Ribu Cherian

Pollution trends over Europe constrain global aerosol forcing as simulated by climate models

An increasing trend in surface solar radiation (solar brightening) has been observed over Europe since the 1990s, linked to economic developments and air pollution regulations and their direct as well as cloud-mediated effects on radiation. Here, we find that the all-sky solar brightening trend (1990–2005) over Europe from seven out of eight models (historical simulations in the Fifth Coupled Model Intercomparison Project) scales well with the regional and global mean effective forcing by anthropogenic aerosols (idealized “present-day” minus “preindustrial” runs). The reason for this relationship is that models that simulate stronger forcing efficiencies and stronger radiative effects by aerosol-cloud interactions show both a stronger aerosol forcing and a stronger solar brightening. The all-sky solar brightening is the observable from measurements (4.06±0.60 W m−2 decade−1), which then allows to infer a global mean total aerosol effective forcing at about −1.30 W m−2 with standard deviation ±0.40 W m−2.

Black carbon indirect radiative effects in a climate model

Abstract The aerosol–cloud interactions due to black carbon (BC) aerosols, as well as the implied climate responses, are examined using an aerosol module in the coupled atmosphere–ocean general circulation model MPI-ESM. BC is simulated to enhance cloud droplet number concentration (CDNC) by 10–15% in the BC emission source regions, especially in the Tropics and mid-latitudes. Higher CDNC and reduced auto-conversion from cloud water to rain water explains the increased cloud water path over the tropical regions (30S–30N) in the model. In the global mean, the cloud water– as well as precipitation changes are negligibly small. The global-mean effective radiative forcing due to aerosol–cloud interactions for BC is estimated at , which is attributable to the increase in CDNC burden and (regionally) cloud water in the model. Global mean temperature and rainfall response were found to be and , respectively, with significantly larger regional changes mainly in the downwind regions from BC sources.

Subgrid-scale variability of clear-sky relative humidity and forcing by aerosol-radiation interactions in an atmosphere model

Atmosphere models with resolutions of several tens of kilometres take subgrid-scale variability in the total specific humidity qt into account by using a uniform probability density function (PDF) to predict fractional cloud cover. However, usually only mean relative humidity, RH, or mean clear-sky relative humidity, RHcls, is used to compute hygroscopic growth of soluble aerosol particles. While previous studies based on limited-area models and also a global model suggest that subgrid-scale variability in RH should be taken into account for estimating radiative forcing due to aerosol–radiation interactions (RFari), here we present the first estimate of RFari using a global atmospheric model with a parameterization for subgrid-scale variability in RH that is consistent with the assumptions in the model. For this, we sample the subsaturated part of the uniform RH-PDF from the cloud cover scheme for its application in the hygroscopic growth parameterization in the ECHAM6-HAM2 atmosphere model. Due to the non-linear dependence of the hygroscopic growth on RH, this causes an increase in aerosol hygroscopic growth. Aerosol optical depth (AOD) increases by a global mean of 0.009 (∼ 7.8% in comparison to the control run). Especially over the tropics AOD is enhanced with a mean of about 0.013. Due to the increase in AOD, net top of the atmosphere clear-sky solar radiation, SWnet,cls, decreases by −0.22 Wm−2 (∼−0.08%). Finally, the RFari changes from −0.15 to −0.19 Wm−2 by about 31 %. The reason for this very disproportionate effect is that anthropogenic aerosols are disproportionally hygroscopic.