Aerosol-cloud interaction

A change in cloud microphysics or cloud properties caused by aerosol, including droplet activation, precipitation changes and adjustments of water content or coverage. The Twomey effect isolates a fixed-water-content optical response; it does not by itself establish the sign or magnitude of every cloud adjustment.

Cloud changes caused by aerosol absorption of radiation and the resulting heating. Heating can evaporate clouds and reduce shortwave reflection, but the effect can reverse when heating changes stability and cloud maintenance differently. The forcing sign depends on the relative positions of cloud and absorbing aerosol and on dynamical adjustment.

Increasing cloud droplet number at fixed liquid-water content and cloud thickness reduces mean droplet size and increases total extinction area, raising shortwave optical depth and cloud albedo. This idealized response produces negative shortwave radiative forcing. Changes in cloud amount, water content and dynamics are separate adjustments.^[ref]

For $\tau \propto N_d^{1/3}$ at fixed liquid-water content and thickness, and $A=\tau /(\tau +7)$, the derivative is $dA/d{N}_d=A(1-A)/(3N_d)$. For a fixed fractional number increase, the initial response is greatest at $A=1/2$. The exact finite-change expression is $A^{\prime }=sA/(1-A+sA)$ with $s=(N_d^{\prime }/N_d{)}^{1/3}$. Uncertain activation of additional particles and changes in water content limit direct application to real clouds.