The relative dispersion of cloud droplets: Its robustness with respect to key cloud properties

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Abstract

Flight data measured in warm convective clouds near Istanbul in June 2008 were used to investigate the relative dispersion of cloud droplet size distribution. The relative dispersion (ε), defined as the ratio between the standard deviation (σ) of the cloud droplet size distribution and cloud droplet average radius (〈r〉), is a key factor in regional and global models. The relationship between ε and the clouds' microphysical and thermodynamic characteristics is examined. The results show that ε is constrained with average values in the range of ∼0.25-0.35. ε is shown not to be correlated with cloud droplet concentration or liquid water content (LWC). However, ε variance is shown to be sensitive to droplet concentration and LWC, suggesting smaller variability of ε in the clouds' most adiabatic regions. A criterion for use of in situ airborne measurement data for calculations of statistical moments (used in bulk microphysical schemes), based on the evaluation of ε, is suggested.

Figures

  • Figure 1. (a) MODIS image of the eastern Mediterranean region on 7 June 2008. (b) The tracks of the five flights. (c) A summary of flight profiles and cloud droplet concentration in airborne measurements carried out on 6–7 June 2008 around Istanbul, Turkey. Black line shows the droplet concentration and colored line shows the height above ground and the temperature.
  • Table 1. Airborne measurements used for the present study. For each of the five airborne measurements used in the present study, the flight date and the corresponding abbreviation used in this paper, number of data points, aerosol loading at the cloud base (see Sect. 2) minimum and maximum temperature, minimum and maximum pressure, cloud base and (estimated) cloud top height are indicated.
  • Figure 2. Cloud droplet size distribution as a function of height above the ground. The contours show the distribution (dN/dlog(D)). The yellow and red lines represent the average and standard deviation of the radius over the entire measurements, respectively. For the purpose of constructing the lines of the average radius and the standard deviation, we divided the measurements into 10 height bins, and for each bin the average was calculated. Note that the vertical axes are not uniform, accounting for the different cloud tops observed in the different flights.
  • Figure 3. (a) Relative dispersion (ε) vs. height above the ground with colors representing the liquid water content (LWC) and (b) ε vs. LWC with colors representing the droplet concentration for the inner cloud data points. Error bars represent standard error of the average ε for each height level (in a) and LWC (in b) with a confidence level of 95 %.
  • Figure 4. (a) Relative dispersion (ε) vs. height above the ground with colors representing the liquid water content (LWC) and (b) ε vs. LWC with colors representing the droplet concentration for the cloud boundary data points. Error bars represent the standard error of the average ε for each height level (in a) and LWC (in b) with a confidence level of 95 %.
  • Figure 5. Relative dispersion vs. average radius for (a) the inner cloud data, and (b) the cloud boundaries. Error bars represent the standard error of the average ε for each 〈r〉 level with a confidence level of 95 %.
  • Figure 7. (a) Histograms of ε for different aerosol loading values. The average aerosol loading for each flight (calculated at cloud base height) is presented. All histograms are based only on measured data associated with Nc > 10 cm −3. (b) Histogram of ε for different height ranges above the cloud base (indicated individually for each histogram by “h” range of the total cloud depth, “H”), excluding data collected during flight TRK3. All histograms are based only on measured data associated withNc > 10 cm −3. The top panel (All data) is based on data collected during all flights. Data collected during flight TRK3 were not used for any of the histograms.
  • Figure 6. Relative dispersion and its variance as a function of cloud liquid water content (LWC) and droplet number (Nc). Relative dispersion (ε), relative dispersion average (AVR (ε)) and relative dispersion variance (STD (ε)) are presented vs. LWC (a) and Nc (b). AVR (ε) and STD (ε) are presented as the average values of 10 number-based size bins.

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CITATION STYLE

APA

Tas, E., Teller, A., Altaratz, O., Axisa, D., Bruintjes, R., Levin, Z., & Koren, I. (2015). The relative dispersion of cloud droplets: Its robustness with respect to key cloud properties. Atmospheric Chemistry and Physics, 15(4), 2009–2017. https://doi.org/10.5194/acp-15-2009-2015

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