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D. R. DAVIS and C. E. DEAN

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David R. Walker and Robert E. Davis

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A climatology of the once-daily (0000 UTC) 1000-hPa error fields of the National Meteorological Center's 80-wave Medium-Range Forecast (MRF) model is studied. An analysis of the error field has been conducted over the contiguous United States and over the Northern Hemisphere from 20° to 80°N for three warm and four cool seasons (9 September 1987 to 6 March 1991). Temporal and spatial mean error fields over various integration lengths are presented.

The skill, as measured by the anomaly correlation, has not significantly changed over the lifetime of the 80-wave MRF model. Anomaly correlation values at 1000 hPa and 500 hPa show that the model is retaining useful information about the anomalies in the height field out to about one week. A reduction in the model biases may reflect an improvement in model physics (longwave radiational calculations, etc). The cool and warm seasons have distinctly different spatial error patterns. The 1000-hPa warm season shows spurious height falls over the southwestern United States that grow with increasing integration length. The 1000-hPa cool season underestimates the intensity of low pressure systems over and east of Hudson Bay and overestimates their strength over the Pacific Northwest.

Principal components analysis of the 429-variable error covariance matrices for the cool and warm seasons identifies 6 orthogonal variables that explain over 60% of the original error variance. MRF model problems appear to be related to problems the model has with simulating the atmosphere's interaction with orographic features (Alberta and Colorado Rockies), storm tracks and baroclinic zones (Gulf Stream region and United States-Canadian border), and persistent atmospheric features (Hudson Bay low, eastern Pacific subtropical high, and desert Southwest heat low).

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Robert E. Davis and David R. Walker

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An automated, year-round synoptic climatology is developed for the western United States from rawinsonde observations from 1979 to 1988. The classification uses thermal, moisture, and flow parameters to characterize seasonal and interannual synoptic-scale variations in hydrodynamic and thermodynamic conditions. Based on twice daily observations for a network of 21 stations from the Pacific coast to the Rockies, a synoptic climatology is developed using air temperature, dewpoint temperature, geopotential height, and the east-west and north-south components of the wind vector at 800-, 700-, 500-, and 250-mb constant pressure surfaces. The 798 variable by 3620 day matrix is reduced to six orthogonal principal components, and the resulting component scores are grouped using a two-stage clustering technique. The 13 synoptic situations represent days experiencing homogeneous weather conditions. These synoptic situations exhibit marked seasonality and interannual variability and depict features observed in the general circulation of the atmosphere. Examples include both summer monsoonal and dry situations, zonal flow situations with a strong polar or subtropical jet, or both and strong ridging or troughing, and meridional flow.

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W. E. Davis, J. M. Thorp, and R. N. Lee

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Precipitation and air concentration data collected during a U.S. Department of Energy–sponsored Frontal Boundary Study (FBS) were used to calculate scavenging ratios. The precipitation data were collected on a 100 km × 100 km surface grid containing 36 sampling sites, while the air concentration data were collected by aircraft. Radar and rawinsonde data were used to position the aircraft into the air mass feeding the precipitation. The coordination of the aircraft data with surface precipitation data allows the calculation of scavenging ratios. During the study, three out of seven events sampled allowed the calculation of scavenging ratios for SO=4, total sulfate, NO3, and total nitrate. The 36 precipitation samplers allowed calculation of multiple scavenging ratios to represent an event. From these scavenging ratios, both a geometric mean and a geometric standard deviation of the scavenging ratio were calculated for each event. The geometric mean (geometric standard deviation) scavenging ratio for total sulfate is 6 × 104 (1.1) for 10 October 1989, 10 × 104 (1.5) for 16–17 October 1989, and 4 × 104 (1.3) for 31 October 1989. For SO=4 the scavenging ratios are 10 × 105 (1.1) for 10 October 1989, 4 × 105 (1.5) for 16–17 October 1989, and 3 × 105 (1.3) for 31 October 1989. The scavenging ratio for NO3 is 3 × 106 (1.2) for 10 October 1989, 20 × 106 (1.4) for 16–17 October 1989, and 0.4 × 106 (1.5) for 31 October 1989. The scavenging ratio for total nitrate is 2 × 106 (1.2) for 10 October 1989, 2 × 106 (1.4) for 16–17 October 1989, and 0.2 × 106 (1.5) for 31 October 1989. The most important finding is the small variation of the numbers within the events as reflected in the geometric standard deviations. These values ranged from 1.1 to 1.5. Based on these results, a single scavenging ratio can be used on a 100 km × 100 km area with a minimum of error.

Two other results were found when comparing these scavenging ratios to total precipitation in the sampler. The comparison revealed that the variation in the scavenging ratio increased with decreasing total precipitation. The increase was up to a factor of 2 for less than 5 mm when compared with greater than 5 mm of total precipitation.

