertical profiles, especially for moisture content, were used to diagnose the relative capacity of each PBL scheme to represent pollutant dispersion and dilution. Stable conditions at night increased suppression of boundary layer mixing by the nonlocal Yonsei University (YSU) scheme when compared with suppression by the local eddy-diffusion component of the Asymmetric Convective Model, version 2 (ACM2), scheme, resulting in decreased wind speed and ambient temperature but moister air with the YSU scheme. The weakening of mixing by the Mellor–Yamada–Nakanishi–Niino (MYNN3) scheme with inland distance suggested that higher-order, nonlocal transport is sensitive to increasing topographic steepness toward the northern part of the valley. Disparities in mixing strengths among PBL schemes were greater in the summer when conditions were generally less stable with moist, warm air blowing inland than in winter when the valley channels cold, stable air from the interior. Increased convection in daytime led to greater entrainment of air from aloft and a thicker PBL with the YSU scheme than with the ACM2 scheme in summer while increasing countergradient transport in the MYNN3 scheme that reduces dilution.

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Peter L. Jackson
and
D. G. Steyn

Abstract

A simple shallow-water model of gap wind in a channel that is based upon hydraulic theory is presented and compared with observations and output from a 3D mesoscale numerical model. The model is found to be successful in simulating gap winds. The speed and depth of gap wind flow is strongly controlled by topography. Horizontal or vertical channel contractions can act to force strong, shallow supercritical flow downwind and light, deep subcritical flow upwind. Force-balance analysis of the hydraulic model output confirms mesoscale model results and indicates that the prime force balance in gap wind is between external pressure gradient and friction for supercritical flow and between external pressure gradient and height pressure gradient for subcritical flow. This force balance changes near channel controls when the balance is between advection and height pressure gradient. Sensitivity analyses show positive sensitivity of gap wind speed to changes in discharge and external pressure gradient, negative sensitivity to changes in friction and boundary layer height at the channel exit, and mixed sensitivity of gap wind speed to changes in reduced gravity.

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Peter L. Jackson
and
D. G. Steyn

Abstract

Gap winds in Howe Sound, British Columbia, are described and placed in context by reviewing studies of similar phenomena in other locations. An observational program consisting of a surface mesonetwork and vertical soundings shows that gap winds vary considerably along and across the channel, as well as vertically. Wind strength generally increases down channel, and strongest winds are found below 1000-m depth. Results from application of a 3D mesoscale numerical model to a gap wind case compare reasonably well with observations. Model output reveals more details of horizontal and especially vertical flow structure than is possible from observations. Model vertical cross sections and Froude number output indicate similarity with hydraulic flow. This is further substantiated by a force-balance analysis of model output.

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Peter L. Jackson
,
Chris J. C. Reason
, and
Shucai Guan

Abstract

A detailed analysis of a simulation of a coastal trapped disturbance (CTD) using the Colorado State University Regional Atmospheric Modeling System (RAMS) is presented. The CTD considered (15–18 May 1985) represents an example of a strong mesoscale trapped event with abrupt gravity current–like transitions in many meteorological parameters, and which was closely tied to the synoptic forcing. Propagation of this event along the west coast of North America occurred from initiation in the Southern California Bight–Baja California coastal region to the northern tip of Vancouver Island, and the event appeared to have no difficulty in negotiating significant bends or gaps in the coastal mountains unlike some other events that have ceased or stalled near Cape Mendocino, Point Arena, and the mouth of the Columbia River.

It is found that warm offshore flow ahead of the CTD, and cool onshore flow in the Southern California Bight–northern Baja California coastal region, both driven by the westward tracking of a synoptic low, are very important for initiation, and subsequent propagation, of the model CTD, similar to observations. Convergence of the initial onshore cool flow in the south combined with warm offshore flow in the north lead to a northward-directed pressure gradient and, hence, a southerly wind transition. The adjustment timescale of the onshore flow to form the southerlies of the CTD is found to be consistent with expectations from theory.

During the propagating stage of the event, the pressure gradient and Coriolis terms were found to be most important for the meridional wind tendency, with advection and diffusion making smaller contributions. Consistent with semigeostrophic theory for CTD, the length scale in the across-mountain direction of the model CTD is much less than the along-mountain scale. Although the model transitions in winds, pressure, and temperature are not as sharp as observed (attributed to the lack of boundary layer structure in the NCEP fields used for model initialization), there is some signature of the gravity current nature of the event.

