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Small-Scale Moist Turbulence in Numerically Generated Convective Clouds

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  • 1 Department of Atmospheric and Oceanic Sciences, McGill University, Montreal, Quebec, Canada
  • 2 Department of Mathematics and Statistics, and Department of Atmospheric and Oceanic Sciences, McGill University, Montreal, Quebec, Canada
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Abstract

The authors present simulations of cloud-free and cloudy, nonprecipitating, convective turbulence at spatial resolutions down to Δx = 2.6 m for a domain size of (1 km)3. The runs are analyzed with attention focused on the dynamical differences between resolutions and the presence or absence of moisture, as well as on the small-scale variability of the liquid water spectra in the cloudy cases. Because of evaporation and condensation, liquid water content does not act like a passive scalar. Much of the evaporation occurs in highly turbulent cloud-top mixing where differences in variances and kurtoses of real-space vorticity probability density functions between cloudy and cloud-free runs are also found. The cloudy cases have higher variance and lower kurtosis values than their cloud-free counterparts. The lower kurtosis values mean fewer high-intensity vortices for the cloudy cases, which is most likely due to the loss of buoyancy as evaporation occurs during entrainment events. This effect is associated with a change in the liquid water content spectra found in regions of cloud decay. Conditional sampling of these regions shows increased small-scale variability, above the background increase due to the bottleneck effect, of liquid water content spectra; this is not found in other cloudy regions. This may help explain recent measurements of enhanced small-scale liquid water content variability in aircraft measurements of stratocumulus clouds.

* Current affiliation: Redeemer University College, Hamilton, Ontario, Canada

Corresponding author address: K. Spyksma, Redeemer University College, Hamilton, ON L9K 1J4, Canada. Email: kspyksma@cs.redeemer.ca

Abstract

The authors present simulations of cloud-free and cloudy, nonprecipitating, convective turbulence at spatial resolutions down to Δx = 2.6 m for a domain size of (1 km)3. The runs are analyzed with attention focused on the dynamical differences between resolutions and the presence or absence of moisture, as well as on the small-scale variability of the liquid water spectra in the cloudy cases. Because of evaporation and condensation, liquid water content does not act like a passive scalar. Much of the evaporation occurs in highly turbulent cloud-top mixing where differences in variances and kurtoses of real-space vorticity probability density functions between cloudy and cloud-free runs are also found. The cloudy cases have higher variance and lower kurtosis values than their cloud-free counterparts. The lower kurtosis values mean fewer high-intensity vortices for the cloudy cases, which is most likely due to the loss of buoyancy as evaporation occurs during entrainment events. This effect is associated with a change in the liquid water content spectra found in regions of cloud decay. Conditional sampling of these regions shows increased small-scale variability, above the background increase due to the bottleneck effect, of liquid water content spectra; this is not found in other cloudy regions. This may help explain recent measurements of enhanced small-scale liquid water content variability in aircraft measurements of stratocumulus clouds.

* Current affiliation: Redeemer University College, Hamilton, Ontario, Canada

Corresponding author address: K. Spyksma, Redeemer University College, Hamilton, ON L9K 1J4, Canada. Email: kspyksma@cs.redeemer.ca

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