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Randolph D. Borys and Melanie A. Wetzel

The Storm Peak Laboratory (SPL), operated by the Atmospheric Sciences Center of the Desert Research Institute, is now located in a newly constructed permanent building at elevation 3210 m (10 530 ft) above mean sea level in the northwestern Colorado Rocky Mountains. The laboratory provides a site for the conduct of basic and applied research in the atmospheric sciences, hands-on instruction in meteorology for students ranging from middle school through graduate school, and high-elevation atmospheric measurement programs for various scientific groups, agencies, and private companies. This article provides a background of the history of SPL, its past and current activities, and a description of the facilities and opportunities available at the laboratory.

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Melanie A. Wetzel and Gary T. Bates

Abstract

Satellite image datasets and regional climate model results are intercompared for evaluation of model accuracy in the simulation of cloud cover. Both monthly average and individual simulation times are analyzed. To provide a consistent comparison, satellite data are first mapped into the model's geographic projection, grid domain, and resolution. It is found that September 1988 monthly average cloud fraction results from the model simulations correspond to observations, in both spatial pattern and magnitude, with bias less than ±20% cloud fraction over the entire inland West. Agreement in the pattern of cloud fraction also is evident for monthly average cloud fraction in July, but there is a negative bias of 10%–30% cloud fraction in the model diagnosis of cloud cover. Correlations between the spatial distributions of model-derived and observed cloud fractions are found to exceed 0.80 for certain geographic regions of the West, and these correlations are largest over mountainous areas during summer. Case studies of a series of daily cloud cover demonstrate the ability of the model to simulate the effects of frontal passage on cloud distribution. The ability of the RegCM1 to simulate daily cloud fraction and diurnal cloud evolution is somewhat weak for the summer convective season. It is anticipated that a more recent version of the regional climate model may improve the simulation of summer season cloud cover, through changes in cloud parameterization and improvements in model resolution.

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John Hallett, Melanie Wetzel, and Steven Rutledge

The NSF Division of Mesoscale Meteorology and the University of Nevada—Reno (UNR) provided support for a two-week field course at the CSU–CHILL radar during 12–24 May 1991. Ten atmospheric science graduate students and two faculty from the Desert Research Institute (University of Nevada) spent the two weeks at the Greeley, Colorado, site of the CSU–CHILL 11-cm dual-polarization Doppler radar. This article describes the purpose, design, facilities, and success of the radar meteorology field course.

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Melanie Wetzel, Randolph Borys, Douglas Lowenthal, and Stephen Brown

The collocation of the Weather Forecast Office and the Desert Research Institute in Reno, Nevada, has fostered a National Weather Service (NWS)–University collaborative effort that provided meteorological forecasting and research support to an experimental aeronautics endeavor. The Earthwinds transglobal ballooning project is an attempt to complete the first nonstop global circumnavigation flight by a lighter-than-air craft, with launch from Stead airfield in Reno. This article describes the cooperative efforts of the Earthwinds operations staff, the NWS office, the Desert Research Institute, and the atmospheric technology industry. Activities included the use of global and regional forecasts, evaluation of local wind and temperature diurnal cycles, numerical simulation of balloon launch trajectories, and onboard atmospheric instrumentation.

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Ramesh Vellore, Darko Koračin, Melanie Wetzel, Steven Chai, and Qing Wang