Decay of the event occurred when the favorable synoptic forcing related to the synoptic low moved to the northwest. There appeared to be no evidence of any topographic influence on the decay or termination of this particular event, unlike for several other cases.

Taken together, this and the previous validation study suggest that RAMS can be usefully employed to better understand the nature of at least these strong CTD events.

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Yanjie Cheng
,
Youmin Tang
,
Peter Jackson
,
Dake Chen
, and
Ziwang Deng

Abstract

El Niño–Southern Oscillation (ENSO) retrospective ensemble-based probabilistic predictions were performed for the period of 1856–2003 using the Lamont-Doherty Earth Observatory, version 5 (LDEO5), model. To obtain more reliable and skillful ENSO probabilistic predictions, first, four ensemble construction strategies were investigated: (i) the optimal initial perturbation with singular vector of sea surface temperature anomaly (SSTA), (ii) the realistic high-frequency anomalous winds, (iii) the stochastic optimal pattern of anomalous winds, and (iv) a combination of the first and the third strategy. Second, verifications were conducted to examine the reliability and resolution of the probabilistic forecasts provided by the four methods. Results suggest that reliability of ENSO probabilistic forecast is more sensitive to the choice of ensemble construction strategy than the resolution, and a reliable and skillful ENSO probabilistic prediction system may not necessarily have the best deterministic prediction skills. Among these ensemble construction methods, the fourth strategy produces the most reliable and skillful ENSO probabilistic prediction, benefiting from the joint contributions of the stochastic optimal winds and the singular vector of SSTA. In particular, the stochastic optimal winds play an important role in improving the ENSO probabilistic predictability for the LDEO5 model.

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Shucai Guan
,
Peter L. Jackson
, and
Chris J. C. Reason

Abstract

The coastal trapped disturbance (CTD) of 15–17 May 1985 represents an example of a strong mesoscale trapped event along the west coast of North America with abrupt transitions in many basic meteorological parameters. In this study, a comparison between observations and a numerical simulation of this event using the Regional Atmospheric Modeling System (RAMS) is presented. The model is shown to realistically reproduce CTD characteristics such as the coastal transition from northerly to southerly flow, as a mesoscale coastal ridge of higher pressure with associated drops in marine-layer temperature propagates northward along the west coast of North America. Simulated sea level pressure and temperature fields near the surface match well with observations, especially at the synoptic scale. The model realistically simulates mesoscale sea level pressure and 6-h pressure changes during the event. The modeled hourly time evolution of sea level pressure and the southerly transitions at a series of coastal stations and buoys also agree reasonably well with observations. The marine boundary layer is not well initialized or very well represented in the model, suggesting that, for this particular case, the details of the boundary layer are not crucial in the evolution of the CTD. It is suggested that the RAMS model can be usefully applied to investigate CTD evolution.

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Timothy D. Finnigan
,
Jason A. Vine
,
Peter L. Jackson
,
Susan E. Allen
,
Gregory A. Lawrence
, and
Douw G. Steyn

Abstract

Strong gap winds in Howe Sound, British Columbia, are simulated using a small-scale physical model. Model results are presented and compared with observations recorded in Howe Sound during a severe gap wind event in December 1992. Hydraulic theory is utilized to explain along-channel variation in wind. Field observations affirm the findings of the physical modeling with both, indicating the presence and location of controls and hydraulic jumps in the wind layer. Hydraulic behavior is found to change as the synoptic pressure gradient and the flow rate increase. In particular, field results indicate two distinct hydraulic situations: one during relatively weak wind, the other, which is more strongly controlled, during the period of peak wind. An additional comparison is made with output from the computer model hydmod of Jackson and Steyn. Numerical simulations, configured for the conditions present in Howe Sound during the December 1992 event, indicate channel hydraulics (and thus spatial wind speed variation) closely resembling the physical model and field results.

