Ulrike Lohmann

Flood or Drought: How Do Aerosols Affect Precipitation?

Aerosols serve as cloud condensation nuclei (CCN) and thus have a substantial effect on cloud properties and the initiation of precipitation. Large concentrations of human-made aerosols have been reported to both decrease and increase rainfall as a result of their radiative and CCN activities. At one extreme, pristine tropical clouds with low CCN concentrations rain out too quickly to mature into long-lived clouds. On the other hand, heavily polluted clouds evaporate much of their water before precipitation can occur, if they can form at all given the reduced surface heating resulting from the aerosol haze layer. We propose a conceptual model that explains this apparent dichotomy.

Tropical Rainfall Trends and the Indirect Aerosol Effect

An atmospheric global climate model coupled to a mixed layer ocean model is used to study changes in tropical rainfall due to the indirect effects of anthropogenic sulfate aerosol. The model is run to equilibrium for present-day (PD) and preindustrial (PI) sulfur emission scenarios. As in two other recent studies, the model generally gives a southward shift of tropical rainfall in the PD run relative to the PI run. This is largely due to a hemispheric asymmetry in the reduction of sea surface temperature (SST) induced by the perturbation of cloud albedo and lifetime. Observed precipitation trends over land for the period 1900‐98 show a complex pattern in the Tropics, but when zonally averaged, a southward shift similar to (but weaker than) the modeled shift is clearly evident. The zonally averaged tropical trends are significant at the 5% level in several latitude bands. The modeled presentday hemispheric contrast in cloud droplet effective radius (which affects cloud albedo) is well supported by one long-term satellite retrieval, but not by another. A third satellite retrieval, which only covers an 8-month period, does show a marked hemispheric contrast in effective radius. Both in the modeled changes and the observed trends, a prominent feature is the drying of the Sahel in North Africa. Modeled dynamical changes in this region are similar to observed changes that have been associated with Sahelian drought. Previous work has identified a near-global, quasi-hemispheric pattern of contrasting SST anomalies (cool in the Northern Hemisphere and warm in the Southern Hemisphere) associated with dry conditions in the Sahel. The present results, combined with this earlier finding, suggest that the indirect effects of anthropogenic sulfate may have contributed to the Sahelian drying trend. More generally, it is concluded that spatially varying aerosol-related forcing (both direct and indirect) can substantially alter low-latitude circulation and rainfall.

Design and performance of a new cloud microphysics scheme developed for the ECHAM general circulation model

A new cloud microphysics scheme including a prognostic treatment of cloud ice (PCI) is developed to yield a more physically based representation of the components of the atmospheric moisture budget in the general circulation model ECHAM. The new approach considers cloud water and cloud ice as separate prognostic variables. The precipitation formation scheme for warm clouds distinguishes between maritime and continental clouds by considering the cloud droplet number concentration, in addition to the liquid water content. Based on several observational data sets, the cloud droplet number concentration is derived from the sulfate aerosol mass concentration as given from the sulfur cycle simulated by ECHAM. Results obtained with the new scheme are compared to satellite observations and in situ measurements of cloud physical and radiative properties. In general, the standard model ECHAM4 and also PCI capture the overall features, and the simulated results usually lie within the range of observed uncertainty. As compared to ECHAM4, only slight improvements are achieved with the new scheme. For example, the overestimated liquid water path and total cloud cover over convectively active regions are reduced in PCI. On the other hand, some shortcomings of the standard model such as underestimated shortwave cloud forcing over the extratropical oceans of the respective summer hemisphere are more pronounced in PCI.

Prediction of the number of cloud droplets in the ECHAM GCM

In this paper a prognostic equation for the number of cloud droplets (CDNC) is introduced into the ECHAM general circulation model. The initial CDNC is based on the mechanistic model of Chuang and Penner [1995], providing a more realistical prediction of CDNC than the empirical method previously used. Cloud droplet nucleation is parameterized as a function of total aerosol number concentration, updraft velocity, and a shape parameter, which takes the aerosol composition and size distribution into account. The total number of aerosol particles is obtained as the sum of marine sulfate aerosols produced from dimethyl sulfide, hydrophylic organic and black carbon, submicron dust, and sea-salt aerosols. Anthropogenic sulfate aerosols only add mass to the preexisting aerosols but do not form new particles. The simulated annual mean liquid water path, column CDNC, and effective radius agree well with observations, as does the frequency distributions of column CDNC for clouds over oceans and the variations of cloud optical depth with effective radius.

