Dynamical Structure and Turbulence in Cirrus Clouds: Aircraft Observations during FIRE

I. Gultepe NASA/Goddard Space Flight Center, Greenbelt, Maryland

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D. O'C. Starr NASA/Goddard Space Flight Center, Greenbelt, Maryland

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Abstract

Aircraft data collected during the First International Satellite Cloud Climatology Project Regional Experiment (FIRE)I are used to examine dynamical processes operating in cirrus cloud systems observed on 19 and 28 October 1986. Measurements from Lagrangian spiral soundings and constant-altitude flight legs are analyzed. Comparisons are made with observations in clear air. Each cirrus case contained a statically stable layer, a conditionally unstable or neutrally stratified layer (ice pseudoadiabatic) in which convection was prevalent, and a neutral layer in which convection was intermittent. The analysis indicates that a mixture of phenomena occurred including small-scale convective cells, gravity waves (λ≈2–9 km), quasi-two-dimensional waves (λ≈10–20 km), and larger two-dimensional mesoscale waves (λ≈100 km). The intermediate-scale waves, observed both in clear air and in the cloud systems, likely played an important role in the development of the cloud systems given the magnitude of the associated vertical air velocity. The spectra of perturbations of wind components for layers where convection was prevalent were characterized by a κ−5/3 power law dependence, while a κ−2/4 dependence was found at other levels in the cloud systems. A steeper spectral slope (κ−3) was found in the more stable cloud-base layer on 19 October. Samples in clear air also showed a (κ−2.4) dependence. Flight-leg-averaged eddy potential heat fluxes (H=±8 W m−2) were comparable to observations in marine stratocumulus clouds. Calculated turbulence dissipation rates agree with previously published studies, which indicate a general enhancement within cloud systems (10−6 to 10−3 m2 s−3 in cloud versus values less than 0.5×10−6 m2 s−3 in clear air).

Abstract

Aircraft data collected during the First International Satellite Cloud Climatology Project Regional Experiment (FIRE)I are used to examine dynamical processes operating in cirrus cloud systems observed on 19 and 28 October 1986. Measurements from Lagrangian spiral soundings and constant-altitude flight legs are analyzed. Comparisons are made with observations in clear air. Each cirrus case contained a statically stable layer, a conditionally unstable or neutrally stratified layer (ice pseudoadiabatic) in which convection was prevalent, and a neutral layer in which convection was intermittent. The analysis indicates that a mixture of phenomena occurred including small-scale convective cells, gravity waves (λ≈2–9 km), quasi-two-dimensional waves (λ≈10–20 km), and larger two-dimensional mesoscale waves (λ≈100 km). The intermediate-scale waves, observed both in clear air and in the cloud systems, likely played an important role in the development of the cloud systems given the magnitude of the associated vertical air velocity. The spectra of perturbations of wind components for layers where convection was prevalent were characterized by a κ−5/3 power law dependence, while a κ−2/4 dependence was found at other levels in the cloud systems. A steeper spectral slope (κ−3) was found in the more stable cloud-base layer on 19 October. Samples in clear air also showed a (κ−2.4) dependence. Flight-leg-averaged eddy potential heat fluxes (H=±8 W m−2) were comparable to observations in marine stratocumulus clouds. Calculated turbulence dissipation rates agree with previously published studies, which indicate a general enhancement within cloud systems (10−6 to 10−3 m2 s−3 in cloud versus values less than 0.5×10−6 m2 s−3 in clear air).

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