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Teddy Holt and Sethu Raman

Abstract

Marine boundary-layer structure and circulation is documented for the 24 February 1986 case of offshore redevelopment of a cyclone during the Genesis of Atlantic Lows Experiment (GALE) Intensive Observing Period (IOP) 9. Mesoscale and satellite information emphasize that the onshore cyclone is not well organized as it moves offshore to the cold shelf waters with redevelopment occurring later over the Gulf Stream region. Within hours of redevelopment, low-level aircraft data were obtained in the region.

Vertical aircraft profiles down by the National Center for Atmospheric Research (NCAR) King Air in the vicinity of redevelopment over the Gulf Stream, as well as the midshelf front region and cold shelf waters, reveal two distinct boundary layers. Over the Gulf Stream region approximately 50 km south-southwest of the redeveloping cyclone, the near-neutral marine boundary layer (−h/L = 6.6) capped by layered stratocumulus is characterized by a low cloud base (360 m), relatively thick stratocumulus cloud layer (800–1200 m) and strong subcloud-layer winds (8–9 m s-1). Associated with the developing cyclone near the Gulf Stream is shallow cyclonic flow with convergence and subsequent acceleration of the wind near the western edge.

Closer to the coast over the cold shelf waters and the midshelf front region, the relatively cloud-free boundary layer (h/L = 44.4) is characterized by a slightly shallower, new-neutral boundary layer (h = 700 to 755 m) with very light and variable winds. Boundary layer flow is strongly divergent west of the midshelf front. Them two regions are approximately 150–200 km west of the Gulf Stream region or redevelopment.

Flux profiles agree with results from other marine boundary layers under similar cloud and stability conditions and emphasize the warming and moistening of the subcloud layer from new the western edge of the Gulf Stream eastward. Temperature and moisture turbulence structure appear less well organized. The mean momentum budget emphasizes the strong baroclinicity in the MABL and the importance of horizontal advection near the western edge of the Gulf Stream. Comparison turbulent kinetic energy (TKE) budgets over the Gulf Stream and over the midshelf front show shear production and dissipation to dominate over the Gulf Stream with strong winds. Turbulent transport over the Gulf Stream is a significant term due primarily to the flux of horizontal velocity variance, which is approximately 5 times that of the flux of vertical velocity variance. Over the midshelf front, all normalized terms in the TKE budget are less active in producing, dissipating and transferring TKE for a given heat flux as compared to the Gulf Stream region, where the effects of the developing cyclone are evident.

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Teddy Holt and Sethu Raman

Abstract

Radiosondes from Soviet ships along with dropsondes and mean and turbulence data from the National Center for Atmospheric Research (NCAR) Electra gust probe aircraft are analyzed to infer the structure of the monsoon marine boundary layer during MONEX 79. Results of mean wind profiles indicate the existence of a jetlike structure in the upper part of the boundary layer during the more suppressed “monsoon-break” conditions. The thermal structure of the monsoon boundary layer during these break conditions is characterized by near-neutral to slightly unstable conditions. There was an approximate balance of form in the monsoon boundary layer between advective acceleration, friction and geostrophic departure. Advective acceleration was found to be a significant term, especially in the lower levels of the boundary layer. This contrasts with typical trade-wind boundary layers in which acceleration is generally negligible.

Results indicate that turbulence statistics associated with wind speed components and temperature in the monsoon boundary layer during MONEX 79 are generally large. Profiles of momentum and virtual temperature flux change sign at altitudes as low as 30 to 50% of the boundary layer height. The turbulent kinetic energy budget indicates that buoyancy is not a dominant source term above, roughly, one-third the boundary layer height. Viscous energy dissipation and turbulent transport are the important sink terms in the lowest one-half of the boundary layer.

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Yihua Wu and Sethu Raman

Abstract

Land-use patterns are a major factor that causes land surface heterogeneities, which in turn influence the development of mesoscale circulations. In the present study, effects of land-use patterns on the formation and structure of mesoscale circulations were investigated using the North Carolina State University mesoscale model linked with the soil–vegetation system. The Midwest type of low-level jet (LLJ) was successfully generated in the model simulation. Characteristics of the LLJ generated in the numerical experiments are consistent with observations. The results suggest that land surface heterogeneities could have significant impacts on the formation and the maintenance of the LLJ.

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Hao Jin and Sethu Raman

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This paper presents a study on air pollutant dispersion from an elevated accidental release from the space shuttle tower at the Kennedy Space Center in Florida under the influence of a stratified onshore flow. The temperature difference between land and ocean can generate a local sea-land circulation and a thermal internal boundary layer. Both play a significant role in the coastal dispersion. Results from a Gaussian dispersion model and those from numerical simulations show that the concentrations obtained from these two distinctly different methods are of the same order of magnitude and have similar patterns. Numerical simulations were performed by combining the Advanced Regional Prediction System with an Eulerian pollutant dispersion model. Numerical sensitivity experiments were conducted to investigate the effects of upwind stability, coastal topography, and calm wind condition. Numerical results also show that the dispersion pattern from a continuous release is significantly different from that of a finite release.

