Review of Roesenfeld et al Review of Roesenfeld et al. (2008); Flood or Drought: How Do Aerosols Affect Precipitation? Science There is an almost linear relationship between the aerosol optical depth and Cloud Condensation Nuclei (CCN) at 0.4% supersaturation. Before anthropogenic sources of aerosols, the CCN between maritime and continental regions were not that different. One can expect the aerosol concentration to continue to increase throughout the 21st century which ties directly into this paper.

Aerosols and Non-inductive Charging Broad DSD (5-50 𝜇m) We have spoken briefly in class about the effects of aerosols on atmospheric electricity Increasing the aerosol concentration stunts precipitation formation More CCN promotes the formation of small cloud droplets which are not as likely to coalesce The result is more supercooled liquid water in the mixed phase region which promotes positive charging on the graupel Narrow DSD (5-35 𝜇m)

Radiative Effect of Aerosols Aerosols have a radiative and microphysical effect Most of the radiative effects cause clouds to produce less precipitation Increased aerosol concentrations scatter more radiation from the sun which leads to less surface heating and less instability Non-scattered radiation is absorbed by low levels of the atmosphere Promotes a low-level inversion

Rosenfeld et al. (2008) developed this model to show how suppressed collision-coalescence can lead better precipitation efficiency. Most of the cloud condensate in the pristine airmass does not reach the 0℃ altitude. The lack of latent heat release in the cloud is detrimental to the lifespan of the thunderstorm cloud. The collision-coalescence is suppressed in the thunderstorm for the hazy conditions. This allows for more water to reach the 0℃ altitude where the condensate can freeze into ice particles and release latent heat. This also means that the ice can melt lower in the cloud and reabsorb latent heat. This causes the thunderstorm to be more efficient at precipitation. The delay of rainout causes enhanced evaporative cooling due to the added cloud water. This enhances the cold pool near the surface and forces the ambient air upwards. The thunderstorm in the hazy conditions is able to convert more CAPE to kinetic energy which enhances the amount of precipitation produced by the cloud. The authors use the pseudo-adiabatic parcel theory to explain this figure.

Assume the parcel ascends from sea level and has a pressure of 960 hPa and a temperature of 22℃ at cloud base. If no precipitation falls from the cloud, the condensate requires 415 J/kg to rise to the -4℃ isotherm where freezing can occur in the atmosphere. Freezing all of the condensate would add thermal buoyancy via latent heat release that would approximately balance the condensate load (d2). When the ice falls from the cloud, the parcel becomes more buoyant due to the release of weight (d3) such that energy released at the top of the cloud is largest (d4). In this case, the cloud is precipitating ice hydrometeors below - 4 ℃. The buoyancy is approximately 1000 J/kg greater relative to (c1) where the cloud is precipitating ice above - 4 ℃ and rain below it. However, if the delay in conversion from cloud water to precipitation is too long it can negatively affect the precipitation efficiency. If the suppression is extreme, it requires an additional 727 J/kg to lift the condensate. Moreover, there is no mechanism for homogenous ice crystals so there would be no precipitation unloading. This would require more energy to keep the ice crystals aloft and suppress the storm and precipitation further.

The previous discussion would lead us to believe that there is some ”goldilocks” aerosol concentration for optimal precipitation efficiency. The amount of released convective energy reaches a maximum at ~1200 cm-3. If more aerosols are added, the convection is suppressed from curve (d) to curve (a) in the previous figure. The microphysical effects on invigorating convection reaches a maximum at medium aerosol concentrations. Increasing the aerosol concentration past this optimum point increases the radiative effects and reduces surface heating. The reduce surface heating results in less instability and a decrease in the amount of released convective energy. On the other end of the spectrum, small concentrations of aerosols lead to a situation like the pristine scenario from slide 2. The rain falls out too early and the lack of latent heat release results in destruction of convection.