#Lee et al. (2016, Climate dynamics) investigate competitions between aerosol-induced changes in vertical temperature gradients and those in horizontal temperature gradients in aerosol-cloud-precipitation interactions. Most of previous studies have focused on either the effects of aerosol-induced changes in the vertical temperature gradient or those in the horizontal temperature gradient but not both. Hence, this study can be a valuable stepping stone to the comprehensive understanding of aerosol-cloud-precipitation interactions.
#Lee et al. (2016, JGR) examine the effect of aerosol on the diurnal cycle of precipitation. This study finds that increasing concentration of aerosol particles can change the rate of depletion of convective energy by convection and thus the cloud life cycle. These changes have substantial impacts on the diurnal cycle of precipitation. This study identifies mechanisms related to these changes and this substantially improves our understanding of aerosol effects on the diurnal cycle of precipitation, considering that our level of understanding of aerosol effects on the diurnal cycle has been very low.
#Lee et al. (2014, JGR) show that the increasing concentration of aerosol particles that act as radiation absorbers can generate circulations. This generation of circulations heavily depends on how surface heat fluxes respond to the increasing concentration of aerosol particles and this study is one of the first studies that show the dependence and associated mechanisms.
#Lee et al. (2014,advances in meteorology) review current studies on aerosol-cloud interactions and discusses how to improve our understanding of these interactions by considering feedbacks among aerosol, cloud dynamics, microphysics and circulations. Environmental instability basically determines the dynamic intensity of clouds and thus acts as one of the most important controls on these feedbacks. This review paper specifically elaborates on how to link the instability to the feedbacks.
#Lee and Feingold (2013,ACP) find that with increasing aerosol (i) the convective mass flux increases, leading to increases in condensation, cloud liquid, and accretion of cloud liquid by precipitation; (ii) autoconversion of cloud water to rain water decreases; (iii) the water path (WP) spatial distribution becomes more homogeneous; and (iv) there is an increase in the frequencies of high and low WP and precipitation rate (P), and a decrease in these frequencies at the mid-range of WP and P. Thus, while aerosol perturbations have a small influence on total precipitation amount, for the case considered, they do have substantial influences on the spatiotemporal distribution of convection and precipitation.
#Lee et al. (2012, JAS) demonstrate that after a sufficiently long period of time, aerosol-perturbed fields of cumulus clouds such as liquid water path (LWP) converge into a similar state to that of aerosol-unperturbed cloud fields. This is because the system tends to adjust itself by modifying its environment in such a way that the effects of the initial aerosol perturbation are countered. This adjustment tends to occur ~ 14 h after the start of simulation which cautions against short-term simulations when attempting to ascertain the effects of aerosol perturbations. This adjustment also indicates that a cloud system can be fairly robust to aerosol perturbation.
#Lee et al. (2012,JAS) also show that even transient aerosol perturbations may linger longer than the duration of the perturbation itself, provided it persists for long enough. Short duration aerosol perturbations are unlikely to have much influences on the system. It raises an important point that aerosol perturbation with a short duration does not have this lingering effect and thus may have a negligible impact on cloud systems.
#There have been many published findings of aerosol-induced invigoration of convective clouds. However, these findings have not explained the exact mechanism which controls aerosol-cloud interactions in convective clouds. My study is able to describe the mechanism which relates aerosols to the gust front and is able to explain how aerosols affect cloud dynamics and invigoration. This mechanism can lead to precipitation enhancement, increasing precipitation frequency and changes in precipitation spatial distributions with increasing aerosols in convective clouds (Lee, Donner, Phillips, Ming, 2008, JGR; Lee, Donner, Phillips, Ming, 2008, QJRMS). Also, my several papers comparing the effects of aerosol on warm clouds (from which the traditional understanding of aerosol-cloud interactions or the Twomey and Albrecht effect was derived) to those on convective clouds have been published over the last few years (Lee, Donner, Phillips, Ming, 2008, JGR; Lee, Donner, Phillips, 2009, ACP; Lee, Donner, Penner, 2010, ACP).