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Randall J. Alliss and Sethu Raman

Abstract

Fields of cloudiness derived from the Geostationary Operational Environmental Satellite VISSR (Visible–Infrared Spin Scan Radiometer) Atmospheric Sounder are analyzed over the Gulf Stream locale (GSL) to investigate seasonal and geographical variations. The GSL in this study is defined as the region bounded from 31° to 38°N and 82° to 66°W. This region covers an area that includes the United States mid-Atlantic coast states, the Gulf Stream, and portions of the Sargasso Sea. Clouds over the GSL are found approximately three-quarters of the time between 1985 and 1993. However, large seasonal variations in the frequency of cloudiness exist. These seasonal variations show a distinct relationship to gradients in sea surface temperature (SST). For example, during winter when large SST gradients are present, large gradients in cloudiness are found. Clouds are observed least often during summer over the ocean portion of the GSL. This minimum coincides with an increase in atmospheric stability due to large-scale subsidence. Cloudiness is also found over the GSL in response to mesoscale convergence areas induced by sea surface temperature gradients. Geographical variations in cloudiness are found to be related to the meteorology of the region. During periods of cold-air advection, which are found most frequently in winter, clouds are found less often between the coastline and the core of the Gulf Stream and more often over the Sargasso Sea. During cyclogenesis, large cloud shields often develop and cover the entire domain.

Satellite estimates of cloudiness are found to be least reliable over land at night during the cold months. In these situations, the cloud retrieval algorithm often mistakes clear sky for low clouds. Satellite-derived cloudiness over land is compared with daytime surface observations of cloudiness. Results indicate that retrieved cloudiness agrees well with surface observations. Relative humidity fields taken from global analyses are compared with satellite cloud heights at three levels in the atmosphere. Cloudiness observed at these levels is found at relative humidities in the 75%–100% range but is also observed at humidities as low as 26%.

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Randall J. Alliss and Sethu Raman

Abstract

Saturation pressure differences, a measure of parcel saturation, are calculated from upper-air soundings and compared to manual surface observations of cloudiness. The saturation pressure level p * (more commonly referred to as the lifted condensation level, LCL), can be calculated for each level in a sounding using the temperature and dewpoint temperatures. Thus, p * of an unsaturated air parcel is found by dry-adiabatic ascent to the pressure level where the parcel is just saturated. The difference between air parcel pressure and saturation pressure level defines the parcel saturation pressure difference. The mean saturation pressure difference between 1000 and 700, 700 and 400, and 400 and 300 mb is calculated and compared to the observed composite cloudiness for those layers. Results indicate that as the absolute value of saturation pressure difference decreases toward zero, the resulting ground observed composite cloud amount increases. However, the mean saturation pressure difference for high clouds ranges from 64 mb under clear skies to 16 mb for overcast conditions. This corresponds to relative humidities between 25% and 76%. Most previous studies do not indicate such large cloud amounts at these humidities. Three empirical relationships that define low, middle, and high clouds are developed based on one year of comparisons. These relationships are then tested on an independent dataset that include a wide variety of cloud cover conditions. Qualitative comparisons are made to manual observations of cloudiness and indicate that the relationships overall slightly overestimate the frequency of cloudiness. Cloudiness derived from the Visible-Infrared Spin Scan Radiometer (VISSR) Atmospheric Sounder (VAS) onboard the Geostationary Environmental Operational Satellite (GOES) 7 using the CO2 slicing technique is also compared to surface observations. Results indicate that the satellite-derived cloudiness overestimates cloudiness compared with surface observations but is also very similar to the saturation pressure difference estimates.

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Teddy R. Holt and Sethu Raman

Abstract

The interaction of oceanic fronts in the vicinity of the Gulf Stream with an atmospheric coastal front during the Genesis of Atlantic Lows Experiment (GALE) is examined using aircraft, satellite, and ship data. The nearshore, midshelf, and Gulf Stream oceanic fronts are readily discernible from low-level aircraft radiometer and satellite imagery data. The three-dimensional (3D) structure of the coastal front is extensively mapped by low-level aircraft transects through the frontal boundary.

Results confirm the existence of the coastal front as a very shallow (depth less than 200 m), spatially inhomogeneous, undulating material surface. Aircraft observations from 2000 to 2200 UTC (late afternoon local time) show a surface location of the coastal front that is aligned over the Gulf Stream oceanic front under conditions of very weak (2 m s−1) onshore flow, but is observed to migrate shoreward for stronger on-shore flow.

Ahead of the front in the warm air, the marine atmospheric boundary layer is characterized as well mixed with broken cumulus and stratocumulus cloud bases observed near 500 m, and tops varying from 1300 to 1900 m. The dominant scale of turbulent eddies is observed to be on the order of the boundary-layer depth. Conditional sampling statistics point to a strong direct circulation ahead of the front dominated by intense, narrow, warm updrafts, and broader, less intense, cool downdrafts.