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Wendell A. Nuss
,
John ML Bane
,
William T. Thompson
,
Teddy Holt
,
Clive E. Dorman
,
F. Martin Ralph
,
Richard Rotunno
,
Joseph B. Klemp
,
William C. Skamarock
,
Roger M. Samelson
,
Audrey M. Rogerson
,
Chris Reason
, and
Peter Jackson

Coastally trapped wind reversals along the U.S. west coast, which are often accompanied by a northward surge of fog or stratus, are an important warm-season forecast problem due to their impact on coastal maritime activities and airport operations. Previous studies identified several possible dynamic mechanisms that could be responsible for producing these events, yet observational and modeling limitations at the time left these competing interpretations open for debate. In an effort to improve our physical understanding, and ultimately the prediction, of these events, the Office of Naval Research sponsored an Accelerated Research Initiative in Coastal Meteorology during the years 1993–98 to study these and other related coastal meteorological phenomena. This effort included two field programs to study coastally trapped disturbances as well as numerous modeling studies to explore key dynamic mechanisms. This paper describes the various efforts that occurred under this program to provide an advancement in our understanding of these disturbances. While not all issues have been solved, the synoptic and mesoscale aspects of these events are considerably better understood.

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Georg j. Mayr
,
David Plavcan
,
Laurence Armi
,
Andrew Elvidge
,
Branko Grisogono
,
Kristian Horvath
,
Peter Jackson
,
Alfred Neururer
,
Petra Seibert
,
James W. Steenburgh
,
Ivana Stiperski
,
Andrew Sturman
,
Željko Večenaj
,
Johannes Vergeiner
,
Simon Vosper
, and
Günther Zängl

Abstract

Strong winds crossing elevated terrain and descending to its lee occur over mountainous areas worldwide. Winds fulfilling these two criteria are called foehn in this paper although different names exist depending on the region, the sign of the temperature change at onset, and the depth of the overflowing layer. These winds affect the local weather and climate and impact society. Classification is difficult because other wind systems might be superimposed on them or share some characteristics. Additionally, no unanimously agreed-upon name, definition, nor indications for such winds exist. The most trusted classifications have been performed by human experts. A classification experiment for different foehn locations in the Alps and different classifier groups addressed hitherto unanswered questions about the uncertainty of these classifications, their reproducibility, and dependence on the level of expertise. One group consisted of mountain meteorology experts, the other two of master’s degree students who had taken mountain meteorology courses, and a further two of objective algorithms. Sixty periods of 48 h were classified for foehn–no foehn conditions at five Alpine foehn locations. The intra-human-classifier detection varies by about 10 percentage points (interquartile range). Experts and students are nearly indistinguishable. The algorithms are in the range of human classifications. One difficult case appeared twice in order to examine the reproducibility of classified foehn duration, which turned out to be 50% or less. The classification dataset can now serve as a test bed for automatic classification algorithms, which—if successful—eliminate the drawbacks of manual classifications: lack of scalability and reproducibility.

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Wayne Higgins
,
Dave Ahijevych
,
Jorge Amador
,
Ana Barros
,
E. Hugo Berbery
,
Ernesto Caetano
,
Richard Carbone
,
Paul Ciesielski
,
Rob Cifelli
,
Miguel Cortez-Vazquez
,
Art Douglas
,
Michael Douglas
,
Gus Emmanuel
,
Chris Fairall
,
David Gochis
,
David Gutzler
,
Thomas Jackson
,
Richard Johnson
,
Clark King
,
Timothy Lang
,
Myong-In Lee
,
Dennis Lettenmaier
,
Rene Lobato
,
Victor Magaña
,
Jose Meiten
,
Kingtse Mo
,
Stephen Nesbitt
,
Francisco Ocampo-Torres
,
Erik Pytlak
,
Peter Rogers
,
Steven Rutledge
,
Jae Schemm
,
Siegfried Schubert
,
Allen White
,
Christopher Williams
,
Andrew Wood
,
Robert Zamora
, and
Chidong Zhang

The North American Monsoon Experiment (NAME) is an internationally coordinated process study aimed at determining the sources and limits of predictability of warm-season precipitation over North America. The scientific objectives of NAME are to promote a better understanding and more realistic simulation of warm-season convective processes in complex terrain, intraseasonal variability of the monsoon, and the response of the warm-season atmospheric circulation and precipitation patterns to slowly varying, potentially predictable surface boundary conditions.

During the summer of 2004, the NAME community implemented an international (United States, Mexico, Central America), multiagency (NOAA, NASA, NSF, USDA) field experiment called NAME 2004. This article presents early results from the NAME 2004 campaign and describes how the NAME modeling community will leverage the NAME 2004 data to accelerate improvements in warm-season precipitation forecasts for North America.

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