Important Paper On Aerosol Forcing Of The Climate System – “Lifting Potential Of Solar‐Heated Aerosol Layers”

As we learn more about aerosols, their role within the climate system is becoming better recognized as very complex. There is an important new paper that further documents this complexity. It is

Boers, R., A. T. de Laat, D. C. Stein Zweers, and R. J. Dirksen (2010), Lifting potential of solar‐heated aerosol layers, Geophys. Res. Lett., 37, L24802, doi:10.1029/2010GL045171.

The abstract reads [hihglight added]

“Absorption of shortwave solar radiation can potentially heat aerosol layers and create buoyancy that can result in the ascent of the aerosol layer over several kilometres altitude within 24–48 hours. Such heating is seasonally dependent with the summer pole region producing the largest lifting in solstice because aerosol layers are exposed to sunshine for close to 24 hours a day. The smaller the Angstrøm parameter, the larger the lifting potential. An important enhancement to lifting is the diffuse illumination of the base of the aerosol layer when it is located above highly reflective cloud layers. It is estimated that aerosol layers residing in the boundary layer with optical properties typical for biomass burning aerosols can reach the extra tropical tropopause within 3–4 day entirely due to diabatic heating as a result of solar shortwave absorption and cross‐latitudinal transport. It is hypothesized that this mechanism can explain the presence and persistence of upper tropospheric/lower stratospheric aerosol layers.”

The conclusion contains the text

“In this paper we explored the potential of lifting of aerosol layers to the upper troposphere/lower stratosphere by means of solar heating. To this end we decoupled the diabatic heating due to solar heating from the adiabatic cooling due to altitude gain. Knowing the latitudinal variation of the potential temperature lapse rate, the daily diabatic heating rate of aerosol layers is translated into a tropospheric altitude gain. For optical properties typical of biomass burning events aerosol layers are lifted by 3–5 km in the course of 3 days. With additional solar heating by diffuse radiation from reflected radiation at the base of the aerosol layer and cross ‐ latitudinal flow these gains can be doubled so that aerosol layer can quickly reach tropopause levels. Neither the presence of pyro ‐ convection nor the onset of complex synoptic scale dynamical systems is required to allow this mechanism to achieve lift. If the aerosol layer decays as a result of sedimentation and/or chemical changes this method of vertical transport will cease to be effective. Sedimentation will remove mostly the larger particles (>10 mm) within a matter of days because of the magnitude their fall speeds. However for the remaining particles the residence time is much longer, and the higher the particles are lifted, the less the risk will be that they fall out due to precipitation which is not so relevant at higher altitudes. Thus, solar absorption can be a powerful mechanism to transport aerosol layers towards higher tropospheric altitudes.”

The importance of this paper is that aerosols can be lofted into the stratosphere, where they will persist much longer than when they are confined to the troposphere and can be scavenged out by precipitation. As with volcanic emissions into the stratosphere, these aerosols can influence weather for weeks and longer. Persistant input of aerosols into the stratopshere by this mechanism can affect climate for years.

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