Abstract

A numerical study using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) was performed to assess the impact of initial and boundary conditions, the parameterization of turbulence transfer and its coupling with cloud-driven radiation, and cloud microphysical processes on the accuracy of mesoscale predictions and forecasts of the cloud-capped marine boundary layer. Aircraft, buoy, and satellite data and the large eddy simulation (LES) results during the Dynamics and Chemistry of Marine Stratocumulus field experiment (DYCOMS II) in July 2001 were used in the assessment. Three of the tested input fields (Eta, NCEP, and ECMWF) show deficiencies, mainly in the thermodynamic structure of the lowest 1500 m of the marine atmosphere. On a positive note, the simulated marine-layer depth showed good agreement with aircraft observations using the Eta fields, while using the NCEP and ECMWF datasets underestimated the marine-layer depth by about 20%–30%. The predicted turbulence kinetic energy (inversion strength) was about 50% of that obtained from the LES results (aircraft observed). As a consequence of moisture overprediction, the predicted liquid water path was twice the observed by 1–2 g kg−1. The sensitivity tests have shown that the selections of turbulence and cloud microphysical schemes significantly influence the turbulence estimates and cloud parameters. Two of the tested turbulence schemes (Eta PBL and Burk–Thompson) did not exhibit the coupling with radiation. The significant differences in the simulated turbulence estimates appear to be a consequence of the use of water-conserving potential temperature variables. The microphysical parameterization, which uses the number concentration of cloud drops in the autoconversion process, simulates a realistic evolution of precipitable hydrometeors in the cloudy marine layer on the positive side, but on the other hand enhances the decoupling in the turbulence structure. This study can provide guidance to operational forecasters concerning accuracy issues of the commonly used large-scale analyses for model initialization, and optimal selection of model parameterizations in order to simulate and forecast the cloudy atmospheric boundary layer over the ocean.

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Melanie A. Wetzel, Randolph D. Borys, and Ling E. Xu

Abstract

Digital data from the National Oceanic and Atmospheric Administration Advanced very High Resolution Radiometer (AVHRR) satellite instrument provides multispectral images in visible near-infrared and thermal infrared wave bands, which have been utilized to develop retrieval techniques for estimating the droplet effective radius and optical depth of land-based fog. The retrieval methods are based on multiple scattering calculations that simulate the increased near-infrared absorption by fog layers with increasing droplet size and liquid water path. The AVHRR thermal window channels are utilized to remove the effects of thermal emission in the near-infrared band.

New instrumentation and field sampling methods have been developed for obtaining detailed vertical profiles of fog droplet size distributions and thermodynamic conditions in fog decks. The in situ measurements derived from the field observations were employed to test the satellite retrieval techniques. Intercomparison shows a close correspondence between field observations and retrieved values of the fog droplet effective radius as well as fog optical depth. Simulated 4-km near-infrared and visible pixel data are also used to test retrievals from GOES-8. The AVHRR and GOES-8 retrievals provide a mapped database of the fog microphysical and depth parameters over the entire region of fog, which may be applied to numerical simulation of fog evolution and pollutant deposition, newcasting of fog visibility hazards, and global monitoring of fog influences on the atmosphere-surface radiation budget.

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Stephen M. Saleeby, William R. Cotton, Douglas Lowenthal, Randolph D. Borys, and Melanie A. Wetzel

Abstract

Pollution aerosols acting as cloud condensation nuclei (CCN) have the potential to alter warm rain clouds via the aerosol first and second indirect effects in which they modify the cloud droplet population, cloud lifetime and size, rainfall efficiency, and radiation balance from increased albedo. For constant liquid water content, an increase in CCN concentration (N CCN) tends to produce an increased concentration of droplets with smaller diameters. This reduces the collision and coalescence rate, and thus there is a local reduction in rainfall. While this process applies to warm clouds, it does not identically carry over to mixed-phase clouds in which crystal nucleation, crystal riming, crystal versus droplet fall speed, and collection efficiency play active roles in determining precipitation amount. Sulfate-based aerosols serve as very efficient cloud nuclei but are not effective as ice-forming nuclei. In clouds where precipitation formation is dominated by the ice phase, N CCN influences precipitation growth by altering the efficiency of droplet collection by ice crystals and the fall trajectories of both droplet and crystal hydrometeors. The temporal and spatial variation in both crystal and droplet populations determines the resultant snowfall efficiency and distribution. Results of numerical simulations in this study suggest that CCN can play a significant role in snowfall production by winter, mixed-phase, cloud systems when liquid and ice hydrometeors coexist. In subfreezing conditions, a precipitating ice cloud overlaying a supercooled liquid water cloud allows growth of precipitation particles via the seeder–feeder process, in which nucleated ice crystals fall through the supercooled liquid water cloud and collect droplets. Enhanced N CCN from sulfate pollution by fossil fuel emissions modifies the droplet distribution and reduces crystal riming efficiency. Reduced riming efficiency inhibits the rate of snow growth, producing lightly rimed snow crystals that fall slowly and advect farther downstream prior to surface deposition. Simulations indicate that increasing N CCN along the orographic barrier of the Park Range in north-central Colorado results in a modification of the orographic cloud such that the surface snow water equivalent amounts are reduced on the windward slopes and enhanced on the leeward slopes. The inhibition of snowfall by pollution aerosols (ISPA) effect has significant implications for water resource distribution in mountainous terrain.

