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Brian E. Potter

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Brian E. Potter and James R. Holton

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Observations by radiosondes, satellites, and aircraft have shown that a minimum in water vapor mixing ratio in the lower tropical stratosphere (typically around 19-km altitude) is a climatological feature of the global water vapor distribution. The processes responsible for the formation and maintenance of this minimum are examined with the aid of a mesoscale dynamical model of tropical convection that includes bulk ice microphysics, radiative transfer, and surface processes.

Model results suggest that convectively generated buoyancy waves may play an important role in dehydrating the tropical lower stratosphere. Vertical parcel displacements produced by buoyancy waves promote the formation of thin ice clouds in the lower stratosphere. Such clouds formed upwind of the convective region in all simulations where ice was allowed to form. When ice was allowed to precipitate, there was a decrease of ≈0.3 ppmm in total water mixing ratio at 19 km after a 30-h simulation.

It is concluded that thin cirrus clouds produced by buoyancy waves may contribute significantly to the formation and maintenance of the observed water vapor minimum in the lower tropical stratosphere.

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Matthew C. Brewer, Clifford F. Mass, and Brian E. Potter

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Despite the significant impacts of the West Coast thermal trough (WCTT) on West Coast weather and climate, questions remain regarding its mesoscale structure, origin, and dynamics. Of particular interest is the relative importance of terrain forcing, advection, and surface heating on WCTT formation and evolution. To explore such questions, the 13–16 May 2007 WCTT event was examined using observations and simulations from the Weather Research and Forecasting (WRF) Model. An analysis of the thermodynamic energy equation for these simulations was completed, as well as sensitivity experiments in which terrain or surface fluxes were removed or modified. For the May 2007 event, vertical advection of potential temperature is the primary driver of local warming and WCTT formation west of the Cascades. The downslope flow that drives this warming is forced by easterly flow associated with high pressure over British Columbia, Canada. When the terrain is removed from the model, the WCTT does not form and high pressure builds over the northwest United States. When the WCTT forms on the east side of the Cascades, diabatic heating dominates over the other terms in the thermodynamic energy equation, with warm advection playing a small role. If surface heat fluxes are neglected, an area of low pressure remains east of the Cascades, though it is substantially attenuated.

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Caroline M. Kiefer, Craig B. Clements, and Brian E. Potter

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Direct measurements of wildland fire plume properties are rare because of difficult access to regions near the fire front and plume. Moisture released from combustion, in addition to added heat, can enhance buoyancy and convection, influencing fire behavior. In this study, a mini unmanned aircraft system (miniUAS) was used to obtain in situ measurements of temperature and relative humidity during a prescribed fire. The miniUAS was successfully maneuvered through the plume and its associated turbulence and provided observations of temperature and humidity profiles from near the centerline of the plume. Within the plume, the water vapor mixing ratio increased by 0.5–3.5 g kg−1 above ambient and was caused by the combustion of fuels. Potential temperature perturbations were on the order of 2–5 K. These results indicate that significant moisture and temperature enhancement can occur and may potentially modify convection dynamics of fire plumes.

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Matthew C. Brewer, Clifford F. Mass, and Brian E. Potter

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Although the West Coast thermal trough (WCTT) is the most important mesoscale feature over the U.S. west coast during the warm season, its initiation, evolution, and structure are not well understood. Originating in the southwest United States, this inverted trough can extend northward into Oregon, Washington, and British Columbia, with large impacts on temperature, wind, humidity, and air quality.

Using NCEP’s North American Regional Reanalysis (NARR), annual and diurnal climatologies of WCTTs reaching the northwest United States were constructed. For the entire year, WCTTs are most frequent along the coast near the California–Oregon border, with weaker maxima west of the Cascade and coastal mountains. Over the coastal region, they occur most often during autumn, while east of the Cascade Mountains, the highest frequency is during summer. There is strong diurnal variability in WCTT frequency during the summer, with little diurnal variation in winter.

Though compositing revealed important seasonal differences in WCTT evolution, some common features emerged. An upper-level ridge moves over the northwest United States and associated high pressure builds in the lower troposphere over southwest Canada, resulting in the development of near-surface easterlies and downslope flow over the western slopes of major terrain barriers of the region. Simultaneously, the WCTT extends northward from California into the Pacific Northwest. As the synoptic configuration changes, the WCTT either moves eastward and merges with the larger thermal low over the Great Basin region, which is most common in summer, or it recedes back into California and dissipates, as often happens in winter.

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

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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.

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Craig B. Clements, Shiyuan Zhong, Scott Goodrick, Ju Li, Brian E. Potter, Xindi Bian, Warren E. Heilman, Joseph J. Charney, Ryan Perna, Meongdo Jang, Daegyun Lee, Monica Patel, Susan Street, and Glenn Aumann

Grass fires, although not as intense as forest fires, present a major threat to life and property during periods of drought in the Great Plains of the United States. Recently, major wildland grass fires in Texas burned nearly 1.6 million acres and destroyed over 730 homes and 1320 other buildings. The fires resulted in the death of 19 people, an estimated loss of 10,000 head of livestock, and more than $628 million in damage, making the 2005/06 fire season the worst on record for the state of Texas.

As an aid to fire management, various models have been developed to describe fire behavior. However, these models strongly emphasize fuels and fail to adequately consider the role of convective dynamics within the atmosphere and its interaction with the fire due to the lack of observational data. To fill this gap, an intensive field measurement campaign called FireFlux was conducted during February 2006 near Houston, Texas. The campaign employed a variety of instrument platforms to collect turbulence data at multiple levels within and immediately downwind of a 155 acre tall-grass prairie burn unit. This paper presents some first-time observations of atmospheric turbulent structures/fluxes associated with intense grass fires and provides a basis to further our understanding of the dynamics of grass fires and their interactions with the atmosphere.

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