#Through the comparison between the warm stratocumulus clouds and thunderstorm-type deep convective clouds for aerosol-cloud interactions, my study finds that ~30% of an increase in the reflection of solar radiation induced by the aerosol increase is offset by a decrease in outgoing longwave radiation in deep convective clouds due to their invigoration, while only ~2% of the increase is offset in stratocumulus clouds due to the absence of invigoration. Note that the traditional understanding of aerosol-cloud interactions was derived from stratocumulus clouds. This comparison indicates a strong dependence of aerosol-cloud interactions on cloud type. The large 30% offset is due to cloud-scale interactions between aerosol, microphysics and dynamics (leading to invigoration), which are not resolvable in climate models. Also, anvil cirrus clouds account for ~ half of the 30% offset in deep convection. This indicates that we need to consider the aerosol-induced changes in cloud-scale interactions and the role of the aerosol-induced changes in cirrus clouds for the better assessment of the effect of aerosol-cloud interactions on climate changes (Lee, Donner, Phillips, 2009, ACP).
#Research I have been involved in with Graham Feingold at ESRL shows that there are invigoration-induced compensation or buffering mechanisms among cloud types or among microphysical processes in a mesoscale cloud system driven by thunderstorm-type deep convective clouds. These mechanisms make the aerosol-induced precipitation difference negligible, which in turn makes it difficult to discern the aerosol effect from the meteorological influences. The research also identifies a different kind of buffering mechanism, which makes a relaxation time needed for aerosol to get back to its unperturbed state nearly identical in spite of the different magnitude of the aerosol perturbation. Despite the negligible aerosol-induced change in precipitation amount, aerosol-induced changes in precipitation spatial distributions and frequency are substantial (Lee and Feingold, 2010, GRL).
#My research finds that there is an additional factor to consider to understand aerosol-cloud interactions in a multiple-cloud system (driven by thunderstorm-type deep convection) as compared to factors considered for a single convective cloud. For example, for a single cloud, the effect of humidity on the interactions and associated invigoration can be explained only by using entrainment. However, my study found that we need to take into account the effect of aerosol on gustiness in addition to entrainment for the understanding of the effect of humidity in a multiple cloud system. This implies that parameterizations for aerosol-cloud interactions based on a single cloud can be misleading in climate models (Lee, 2011, ACP).
#Most of previous studies on aerosol-cloud interactions have focused on clouds just over an area polluted by aerosol under an assumption that only clouds directly over the polluted area are affected by aerosol. My study shows that this assumption can be misleading in the assessment of the effect of aerosol on clouds and precipitation, since the effect of pollution can extend onto clouds away from the polluted area. The pollution enhances precipitation over the polluted area by invigorating thunderstorm-type deep convection. However, the invigorated convection intensifies large-scale circulations, which intensifies downdrafts over areas which are away from the pollution and much larger than the polluted area. These intensified downdrafts suppress clouds and precipitation over these areas away from the pollution. This indicates changes in precipitation and thus hydrologic spatial distributions induced by aerosols over a much larger domain than a polluted area (Lee, 2012, JAS).
#I have also been involved in the comparison of a global-climate model simulation to a cloud system resolving model (CSRM) simulation for long-term thin stratocumulus clouds lasting 20 days. In this work, I performed an in-depth analysis of deficiencies of cloud parameterizations in climate models. Also, I found that the impact of rising sea-surface temperatures due to global warming on cloud type would not be correctly simulated in climate models by comparing the results of a climate model to those of a CSRM. This is found to be one of causes of the overestimation of aerosol indirect effect in climate models. I proposed a new philosophy of cloud parameterizations in climate models based on this study. It should be pointed out that I performed a 20-day long-term CSRM simulation with very high resolution for this study. This kind of long-term simulation is the first of its type (Lee, Penner, 2010, ACP).
