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- Author or Editor: P. Kollias x
- Journal of the Atmospheric Sciences x
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Abstract
Observations from a 94-GHz radar are used to define the vertical structure of marine fair-weather cumuli. Doppler spectra obtained from the radar provide mean vertical velocities as well as detailed spectral shapes that can be used to infer small-scale vertical velocity shear, illuminate cloud microphysical processes, and provide estimates of turbulence dissipation rates. These new observations facilitate the analysis and understanding of in-cloud circulations and the physical processes involved, since the cloud boundaries and dimensions are mapped along with the internal structure of the clouds. Coincident observations from a 915-MHz radar (wind profiler) were used to further define the turbulence structure in and around the clouds. The observations document the detailed vertical and horizontal dimensions of updraft and downdraft circulations in the clouds observed. The two cumuli studied in detail have similar circulation patterns—an updraft core surrounded by downdrafts. Although the clouds have a horizontal depth of only about 700 m, updraft velocities of about 5.5 m s−1 were observed. These updrafts, which are only about 400 m across, exhibit characteristics that are consistent with adiabatic ascent, and penetrate about 150 m into the capping inversion. No penetrating downdrafts are observed within the updraft cores. The downdrafts that flank the updraft on the downwind side of the cloud are relatively narrow (less than 100 m) and extend from cloud top to cloud base. The downdraft on the upwind side of the cloud is about 150 m across and penetrates about 200 m into the detraining cloud mass observed in this part of the cloud. This downdraft appears to be driven by the cooling associated with entrainment mixing at cloud top penetrating through detraining, dynamically inactive parts of the cloud matter. Analysis of the Doppler spectrum at the updraft–downdraft interfaces indicates large Doppler spectral widths due to turbulence and sharp shear zones in the radar sampling volume. Large Doppler spectral widths in the detraining upwind part of the cloud are consistent with the presence of larger droplets. The updraft core structure in one of the clouds has a structure that is consistent with the idea that cumulus clouds are composed of successive bubbles that emerge from the subcloud layer. Thus these small cumuli should be considered as convective complexes rather than simple growing elements that later decay into passive clouds. This study illustrates the potential of millimeter-wavelength radars for studying small cumuli.
Abstract
Observations from a 94-GHz radar are used to define the vertical structure of marine fair-weather cumuli. Doppler spectra obtained from the radar provide mean vertical velocities as well as detailed spectral shapes that can be used to infer small-scale vertical velocity shear, illuminate cloud microphysical processes, and provide estimates of turbulence dissipation rates. These new observations facilitate the analysis and understanding of in-cloud circulations and the physical processes involved, since the cloud boundaries and dimensions are mapped along with the internal structure of the clouds. Coincident observations from a 915-MHz radar (wind profiler) were used to further define the turbulence structure in and around the clouds. The observations document the detailed vertical and horizontal dimensions of updraft and downdraft circulations in the clouds observed. The two cumuli studied in detail have similar circulation patterns—an updraft core surrounded by downdrafts. Although the clouds have a horizontal depth of only about 700 m, updraft velocities of about 5.5 m s−1 were observed. These updrafts, which are only about 400 m across, exhibit characteristics that are consistent with adiabatic ascent, and penetrate about 150 m into the capping inversion. No penetrating downdrafts are observed within the updraft cores. The downdrafts that flank the updraft on the downwind side of the cloud are relatively narrow (less than 100 m) and extend from cloud top to cloud base. The downdraft on the upwind side of the cloud is about 150 m across and penetrates about 200 m into the detraining cloud mass observed in this part of the cloud. This downdraft appears to be driven by the cooling associated with entrainment mixing at cloud top penetrating through detraining, dynamically inactive parts of the cloud matter. Analysis of the Doppler spectrum at the updraft–downdraft interfaces indicates large Doppler spectral widths due to turbulence and sharp shear zones in the radar sampling volume. Large Doppler spectral widths in the detraining upwind part of the cloud are consistent with the presence of larger droplets. The updraft core structure in one of the clouds has a structure that is consistent with the idea that cumulus clouds are composed of successive bubbles that emerge from the subcloud layer. Thus these small cumuli should be considered as convective complexes rather than simple growing elements that later decay into passive clouds. This study illustrates the potential of millimeter-wavelength radars for studying small cumuli.
Abstract
Turbulence and drizzle-rate measurements from a large dataset of marine and continental low stratiform clouds are presented. Turbulence peaks at cloud base over land and near cloud top over the ocean. For both regions, eddy dissipation rate values of 10−5–10−2 m2 s−3 are observed. Surface-based measurements of cloud condensation nuclei number concentration N CCN and liquid water path (LWP) are used to estimate the precipitation susceptibility S 0. Results show that positive S 0 values are found at low turbulence, consistent with the principle that aerosols suppress precipitation formation, whereas S 0 is smaller, and can be negative, in a more turbulent environment. Under similar macrophysical conditions, especially for medium to high LWP, high (low) turbulence is likely to lessen (promote) the suppression effect of high N CCN on precipitation. Overall, the turbulent effect on S 0 is stronger in continental than marine stratiform clouds. These observational findings are consistent with recent analytical prediction for a turbulence-broadening effect on cloud droplet size distribution.
Abstract
Turbulence and drizzle-rate measurements from a large dataset of marine and continental low stratiform clouds are presented. Turbulence peaks at cloud base over land and near cloud top over the ocean. For both regions, eddy dissipation rate values of 10−5–10−2 m2 s−3 are observed. Surface-based measurements of cloud condensation nuclei number concentration N CCN and liquid water path (LWP) are used to estimate the precipitation susceptibility S 0. Results show that positive S 0 values are found at low turbulence, consistent with the principle that aerosols suppress precipitation formation, whereas S 0 is smaller, and can be negative, in a more turbulent environment. Under similar macrophysical conditions, especially for medium to high LWP, high (low) turbulence is likely to lessen (promote) the suppression effect of high N CCN on precipitation. Overall, the turbulent effect on S 0 is stronger in continental than marine stratiform clouds. These observational findings are consistent with recent analytical prediction for a turbulence-broadening effect on cloud droplet size distribution.
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. Between updrafts, nearly all ice falls out and/or sublimates while the cloud liquid diminishes but does not completely evaporate. The persistence of liquid water throughout these cloud cycles suggests that ice-forming nuclei, and thus ice crystal, concentrations must be limited and that water vapor is plentiful. These details are brought together within the context of a conceptual model relating cloud-scale dynamics and microphysics.
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. Between updrafts, nearly all ice falls out and/or sublimates while the cloud liquid diminishes but does not completely evaporate. The persistence of liquid water throughout these cloud cycles suggests that ice-forming nuclei, and thus ice crystal, concentrations must be limited and that water vapor is plentiful. These details are brought together within the context of a conceptual model relating cloud-scale dynamics and microphysics.
