Aerosol indirect effect from turbulence-induced broadening of cloud-droplet size distributions

作者：Chandrakar Kamal Kant; Cantrell Will; Chang Kelken; Ciochetto David; Niedermeier Dennis; Ovchinnikov Mikhail; Shaw Raymond A; Yang Fan

来源：Proceedings of the National Academy of Sciences, 2016, 113(50): 14243-14248.

DOI：10.1073/pnas.1612686113

<jats:p>The influence of aerosol concentration on the cloud-droplet size distribution is investigated in a laboratory chamber that enables turbulent cloud formation through moist convection. The experiments allow steady-state microphysics to be achieved, with aerosol input balanced by cloud-droplet growth and fallout. As aerosol concentration is increased, the cloud-droplet mean diameter decreases, as expected, but the width of the size distribution also decreases sharply. The aerosol input allows for cloud generation in the limiting regimes of fast microphysics (<jats:inline-formula><m:math xmlns:m="http://www.w3.org/1998/Math/MathML" overflow="scroll"><m:mrow><m:msub><m:mi>τ</m:mi><m:mi>c</m:mi></m:msub><m:mo>&lt;</m:mo><m:msub><m:mi>τ</m:mi><m:mi>t</m:mi></m:msub></m:mrow></m:math></jats:inline-formula>) for high aerosol concentration, and slow microphysics (<jats:inline-formula><m:math xmlns:m="http://www.w3.org/1998/Math/MathML" overflow="scroll"><m:mrow><m:msub><m:mi>τ</m:mi><m:mi>c</m:mi></m:msub><m:mo>&gt;</m:mo><m:msub><m:mi>τ</m:mi><m:mi>t</m:mi></m:msub></m:mrow></m:math></jats:inline-formula>) for low aerosol concentration; here, <jats:inline-formula><m:math xmlns:m="http://www.w3.org/1998/Math/MathML" overflow="scroll"><m:msub><m:mi>τ</m:mi><m:mi>c</m:mi></m:msub></m:math></jats:inline-formula> is the phase-relaxation time and <jats:inline-formula><m:math xmlns:m="http://www.w3.org/1998/Math/MathML" overflow="scroll"><m:msub><m:mi>τ</m:mi><m:mi>t</m:mi></m:msub></m:math></jats:inline-formula> is the turbulence-correlation time. The increase in the width of the droplet size distribution for the low aerosol limit is consistent with larger variability of supersaturation due to the slow microphysical response. A stochastic differential equation for supersaturation predicts that the standard deviation of the squared droplet radius should increase linearly with a system time scale defined as <jats:inline-formula><m:math xmlns:m="http://www.w3.org/1998/Math/MathML" overflow="scroll"><m:mrow><m:msubsup><m:mi>τ</m:mi><m:mi>s</m:mi><m:mrow><m:mo>−</m:mo><m:mn>1</m:mn></m:mrow></m:msubsup><m:mo>=</m:mo><m:mrow><m:msubsup><m:mi>τ</m:mi><m:mi>c</m:mi><m:mrow><m:mo>−</m:mo><m:mn>1</m:mn></m:mrow></m:msubsup><m:mo>+</m:mo><m:msubsup><m:mi>τ</m:mi><m:mi>t</m:mi><m:mrow><m:mo>−</m:mo><m:mn>1</m:mn></m:mrow></m:msubsup></m:mrow></m:mrow></m:math></jats:inline-formula>, and the measurements are in excellent agreement with this finding. The result underscores the importance of droplet size dispersion for aerosol indirect effects: increasing aerosol concentration changes the albedo and suppresses precipitation formation not only through reduction of the mean droplet diameter but also by narrowing of the droplet size distribution due to reduced supersaturation fluctuations. Supersaturation fluctuations in the low aerosol/slow microphysics limit are likely of leading importance for precipitation formation.</jats:p>

• 出版日期2016-12-13