#Thin stratocumulus clouds cover around 30% of the globe and, thus, aerosol effects on these clouds play an important role in climate change. My study found that the traditional understanding of aerosol indirect effects was not applicable and a new approach, which takes into account feedbacks among microphysics, aerosols, and dynamics, was necessary for the correct assessment of aerosol effects on these clouds and thus climate. The mechanisms of feedbacks in my study proposed the need to model supersaturation in these warm clouds as an effort to minimize the overestimation of aerosol indirect effect in climate models, and this was a first for the area of aerosol-cloud interactions in clouds. Most climate modelers do not yet properly include supersaturation even for ice clouds, though the importance of including these supersaturations is firmly established. One of my papers associated with this area was selected as an American Geophysical Union Journal Highlight (Lee, Penner, Saleeby, 2009, JGR).
References
Lee, S. S., J. M. Guo and Z. Li, 2016, Delayed Diurnal Changes in Precipitation and Lightning by Air Pollution over Pearl River Delta. Part II: Model Simulations, JGR, in review.
Lee, S. S., B. G. Kim, S. S. Yum, et al., 2016, Effects of aerosol on evaporation, freezing and precipitation in a multiple cloud system, Climate dynamics, DOI: 10.1007/s00382-016-3128-1
Lee, S. S., G. Feingold, I. Koren, H. Yu, T. Yamaguchi, and A. McComiskey, 2014, Effect of gradients in biomass burning aerosol on circulations and clouds, J. Geophys. Res, 119, 9948-9964.
Lee, S. S., W.-K. Tao, and C. H. Jung, 2014, Aerosol effects on instability, circulations, clouds and precipitation, Advances in Meteorology, 2014, Article ID 683950.
Lee, S. S., and G. Feingold, 2013, Aerosol effects on the cloud-field properties of tropical convective clouds, Atmos. Chem. Phys., 13, 6713-6726.
Lee, S. S., 2012, Effect of aerosol on circulations and precipitation in deep convective clouds, J. Atmos. Sci., 69, 1957-1974.
Lee, S. S., and G. Feingold, P. Y. Chuang, 2012, Effect of aerosol on cloud-environment interactions in trade cumulus, Journal of the Atmospheric Sciences, in press.
Lee, S. S., L. J. Donner, V T J Phillips, and Y. Ming, 2008: The dependence of aerosol effects on clouds and precipitation on cloud-system organization, shear and stability. J. Geophys. Res., 113, D16202, doi:10.1029/2007JD009224
Lee, S. S., L. J. Donner,V T J Phillips, and Y. Ming, 2008: Examination of aerosol effects on precipitation in deep convective clouds during the 1997 ARM summer experiment. Quarterly Journal of the Royal Meteorological Society, 134(634), doi: 10.1002/qj.287
Lee, S. S., L. J. Donner, and V T J Phillips, 2009: Sensitivity of aerosol and cloud effects on radiation to cloud types: comparison between deep convective clouds and warm stratiform clouds over one-day period. Atmos. Chem. Phys., 9, 2555-2575.
Lee, S. S., L. J. Donner, and J. E. Penner, 2010, Thunderstorm and stratocumulus: How does their contrasting morphology affect their interactions with aerosols?, Atmos. Chem. Phys., 10, 6819-6837.
Lee, S. S., and G. Feingold, 2010, Precipitating cloud-system response to aerosol perturbations, Geophys. Res. Lett., 37, doi:10.1029/2010GL045596.
Lee, S. S., 2011, Dependence of aerosol-precipitation interactions on humidity in a multiple-cloud system, Atmos. Chem. Phys., 11, 2179-2196.
Lee, S. S., 2011, Effect of aerosol on circulations and precipitation in deep convective clouds, Journal of the Atmospheric Sciences, minor revision, in review.
Lee S. S., and J. E. Penner, 2010, Comparison of a global-climate model simulation to a cloud-system resolving model simulation for long-term thin stratocumulus clouds in a response to the transition from the preindustrial condition to the present-day condition, Atmos. Chem. Phys, 10, 6371-6389.
Lee S. S., J. E. Penner, and S. M. Saleeby, 2009, Aerosol effects on liquid-water path of thin stratocumulus clouds, J. Geophys. Res., 114, D07204, doi:10.1029/2008JD010513
Rosenfeld, D., U. Lohmann, G. B. Raga, et al., 2008, Flood or drought: How do aerosols affect precipitation?, Science, 321, 1309-1313.