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  • 2020-2022  (6)
  • 2010-2014  (1)
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  • 1
    Publication Date: 2014-01-01
    Description: The predictability of North Pacific cyclones can vary widely, from highly accurate prediction of storm intensity and location to forecast position errors of hundreds of kilometers and central pressure errors of tens of hectopascals. In this study, a Weather Research and Forecasting Model (WRF) ensemble Kalman filter is used to investigate predictability of landfalling cyclones on the west coast of North America over two winter seasons (2008/09 and 2009/10). Predictability is defined as the ensemble spread of cyclone central pressure at the final forecast time (24 h) where large spread means low predictability. Both ensemble spread and ensemble initial-condition sensitivity are examined for a wide variety of cyclones that occurred during the two seasons. Storms that are deepening and track from the southwest exhibit the largest ensemble initial-condition sensitivity and highest ensemble spread compared to decaying storms and storms that track from other directions. Storms that end south of 40°N, typically slow moving storms from the northwest, exhibit higher predictability regardless of whether or not they are deepening or decaying. Cyclones with large ensemble spread and low sensitivity are mature cyclones whose low predictability likely results from large initial-condition spread instead of large perturbation growth. These results highlight particular synoptic situations and cyclone characteristics that are associated with low predictability and can potentially be used to improve forecasts through improved observational coverage.
    Print ISSN: 0027-0644
    Electronic ISSN: 1520-0493
    Topics: Geography , Geosciences , Physics
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  • 2
    Publication Date: 2020-04-07
    Description: We analyze the temporal structure of the climatological seasonal cycle in surface air temperature across the globe. We find that, over large regions of Earth, the seasonal cycle of surface temperature departs from an annual harmonic: the duration of fall and spring differ by as much as 2 months. We characterize this asymmetry by the metric ASYM, defined as the phase lag of the seasonal maximum temperature relative to the summer solstice minus the phase lag of the seasonal minimum temperature relative to winter solstice. We present a global analysis of ASYM from weather station data and atmospheric reanalysis and find that ASYM is well represented in the reanalysis. ASYM generally features positive values over land and negative values over the ocean, indicating that spring has a longer duration over the land domain whereas fall has a longer duration over the ocean. However, ASYM also features more positive values over North America compared to Europe and negative values in the polar regions over ice sheets and sea ice. Understanding the root cause of the climatological ASYM will potentially further our understanding of controls on the seasonal cycle of temperature and its future/past changes. We explore several candidate mechanisms to explain the spatial structure of ASYM including 1) modification of the seasonal cycle of surface solar radiation by the seasonal evolution of cloud thickness, 2) differences in the seasonal cycle of the atmospheric boundary layer depth over ocean and over land, and 3) temperature advection by the seasonally evolving atmospheric circulation.
    Print ISSN: 0894-8755
    Electronic ISSN: 1520-0442
    Topics: Geography , Geosciences , Physics
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  • 3
    Publication Date: 2020-12-04
    Description: High-resolution numerical model simulations of six different cases during the 2015-16 Olympic Mountains Experiment (OLYMPEX) are used to examine dynamic and microphysical precipitation processes on both the full barrier-scale and smaller sub-barrier scale ridges and valleys. The degree to which stratiform precipitation within mid-latitude cyclones is modified over the coastal Olympic Mountains range was found to be strongly dependent on the synoptic environment within a cyclone’s prefrontal and warm sectors. In prefrontal sectors, barrier-scale ascent over stably stratified flow resulted in enhanced ice production aloft at the coast and generally upstream of higher terrain. At low levels, stable flow orientated transverse to sub-barrier scale windward ridges generated small-scale mountain waves, which failed to produce enough cloud water to appreciably enhance precipitation on the scale of the windward ridges. In moist-neutral warm sectors, the upstream side of the barrier exhibited broad ascent oriented along the windward ridges with lesser regions of adjacent downward motion. Significant quantities of cloud water were produced over coastal foothills with further production of cloud water on the lower-windward slopes. Ice production above the melting layer occurred directly over the barrier where the ice particles were further advected downstream by cross-barrier winds and spilled over into the lee. The coastal foothills were found to be essential for the production and maintenance of cloud water upstream of the primary topographic barrier, allowing additional time for hydrometeors to grow to precipitation size by autoconversion and collection before falling out on the lower-windward slopes.
