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Abstract
The structure and evolution of the extratropical marine atmosphere boundary layer (MABL) depend largely on the variability of stratus and stratocumulus clouds. Stratus clouds are generally associated with a well-mixed MABL, whereas daytime observations of stratocumulus-topped boundary layers generally indicate that the cloud and subcloud layers are decoupled. In the Atlantic Stratocumulus Transition Experiment, aircraft measurements show a surface-based mixed layer separated from the base of the stratocumulus by a layer that is stable to dry turbulent mixing. This layer forms due to shortwave heating of the stratocumulus clouds. Cumulus clouds often develop in this transition layer and they play a fundamental role in the redistribution of heat in the decoupled stratcumulus-capped boundary layer. They are, however, very sensitive to small changes in the heat and moisture in the boundary layer and are generally transient features that depend directly on the surface sensible and latent heat fluxes. The cumulus contribute a bimodal drop-size distribution to the stratocumulus layer skewed to the smallest sizes but may contain many large drops. Clouds increase at night in response to the combined effect of convection, which can transport drops to the top of the MABL, and outgoing longwave radiation, which cools the boundary layer. The relationship between the cumulus clouds and the latent heat flux is complex. Small cumulus may enhance the flux, but as more water vapor is redistributed vertically by an increase in convective activity the latent heat flux decreases.
This study illustrates the need for boundary-layer models to properly handle the occurrence of intermittent cumulus to predict the diurnal evolution of the stratocumulus-capped MABL.
Abstract
The structure and evolution of the extratropical marine atmosphere boundary layer (MABL) depend largely on the variability of stratus and stratocumulus clouds. Stratus clouds are generally associated with a well-mixed MABL, whereas daytime observations of stratocumulus-topped boundary layers generally indicate that the cloud and subcloud layers are decoupled. In the Atlantic Stratocumulus Transition Experiment, aircraft measurements show a surface-based mixed layer separated from the base of the stratocumulus by a layer that is stable to dry turbulent mixing. This layer forms due to shortwave heating of the stratocumulus clouds. Cumulus clouds often develop in this transition layer and they play a fundamental role in the redistribution of heat in the decoupled stratcumulus-capped boundary layer. They are, however, very sensitive to small changes in the heat and moisture in the boundary layer and are generally transient features that depend directly on the surface sensible and latent heat fluxes. The cumulus contribute a bimodal drop-size distribution to the stratocumulus layer skewed to the smallest sizes but may contain many large drops. Clouds increase at night in response to the combined effect of convection, which can transport drops to the top of the MABL, and outgoing longwave radiation, which cools the boundary layer. The relationship between the cumulus clouds and the latent heat flux is complex. Small cumulus may enhance the flux, but as more water vapor is redistributed vertically by an increase in convective activity the latent heat flux decreases.
This study illustrates the need for boundary-layer models to properly handle the occurrence of intermittent cumulus to predict the diurnal evolution of the stratocumulus-capped MABL.
Abstract
Tropical cyclone (TC) outflow and its relationship to TC intensity change and structure were investigated in the Office of Naval Research Tropical Cyclone Intensity (TCI) field program during 2015 using dropsondes deployed from the innovative new High-Definition Sounding System (HDSS) and remotely sensed observations from the Hurricane Imaging Radiometer (HIRAD), both on board the NASA WB-57 that flew in the lower stratosphere. Three noteworthy hurricanes were intensively observed with unprecedented horizontal resolution: Joaquin in the Atlantic and Marty and Patricia in the eastern North Pacific. Nearly 800 dropsondes were deployed from the WB-57 flight level of ∼60,000 ft (∼18 km), recording atmospheric conditions from the lower stratosphere to the surface, while HIRAD measured the surface winds in a 50-km-wide swath with a horizontal resolution of 2 km. Dropsonde transects with 4–10-km spacing through the inner cores of Hurricanes Patricia, Joaquin, and Marty depict the large horizontal and vertical gradients in winds and thermodynamic properties. An innovative technique utilizing GPS positions of the HDSS reveals the vortex tilt in detail not possible before. In four TCI flights over Joaquin, systematic measurements of a major hurricane’s outflow layer were made at high spatial resolution for the first time. Dropsondes deployed at 4-km intervals as the WB-57 flew over the center of Hurricane Patricia reveal in unprecedented detail the inner-core structure and upper-tropospheric outflow associated with this historic hurricane. Analyses and numerical modeling studies are in progress to understand and predict the complex factors that influenced Joaquin’s and Patricia’s unusual intensity changes.
Abstract
Tropical cyclone (TC) outflow and its relationship to TC intensity change and structure were investigated in the Office of Naval Research Tropical Cyclone Intensity (TCI) field program during 2015 using dropsondes deployed from the innovative new High-Definition Sounding System (HDSS) and remotely sensed observations from the Hurricane Imaging Radiometer (HIRAD), both on board the NASA WB-57 that flew in the lower stratosphere. Three noteworthy hurricanes were intensively observed with unprecedented horizontal resolution: Joaquin in the Atlantic and Marty and Patricia in the eastern North Pacific. Nearly 800 dropsondes were deployed from the WB-57 flight level of ∼60,000 ft (∼18 km), recording atmospheric conditions from the lower stratosphere to the surface, while HIRAD measured the surface winds in a 50-km-wide swath with a horizontal resolution of 2 km. Dropsonde transects with 4–10-km spacing through the inner cores of Hurricanes Patricia, Joaquin, and Marty depict the large horizontal and vertical gradients in winds and thermodynamic properties. An innovative technique utilizing GPS positions of the HDSS reveals the vortex tilt in detail not possible before. In four TCI flights over Joaquin, systematic measurements of a major hurricane’s outflow layer were made at high spatial resolution for the first time. Dropsondes deployed at 4-km intervals as the WB-57 flew over the center of Hurricane Patricia reveal in unprecedented detail the inner-core structure and upper-tropospheric outflow associated with this historic hurricane. Analyses and numerical modeling studies are in progress to understand and predict the complex factors that influenced Joaquin’s and Patricia’s unusual intensity changes.
