P. Ola G. Persson

The Fronts and Atlantic Storm-Track Experiment (FASTEX): Scientific Objectives and Experimental Design

The Fronts and Atlantic Storm-Track Experiment (FASTEX) will address the life cycle of cyclones evolving over the North Atlantic Ocean in January and February 1997. The objectives of FASTEX are to improve the forecasts of end-of-storm-track cyclogenesis (primarily in the eastern Atlantic but with applicability to the Pacific) in the range 24 to 72 h, to enable the testing of theoretical ideas on cyclone formation and development, and to document the vertical and the mesoscale structure of cloud systems in mature cyclones and their relation to the dynamics. The observing system includes ships that will remain in the vicinity of the main baroclinic zone in the central Atlantic Ocean, jet aircraft that will fly and drop sondes off the east coast of North America or over the central Atlantic Ocean, turboprop aircraft that will survey mature cyclones off Ireland with dropsondes, and airborne Doppler radars, including ASTRAIA/ELDORA. Radiosounding frequency around the North Atlantic basin will be increased, as ...

The Mesoscale and Microscale Structure and organization of Clouds and precipitation in Midlatitude Cyclones. Part V: The Substructure of Narrow Cold-Frontal Rainbands

Abstract The organization and structure of a narrow cold-frontal rainband (NCFR) on the small mesoscale and the microscale have been investigated through quantitative radar reflectivity, Doppler radar observations, airborne observations and surface measurements. The NCFR was composed of small mesoscale regions of heavy precipitation called “precipitation cores” (PCs) oriented at an angle to the synoptic-scale cold front; in horizontal cross section the PCs were roughly elliptical in shape. Areas of lighter precipitation called “gap regions” (GRs) separated the PCs. The PCs were so oriented that their loading edges were regions of strong low-level convergence. The weather associated with the passage of a PC resembled that of a squall-line gust front, with concurrent windshifts and pressure checks occurring ∼5 min before heavy precipitation and a fall in temperature. The changes in surface weather that accompanied the passage of a GR were more variable but tended to be less marked than for PCs. Thus, the se...

Vertical Motions in Arctic Mixed-Phase Stratiform Clouds

Abstract The characteristics of Arctic mixed-phase stratiform clouds and their relation to vertical air motions are examined using ground-based observations during the Mixed-Phase Arctic Cloud Experiment (MPACE) in Barrow, Alaska, during fall 2004. The cloud macrophysical, microphysical, and dynamical properties are derived from a suite of active and passive remote sensors. Low-level, single-layer, mixed-phase stratiform clouds are typically topped by a 400–700-m-deep liquid water layer from which ice crystals precipitate. These clouds are strongly dominated (85% by mass) by liquid water. On average, an in-cloud updraft of 0.4 m s−1 sustains the clouds, although cloud-scale circulations lead to a variability of up to ±2 m s−1 from the average. Dominant scales-of-variability in both vertical air motions and cloud microphysical properties retrieved by this analysis occur at 0.5–10-km wavelengths. In updrafts, both cloud liquid and ice mass grow, although the net liquid mass growth is usually largest. Betwee...

Coastal Orographic Rainfall Processes Observed by Radar during the California Land-Falling Jets Experiment

Abstract Radar and rain gauge observations collected in coastal mountains during the California Land-Falling Jets Experiment (CALJET) are used to diagnose the bulk physical properties of rainfall during a wet winter season (January–March 1998). Three rainfall types were clearly distinguishable by differences in their vertical profiles of radar reflectivity and Doppler vertical velocity: nonbright band, bright band, and hybrid (seeder–feeder). The contribution of each rainfall type to the total rainfall observed at the radar site (1841 mm) was determined by a new, objective algorithm. While hybrid rain occurred most often, nonbrightband rain (NBB rain) contributed significantly (28%) to the total. This paper focuses on characterizing NBB rain because of the need to document this key physical process and because of its impact on Weather Surveillance Radar-1988 Doppler (WSR-88D) precipitation surveillance capabilities. NBB rain is a quasi-steady, shallow rain process that does not exhibit a radar bright band...

A bulk turbulent air-sea flux algorithm for high-wind, spray conditions

Abstract Sensible and latent heat can cross the air–sea interface by two routes: as interfacial fluxes controlled by molecular processes right at the interface, and as spray fluxes from the surface of sea spray droplets. Once the 10-m wind speed over the ocean reaches approximately 11–13 m s−1, the spray sensible and latent heat fluxes become significant fractions (i.e., 10% or greater) of the corresponding interfacial fluxes. The analysis here establishes that result by combining the Tropical Ocean-Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (COARE) version 2.6 bulk interfacial flux algorithm with a microphysical spray model to partition measured heat fluxes from two good high-wind datasets into spray and interfacial flux contributions. The measurements come from the Humidity Exchange over the Sea (HEXOS) experiment and the Fronts and Atlantic Storm-Tracks Experiment (FASTEX); wind speeds in these two datasets span 5 to 20 m s−1. After the measured heat fluxes are separated into spray ...

Modification of Fronts and Precipitation by Coastal Blocking during an Intense Landfalling Winter Storm in Southern California: Observations during CALJET

Abstract The California Land-falling Jets Experiment (CALJET) was carried out along the California coast, and up to 1000-km offshore, during the winter of 1997/98 to study the underlying physical processes that cause flooding rains and high winds in the orographically complex coastal zone and to explore the impact of potential future observing systems on short-term (<24 h) quantitative precipitation and wind forecasts during the landfall of winter storms from the data-sparse eastern Pacific Ocean. Using the suite of experimental and operational observing systems that were available during CALJET, this study documented the mesoscale modification of an intense landfalling cyclone by the steep coastal orography on 3 February 1998. This storm heavily impacted the populous and highly vulnerable coastal zone of southern California with flooding rains, strong winds, and major beach erosion. A pair of landfalling cold-frontal zones produced most of the damaging weather, while the primary cyclone circulation remai...

Evaluations of the von Kármán constant in the atmospheric surface layer

The von Karman constant

Onset and end of the summer melt season over sea ice: thermal structure and surface energy perspective from SHEBA

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The Impact of a Prominent Rain Shadow on Flooding in California's Santa Cruz Mountains: A CALJET Case Study and Sensitivity to the ENSO Cycle

relates the flow speed profile in a wall-bounded shear flow to the stress at the surface. Recent laboratory studies in aerodynamically smooth flow report

Parameterizing turbulent exchange over sea ice in winter

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