A Pseudo-aerosol Convective Invigoration Effect Caused by Aerosol-meteorology Correlation

Varble, A., Pacific Northwest National Laboratory

Cloud-Aerosol-Precipitation Interactions

Convective Processes

Varble A. 2018. "Erroneous attribution of deep convective invigoration to aerosol concentration." Journal of the Atmospheric Sciences, 75(4), 10.1175/JAS-D-17-0217.1.


(a) Minimum cloud top temperature (CTT) as a function of CN category for warm cloud base, cold cloud top systems, (b) the same as (a) but CAPE > 0, LCL > 15 C, and LNB < LCL, (c) CAPE as a function of CN category, and (d) LNB as a function of CN category. Panels c-d use the same samples as panel b. Diamonds represent mean values for all minimum CTTs < -4 C, squares represent mean values for minimum CTTs < -36 C, and standard errors of are shown with vertical lines.


The mean of the minimum cloud top temperature (CTT, shaded) for warm cloud base, cold cloud top systems as a function of (left) LNB and CN bins and (right) CAPE and CN bins. The number of samples (#), mean CTT minimum value (mu), and standard error of the mean CTT minimum (SE) are written in black in each bin.


(a) Minimum cloud top temperature (CTT) as a function of CN category for warm cloud base, cold cloud top systems, (b) the same as (a) but CAPE > 0, LCL > 15 C, and LNB < LCL, (c) CAPE as a function of CN category, and (d) LNB as a function of CN category. Panels c-d use the same samples as panel b. Diamonds represent mean values for all minimum CTTs < -4 C, squares represent mean values for minimum CTTs < -36 C, and standard errors of are shown with vertical lines.

The mean of the minimum cloud top temperature (CTT, shaded) for warm cloud base, cold cloud top systems as a function of (left) LNB and CN bins and (right) CAPE and CN bins. The number of samples (#), mean CTT minimum value (mu), and standard error of the mean CTT minimum (SE) are written in black in each bin.

Science

Previous studies have hypothesized that increasing the number of particles in the atmosphere that can serve as condensation nuclei for cloud droplets may increase the dynamical intensity of deep, moist convection, and thereby increase cloud top height, for clouds with a warm cloud base and ice. This study shows that previous studies claiming to observe this effect at the ARM Southern Great Plains (SGP) site in northern Oklahoma are in fact observing an increase in cloud top height because of changes in the temperature profile that correlates with changes in the condensation nuclei concentration. The correlation between thermodynamics and condensation nuclei conditions could be caused by a correlation with prior rainfall in the region, which may lower condensation nuclei concentrations through wet deposition, cool low levels through evaporation, and warm upper levels through condensation.

Impact

These results suggest that the effect of a four-fold increase in aerosol concentration on convective cloud top heights can be masked by temperature changes of ~1 degree Celsius at a mid-latitude, continental observing site with relatively frequent convectively instability. This implies that the frequently used method of analyzing subsets of aerosol data within large meteorological binned values is not a valid method for discerning an aerosol indirect effect on convective cloud tops in some conditions. It also suggests that correlations between aerosol concentrations and meteorological factors in convectively unstable situations may be quite limited but still explain correlations between aerosol concentrations and cloud top heights because the sensitivity of deep convective cloud top to thermodynamic conditions is so much larger than its sensitivity to aerosol concentration. This highlights the need for more careful and detailed accounting of convective meteorological variables in studies analyzing aerosol indirect effects on deep convection.

Summary

This study uses 14 years of observations in warm cloud base, convectively unstable environments to show that surface condensation nuclei (CN) concentration at the ARM SGP site statistically significantly correlates with convective available potential energy (CAPE) and the level of neutral buoyancy (LNB) associated with a buoyancy parcel of air lifted from the level of maximum CAPE. Accounting for correlations between CN concentration and thermodynamic conditions eliminates the positive correlation between CN concentration and convective cloud top height. This correlation can also be eliminated by simply removing a cloud top temperature mode centered at -10 Celsius, since these clouds likely do not contain ice. The statistically significant correlation between CN concentration and convection-related thermodynamic variables does not appear to be related to the diurnal cycle, seasonality, or synoptic conditions. However, it correlates with regional rainfall accumulation in the 6-hour period prior to convectively unstable, warm-cloud-base conditions at the SGP site.