Behind the coastal front in the cold air, visibility is much reduced by low-level fractus and layered stratocumulus clouds. The shallow subcloud layer is observed to be generally moister and more statically stable than ahead of the front. It is also characterized by an indirect circulation with more prevalent cool updrafts and warm downdrafts, particularly for the near–cloud-base region.

However, behind the front there exists a strong thermodynamic coupling of atmosphere and ocean as evidenced by the distinctly different atmospheric regimes present over the oceanic nearshore and midshelf front regions. Over the nearshore region, the horizontal wind structure is dominated by 100-m waves imbedded in a weaker 1–2-km circulation. Warm updrafts are observed over the nearshore waters, but the smaller air-sea temperature difference effectively limits large temperature perturbations. Hence, much smaller sensible heat flux is evident over the nearshore region as compared to the oceanic midshelf region. Over the midshelf region, turbulent eddies on the scale of 1.5 times the depth of the front (120 m) are solely responsible for the larger positive beat flux. The transition zone of the coastal front aloft near 150 m is remarkably confined to just the oceanic nearshore shelf, located between the nearshore waters and the midshelf region.

The frontal surface itself is observed to play an important role in the 3D atmospheric circulation in the vicinity of the front. The front causes a decoupling of the region just above the frontal surface by inhibiting the vertical transfer of fluxes from the surface. Cospectra for regions just above the front show no contributions from smaller waves generated by near-surface processes (on the order of 100–500 m) that are evident just ahead of the front. This suggests a decoupling due to the frontal boundary. Associated with this decoupling and the subsequent stabilization of the region above the front is the occurrence of buoyancy waves. These waves of wavelength approximately 840 m are believed to be a result of penetrating thermals and/or instabilities present along the frontal surface.

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Ching-Yuang Huang and Sethu Raman

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Cold air advection over the Gulf Stream off the Carolinas and the Appalachian Mountains is studied using idealized two-dimensional cases for the Genesis of Atlantic Lows Experiment (GALE) lop 2 conditions. An anelastic hydrostatic mesoscale model is used. Turbulent transfer in the planetary boundary layer, diurnal heating, cloud dynamics, atmospheric longwave and shortwave radiation and subgrid cumulus parameterization are included in the model.

Model results show that the geometry of the oceanic and coastal rainbands depends on the direction of the ambient flow (onshore or offshore). For onshore flows, the rainbands remain in the vicinity of the oceanic baroclinic zone. The rainbands become, transient and migrate downwind of the Gulf Stream front for offshore flows. Depths of the marine boundary layer (MBL) and the cloud (or rain) bands depend more on the ambient flow speed than its direction. The rainbands develop primarily in response to the strong low level convergence.

As expected, southward winds are produced at the eastern side of the Appalachian Mountains for onshore conditions. A significant amount of the turning, however, results from the baroclinic zone over the ocean. Upstream influence of the mountain intensifies the updrafts'in the MBL and moves the oceanic rainbands further offshore. The effects of the atmospheric longwave and shortwave radiation, subgrid cloud heating and diurnal ground heating are of secondary importance in influencing the structure of the MBL as compared to the surface turbulent beat fluxes. Diurnal effects can change the coastal inland flow regime considerably, resulting in a local breeze and the formation of another cloud (or rain) band.

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Ching-Yuang Huang and Sethu Raman

Abstract

A three-dimensional mesoscale planetary boundary layer (PBL) numerical model is used to investigate mesoscale circulations over the Carolina coastal and Gulf Stream baroclinic zones. Idealized ambient onshore and offshore flows are investigated, which represent the synoptic conditions during the Intensive Observation Period-2 (IOP-2) of the 1986 Genesis of Atlantic Lows Experiment (GALE). For the easterly onshore flow, a confluence zone appears west of the Gulf Stream in response to the effect of the oceanic baroclinicity. The confluence zone is nearly parallel to the coastline and the SST isotherms, with northeasterly (southwesterly) flow to the west (east). A shallow coastal front forms below 2 km as the cyclonic shear of the ageostrophic flow becomes strong. Quasi-stationary rainbands are produced by cumulus convection along the coastal front. The northern part of the front and the rainbands later encroach inland as the cold air intensity over ground weakens due to onshore warm air advection. The modeled coastal circulation is in agreement with the observations, suggesting that differential boundary-layer modification may be the main mechanism for the formation of the coastal front. The existence of an onshore ambient flow appears to be a necessary condition for the presence of the Coastal front. For the northerly offshore ambient flow, the rainband therefore appears along the eastern edge of the Gulf Stream, which then moves slowly downstream in response to the generated atmospheric baroclinicity. For both flows, the development of the rainbands is sensitive to variations in eddy Prandtl number, and their growth rate can be explained in terms of conditional symmetric instability.

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