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R. E. Davis, J. T. Sherman, and J. Dufour

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Over the past decade more than 1200 autonomous floats have been deployed worldwide. In addition to velocity as marked by lateral movement, many of these floats measured quantities like profiles of temperature and salinity, temperature microstructure, and time series of vertical velocity. The authors' laboratory's implementation of profile measurements in what is called a Profiling Autonomous Lagrangian Circulation Explorer is described. Biofouling and degradation of antifouling coatings on the conductivity sensor both cause drifts that mean accurate salinity measurements will depend on corrections based on known temperature–salinity relations. A second generation autonomous float called the Sounding Oceanographic Lagrangian Observer (SOLO) has been developed to provide enhanced reliability and to provide complete two-way depth control. A dual hydraulic-pneumatic buoyancy system reduces the energy cost of vertical cycling and buoyancy generation at the surface. A SOLO Vertical Current Meter has been found capable of measuring vertical velocity with errors of O(3 m day−1) on month timescales.

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GORDON E. DUNN, WALTER R. DAVIS, and PAUL L. MOORE

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James Montesi, Kelly Elder, R. A. Schmidt, and Robert E. Davis

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To determine how elevation affects the sublimation rate from intercepted snow within a subalpine forest canopy, a cut subalpine fir and an artificial conifer were weighed at each of two elevations (3230 and 2920 m) at a U.S. continental site (39°53′N, 105°54′W) from 1 January to 1 May 2001. Measured stand characteristics included canopy density (67% and 75%) and basal area (43.4 and 24.1 m2 ha−1) for the higher and lower elevations, respectively. Temperature, relative humidity, net radiation, wind speed, and mass of snow on suspended trees provided data to determine whether sublimation rates of intercepted snow are more rapid at higher elevations associated with increased wind speed. Measurements showed the unexpected result that wind speed during sublimation periods was lower at higher elevations, probably because of terrain sheltering. The analysis examined 21 storm-free periods ranging in duration from 9 to 53 h. Sublimation rates per unit mass of intercepted snow were significantly larger at the lower-elevation site associated with warmer temperatures, lower relative humidity, and greater wind speeds. Application of meteorological data to an ice sphere model indicated that predicted mean sublimation rates of an ice sphere index were 23% ± 7% more rapid at the lower elevation due to weather factors alone. However, greater snowfall at higher elevations produced greater interception, resulting in substantially more snow being sublimated back to the atmosphere at the upper site. Over the study period, sublimation of snow intercepted by the test trees amounted to 20%–30% of total snowfall accumulated at the sites during the 21 storms selected for analysis.

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GORDON E. DUNN, WALTER R. DAVIS, and PAUL L. MOORE

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D. A. Ahijevych, C. A. Davis, R. E. Carbone, and J. D. Tuttle

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The western and central United States experience a pronounced diurnal cycle in rainfall during the warm season. Over the higher terrain west of 105°W, most precipitation occurs in the afternoon, whereas the central United States experiences more nocturnal events. This coherent phase transition between the Rocky Mountains and the U.S. Great Plains is well defined for all warm seasons between 1996 and 2003, provided that the rainfall observations are remapped relative to the elevated terrain in the western United States prior to north–south averaging. Due to the westward shift of the Continental Divide north of 42°N and its intersection with the warm season storm track for 2002, the diurnal coherence greatly improves after remapping the 2002 rainfall observations. This speaks to the long-range influence of orography on precipitation frequency and suggests that the primary east–west corridor of precipitation for an individual warm season intersects the cordillera over a fairly narrow latitude range.

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R. E. Davis, L. A. Regier, J. Dufour, and D. C. Webb

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The autonomous Lagrangian circulation explorer (ALACE) is a subsurface float that cycles vertically from a depth where it is neutrally buoyant to the surface where it is located by, and relays data to, System Argos satellites. ALACEs are intended to permit exploration of large-scale low-frequency currents and to provide repeated vertical profiles of mean variables. ALACEs periodically change their buoyancy by pumping hydraulic fluid from an internal reservoir to an external bladder, thereby increasing float volume and buoyancy. Because positioning and data relay are accomplished by satellite, ALACEs are autonomous of acoustic tracking networks and are suitable for global deployment in arrays of any size. While providing only a sequence of displacements between surfacing intervals, ALACEs are efficient in gathering the widely spaced long-term observations needed to map large-scale average flow.

The primary technical challenges met in the ALACE design are maximizing energy efficiency to achieve a lifetime of 50 surfacing cycles over several year., achieving reliable satellite communication with minimal surface buoyancy, and developing overall system reliability in an instrument that cannot be recovered or diagnosed after most failures. This paper describes the ALACE system, design specifications, and some field experiences. The singular failure of a simple dynamical model to predict the surface following behavior of scale models in laboratory tests serves as a cautionary note in using simple models to infer the dynamics of surface floats in various oceanographic applications. The limitations of interpreting the sequence of net displacements between surface positions, including errors caused by surface drift, are also discussed.

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