Canadian Aerosol Module: A size‐segregated simulation of atmospheric aerosol processes for climate and air quality models 1. Module development

A size-segregated multicomponent aerosol algorithm, the Canadian Aerosol Module (CAM), was developed for use with climate and air quality models. It includes major aerosol processes in the atmosphere: generation, hygroscopic growth, coagulation, nucleation, condensation, dry deposition/sedimentation, below-cloud scavenging, aerosol activation, a cloud module with explicit microphysical processes to treat aerosol-cloud interactions and chemical transformation of sulphur species in clear air and in clouds. The numerical solution was optimized to efficiently solve the complicated size-segregated multicomponent aerosol system and make it feasible to be included in global and regional models. An internal mixture is assumed for all types of aerosols except for soil dust and black carbon which are assumed to be externally mixed close to sources. To test the algorithm, emissions to the atmosphere of anthropogenic and natural aerosols are simulated for two aerosol types: sea salt and sulphate. A comparison was made of two numerical solutions of the aerosol algorithm: process splitting and ordinary differential equation (ODE) solver. It was found that the process-splitting method used for this model is within 15% of the more accurate ODE solution for the total sulphate mass concentration and <1% accurate for sea-salt concentration. Furthermore, it is computationally more than 100 times faster. The sensitivity of the simulated size distributions to the number of size bins was also investigated. The diffusional behavior of each individual process was quantitatively characterized by the difference in the mode radius and standard deviation of a lognormal curve fit of distributions between the approximate solution and the 96-bin reference solution. Both the number and mass size distributions were adequately predicted by a sectional model of 12 bins in many situations in the atmosphere where the sink for condensable matter on existing aerosol surface area is high enough that nucleation of new particles is negligible. Total mass concentration was adequately simulated using lower size resolution of 8 bins. However, to properly resolve nucleation mode size distributions and minimize the numerical diffusion, a sectional model of 18 size bins or greater is needed. The number of size bins is more important in resolving the nucleation mode peaks than in reducing the diffusional behavior of aerosol processes. Application of CAM in a study of the global cycling of sea-salt mass accompanies this paper

Indirect effect of sulfate and carbonaceous aerosols: A mechanistic treatment

The indirect effect of anthropogenic aerosols, whereby aerosol particles change cloud optical properties, is the most uncertain component of climate forcing over the past 100 years. Here we use a mechanistic treatment of droplet nucleation and a prognostic treatment of the number of cloud droplets to study the indirect aerosol effect from changes in carbonaceous and sulfate aerosols. Cloud droplet nucleation is parameterized as a function of total aerosol number concentration, updraft velocity, and an activation parameter, which takes into account the mechanism of sulfate aerosol formation. Where previous studies focussed either on sulfate aerosols or carbonaceous aerosols only, here we estimate the combined effect. The combined indirect aerosol effect amounts to −1.1 W m−2 for an internally mixed aerosol and −1.5 W m−2 for an externally mixed aerosol compared to −1.4 W m−2, which we obtained by empirically relating sulfate mass to cloud droplet number. In the case of an internally mixed aerosol, the contribution from increasing carbonaceous and sulfate aerosols is close to being additive as the individual simulations yield an indirect effect of −0.4 W m−2 due to anthropogenic sulfate aerosols and −0.9 W m−2 due to anthropogenic carbonaceous aerosols. The contribution of anthropogenic sulfate to the indirect effect is close to zero if an externally mixed aerosol is assumed, while the contribution of carbonaceous aerosols increases to −1.3 W m−2. The effect of sulfate in the external mixture approach is much smaller than that of carbonaceous aerosols because its burden only increases by a third of that of carbonaceous aerosols and because the mode radius of sulfate is much larger than that of black and organic carbon.