    Print ISSN: 0027-0644
    Electronic ISSN: 1520-0493
    Topics: Geography , Geosciences , Physics
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  • 4
    Publication Date: 2020-12-01
    Print ISSN: 0882-8156
    Electronic ISSN: 1520-0434
    Topics: Geography , Physics
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  • 5
    Publication Date: 2021-08-01
    Description: This article provides an overview of the experimental design, execution, education and public outreach, data collection, and initial scientific results from the Remote Sensing of Electrification, Lightning, and Mesoscale/Microscale Processes with Adaptive Ground Observations (RELAMPAGO) field campaign. RELAMPAGO was a major field campaign conducted in the Córdoba and Mendoza provinces in Argentina and western Rio Grande do Sul State in Brazil in 2018–19 that involved more than 200 scientists and students from the United States, Argentina, and Brazil. This campaign was motivated by the physical processes and societal impacts of deep convection that frequently initiates in this region, often along the complex terrain of the Sierras de Córdoba and Andes, and often grows rapidly upscale into dangerous storms that impact society. Observed storms during the experiment produced copious hail, intense flash flooding, extreme lightning flash rates, and other unusual lightning phenomena, but few tornadoes. The five distinct scientific foci of RELAMPAGO—convection initiation, severe weather, upscale growth, hydrometeorology, and lightning and electrification—are described, as are the deployment strategies to observe physical processes relevant to these foci. The campaign’s international cooperation, forecasting efforts, and mission planning strategies enabled a successful data collection effort. In addition, the legacy of RELAMPAGO in South America, including extensive multinational education, public outreach, and social media data gathering associated with the campaign, is summarized.
    Print ISSN: 0003-0007
    Electronic ISSN: 1520-0477
    Topics: Geography , Physics
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  • 6
    Publication Date: 2021-08-01
    Description: The Cloud, Aerosol, and Complex Terrain Interactions (CACTI) field campaign was designed to improve understanding of orographic cloud life cycles in relation to surrounding atmospheric thermodynamic, flow, and aerosol conditions. The deployment to the Sierras de Córdoba range in north-central Argentina was chosen because of very frequent cumulus congestus, deep convection initiation, and mesoscale convective organization uniquely observable from a fixed site. The C-band Scanning Atmospheric Radiation Measurement (ARM) Precipitation Radar was deployed for the first time with over 50 ARM Mobile Facility atmospheric state, surface, aerosol, radiation, cloud, and precipitation instruments between October 2018 and April 2019. An intensive observing period (IOP) coincident with the RELAMPAGO field campaign was held between 1 November and 15 December during which 22 flights were performed by the ARM Gulfstream-1 aircraft. A multitude of atmospheric processes and cloud conditions were observed over the 7-month campaign, including numerous orographic cumulus and stratocumulus events; new particle formation and growth producing high aerosol concentrations; drizzle formation in fog and shallow liquid clouds; very low aerosol conditions following wet deposition in heavy rainfall; initiation of ice in congestus clouds across a range of temperatures; extreme deep convection reaching 21-km altitudes; and organization of intense, hail-containing supercells and mesoscale convective systems. These comprehensive datasets include many of the first ever collected in this region and provide new opportunities to study orographic cloud evolution and interactions with meteorological conditions, aerosols, surface conditions, and radiation in mountainous terrain.
    Print ISSN: 0003-0007
    Electronic ISSN: 1520-0477
    Topics: Geography , Physics
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  • 7
    Publication Date: 2021-09-22
    Description: Over mountainous terrain, windward enhancement of stratiform precipitation results from a combination of warm-rain and ice-phase processes. In this study, ice-phase precipitation processes are investigated within frontal systems during the Olympic Mountains Experiment (OLYMPEX). An enhanced layer of radar reflectivity (ZH) above the melting level bright band (i.e., a secondary ZH maximum) is observed over both the windward slopes of the Olympic Mountains and the upstream ocean, with a higher frequency of occurrence and higher ZH values over the windward slopes indicating an orographic enhancement of ice-phase precipitation processes. Aircraft-based in situ observations are evaluated for the 01-02 and 03 December 2015 orographically-enhanced precipitation events. Above the secondary ZH maximum, the hydrometeors are primarily horizontally oriented dendritic and branched crystals. Within the secondary ZH maximum, there are high concentrations of large (〉 ~2 mm diameter) dendrites, plates, and aggregates thereof, with a significant degree of riming. In both events, aggregation and riming appear to be enhanced within a turbulent layer near sheared flow at the top of a low-level jet impinging on the terrain and forced to rise above the melting level. Based on windward ground sites at low-, mid-, and high-elevations, secondary ZH maxima periods during all of OLYMPEX are associated with increased rain rates and larger mass-weighted mean drop diameters compared to periods without a secondary ZH maximum. This result suggests that precipitation originating from secondary ZH maxima layers may contribute to enhanced windward precipitation accumulations through the formation of large, dense particles that accelerate fallout.
    Print ISSN: 0022-4928
    Electronic ISSN: 1520-0469
    Topics: Geography , Geosciences , Physics
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