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Melanie A. Wetzel, Steven K. Chai, Marcin J. Szumowski, William T. Thompson, Tracy Haack, Gabor Vali, and Robert Kelly

Abstract

A field project was carried out offshore of central Oregon during August 1999 to evaluate mesoscale model simulations of coastal stratiform cloud layers. Procedures for mapping cloud physical parameters such as cloud optical depth, droplet effective radius, and liquid water path retrieved from Geostationary Operational Environmental Satellite (GOES) Imager multichannel data were developed and implemented. Aircraft measurements by the University of Wyoming provided in situ verification for the satellite retrieval parameters and for the forecast model simulations of the U.S. Navy's nonhydrostatic mesoscale prediction system, the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS). Case studies show that the satellite retrieval methods are valid within the range of uncertainty associated with aircraft measurements of the microphysical parameters and demonstrate how the gridded cloud parameters retrieved from satellite data can be utilized for mesoscale model verification. Satellite-derived products with applications to forecasting, such as temporal trends and composites of droplet size and liquid water path, are also discussed.

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Melanie Wetzel, Michael Meyers, Randolph Borys, Ray McAnelly, William Cotton, Andrew Rossi, Paul Frisbie, David Nadler, Douglas Lowenthal, Stephen Cohn, and William Brown

Abstract

Short-term forecasting of precipitation often relies on meteorological radar coverage to provide information on the intensity, extent, and motion of approaching mesoscale features. However, in significant portions of mountainous regions, radar coverage is lacking because of topographic blocking, and the absence of radar signatures in sections of the radar scan produces uncertain or even misleading information to the public and operational forecasters. In addition, echo characteristics within the radar volume scan are often influenced by the vertical extent and type of precipitation. Each of these conditions limits the opportunity for accurate snowfall prediction and studies of precipitation climatology. To improve both short-term forecasting and postevent verification studies, much greater use can be made of specifically sited surface observations, tailored graphical output from mesoscale models, satellite remote sensing, and case study knowledge of local topographic influences. In this paper, methods to support snowfall forecasts and verification in radar-limited mountainous terrain are demonstrated that include matching the output parameters and graphics from high-resolution mesoscale models to surface mesonets and snowfall observations, analysis of continuous and event-based measurements of snow density, application of multispectral satellite data for verification and trend analysis, and characterization of orographic influences in different winter storm scenarios. The advantages of improved wintertime quantitative precipitation forecasting (QPF) in mountain regions include public safety responsibilities that are critical to National Weather Service (NWS) operations, and are relevant to any mountainous region with radar scan limitations or during periods of radar data outages.

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Melanie Wetzel, David Dempsey, Sandra Nilsson, Mohan Ramamurthy, Steve Koch, Jennie Moody, David Knight, Charles Murphy, David Fulker, Mary Marlino, Michael Morgan, Doug Yarger, Dan Vietor, and Greg Cox

An education-oriented workshop for college faculty in the atmospheric and related sciences was held in Boulder, Colorado, during June 1997 by three programs of the University Corporation for Atmospheric Research. The objective of this workshop was to provide faculty with hands-on training in the use of Web-based instructional methods for specific application to the teaching of satellite remote sensing in their subject areas. More than 150 faculty and associated scientists participated, and postworkshop evaluation showed it to have been a very successful integration of information and activities related to computer-based instruction, educational principles, and scientific lectures.

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