Impact of sulfate aerosols on albedo and lifetime of clouds: A sensitivity study with the ECHAM4 GCM

A coupled sulfur chemistry-cloud microphysics scheme (COUPL) is used to study the impact of sulfate aerosols on cloud lifetime and albedo. The cloud microphysics scheme includes precipitation formation, which depends on the cloud droplet number concentration (CDNC) and on the liquid water content. On the basis of different observational data sets, CDNC is proportional to the sulfate aerosol mass, which is calculated by the model. Cloud cover is a function of relative humidity only. Additional sensitivity experiments with another cloud cover parameterization (COUPL-CC), which also depends on cloud water, and with a different autoconversion rate of cloud droplets (COUPL-CC-Aut) are conducted to investigate the range of the indirect effect due to uncertainties in cloud physics. For each experiment, two simulations, one using present-day and one using preindustrial sulfur emissions are carried out. The increase in liquid water path, cloud cover, and shortwave cloud forcing due to anthropogenic sulfur emissions depends crucially upon the parameterization of cloud cover and autoconversion of cloud droplets. In COUPL the liquid water path increases by 17% and cloud cover increases by 1% because of anthropogenic sulfur emissions, yielding an increase in shortwave cloud forcing of -1.4 W m -2 . In COUPL-CC the liquid water path increases by 32%, cloud cover increases by 3% and thus shortwave cloud forcing increases by -4.8 W m -2 . This large effect is caused by the strong dependence of cloud cover on cloud water and of the autoconversion rate on CDNC, cloud water, and cloud cover. Choosing a different autoconversion rate (COUPL-CC-Aut) with a reduced dependence on CDNC and cloud water results in an increase of liquid water path by only 11% and of cloud cover by 1%, and the increase in shortwave cloud forcing amounts to -2.2 W m -2 . These results clearly show that the uncertainties linked to the indirect aerosol effect are higher than was previously suggested.

Sensitivity Studies of the Importance of Dust Ice Nuclei for the Indirect Aerosol Effect on Stratiform Mixed-Phase Clouds

Abstract New parameterizations of contact freezing and immersion freezing in stratiform mixed-phase clouds (with temperatures between 0° and −35°C) for black carbon and mineral dust assumed to be composed of either kaolinite (simulation KAO) or montmorillonite (simulation MON) are introduced into the ECHAM4 general circulation model. The effectiveness of black carbon and dust as ice nuclei as a function of temperature is parameterized from a compilation of laboratory studies. This is the first time that freezing parameterizations take the chemical composition of ice nuclei into account. The rather subtle differences between these sensitivity simulations in the present-day climate have significant implications for the anthropogenic indirect aerosol effect. The decrease in net radiation in these sensitivity simulations at the top of the atmosphere varies from 1 ± 0.3 to 2.1 ± 0.1 W m−2 depending on whether dust is assumed to be composed of kaolinite or montmorillonite. In simulation KAO, black carbon has a ...

Solid Ammonium Sulfate Aerosols as Ice Nuclei: A Pathway for Cirrus Cloud Formation

Laboratory measurements support a cirrus cloud formation pathway involving heterogeneous ice nucleation by solid ammonium sulfate aerosols. Ice formation occurs at low ice-saturation ratios consistent with the formation of continental cirrus and an interhemispheric asymmetry observed for cloud onset. In a climate model, this mechanism provides a widespread source of ice nuclei and leads to fewer but larger ice crystals as compared with a homogeneous freezing scenario. This reduces both the cloud albedo and the longwave heating by cirrus. With the global ammonia budget dominated by agricultural practices, this pathway might further couple anthropogenic activity to the climate system.

Can the direct and semi‐direct aerosol effect compete with the indirect effect on a global scale?

During the Indian Ocean Experiment (INDOEX) Ackerman et al. [2000] found that black carbon (BC) aerosols can reduce cloud cover and liquid water path (“semi-direct” aerosol effect). Absorption of solar radiation by BC leads to a heating of the air which can result in an evaporation of cloud droplets. This warming can partially offset the cooling due to the indirect aerosol effect. We use the ECHAM4 general circulation model to investigate whether this effect is discernible on a global scale. The results of this sensitivity study, where the absorption of BC only takes place in clear air and interstitially, indicate that the zonal mean reduction in cloud cover and liquid water path in highly polluted regions of the Northern Hemisphere can amount to 4% and 10 g m−2, respectively. However, the indirect aerosol effects of increasing cloud lifetime and cloud albedo dominate.