Yiran Peng

Importance of vertical velocity variations in the cloud droplet nucleation process of marine stratus clouds

[1] Eleven cloud cases through marine stratus, obtained during two field experiments in the North Atlantic Ocean, are used to study the sensitivity of cloud droplet nucleation to the vertical gust velocity. Selected cloud microphysical data, size-distributed aerosol properties and particle chemistry are applied in an adiabatic parcel model. The nucleated cloud droplet number concentrations (N) predicted using the probability density function (PDF) of the measured in-cloud vertical velocities are compared to predictions using a characteristic velocity value. In this study, the model-predicted N from the PDF of the measured in-cloud vertical velocities agrees with the observed maximum N (Nmax )t o within 8.6%. The average N (Navg) can be related to Nmax using a power law (Leaitch et al., 1996). If a relationship between Nmax and Navg based on the measurements is applied to obtain the average N from the model-predicted N, then the model-predicted average N agrees with the observed average N to within 13.3%. When a characteristic vertical velocity (0.8 times the standard deviation of the vertical velocity distribution in this study) is used in the parcel model to simulate N, the model-predicted N agrees with the observed maximum N to within 5.7% and the model-predicted average N agrees with the observed average N within 8.8%. This indicates that using a characteristic value of the vertical velocity distribution instead of its PDF is a good approximation for simulating the nucleated cloud droplet number of marine stratus on a cloud scale.

Dispersion bias, dispersion effect, and the aerosol-cloud conundrum

This work examines the influences of relative dispersion (the ratio of the standard deviation to the mean radius of the cloud droplet size distribution) on cloud albedo and cloud radiative forcing, derives an analytical formulation that accounts explicitly for the contribution from droplet concentration and relative dispersion, and presents a new approach to parameterize relative dispersion in climate models. It is shown that inadequate representation of relative dispersion in climate models leads to an overestimation of cloud albedo, resulting in a negative bias of global mean shortwave cloud radiative forcing that can be comparable to the warming caused by doubling CO2 in magnitude, and that this dispersion bias is likely near its maximum for ambient clouds. Relative dispersion is empirically expressed as a function of the quotient between cloud liquid water content and droplet concentration (i.e., water per droplet), yielding an analytical formulation for the first aerosol indirect effect. Further analysis of the new expression reveals that the dispersion effect not only offsets the cooling from the Twomey effect, but is also proportional to the Twomey effect in magnitude. These results suggest that unrealistic representation of relative dispersion in cloud parameterization in general, and evaluation of aerosol indirect effects in particular, is at least in part responsible for several outstanding puzzles of the aerosol‐cloud conundrum: for example, overestimation of cloud radiative cooling by climate models compared to satellite observations; large uncertainty and discrepancy in estimates of the aerosol indirect effect; and the lack of interhemispheric difference in cloud albedo.

Potential near‐future carbon uptake overcomes losses from a large insect outbreak in British Columbia, Canada

The current capacity of northern high-latitude forests to sequester carbon has been suggested to be undermined by the potential increase in fire and insect outbreaks. Here we investigate the response of the terrestrial ecosystems in the province of British Columbia (BC), Canada, to the recent large mountain pine beetle (MPB) outbreak that started in 1999 as well as changing climate and continually increasing atmospheric CO2 concentration up to 2050, in a combined framework, using a process-based model. Model simulations suggest that the recent MPB outbreak results in BCs forests accumulating 328 Tg less carbon over the 1999–2020 period. Over this same period changing climate and increasing atmospheric CO2 concentration, however, yield enhanced carbon uptake equal to a cumulative sink of around 900–1060 Tg C, depending on the future climate change scenario, indicating that the reduced carbon uptake by land due to the MPB disturbance may already be surpassed by 2020.

Numerical simulation of clouds and precipitation depending on different relationships between aerosol and cloud droplet spectral dispersion

ABSTRACT The aerosol effects on clouds and precipitation in deep convective cloud systems are investigated using the Weather Research and Forecast (WRF) model with the Morrison two-moment bulk microphysics scheme. Considering positive or negative relationships between the cloud droplet number concentration (N c ) and spectral dispersion (ɛ), a suite of sensitivity experiments are performed using an initial sounding data of the deep convective cloud system on 31 March 2005 in Beijing under either a maritime (‘clean’) or continental (‘polluted’) background. Numerical experiments in this study indicate that the sign of the surface precipitation response induced by aerosols is dependent on the ɛ−N c relationships, which can influence the autoconversion processes from cloud droplets to rain drops. When the spectral dispersion ɛ is an increasing function of N c , the domain-average cumulative precipitation increases with aerosol concentrations from maritime to continental background. That may be because the existence of large-sized rain drops can increase precipitation at high aerosol concentration. However, the surface precipitation is reduced with increasing concentrations of aerosol particles when ɛ is a decreasing function of N c . For the ɛ−N c negative relationships, smaller spectral dispersion suppresses the autoconversion processes, reduces the rain water content and eventually decreases the surface precipitation under polluted conditions. Although differences in the surface precipitation between polluted and clean backgrounds are small for all the ɛ−N c relationships, additional simulations show that our findings are robust to small perturbations in the initial thermal conditions.

New understanding and quantification of the regime dependence of aerosol-cloud interaction for studying aerosol indirect effects

In this study, aerosol indirect effects suffer from large uncertainty in climate models and among observations. This study focuses on two plausible factors: regime dependence of aerosol-cloud interactions and the effect of cloud droplet spectral shape. We show, using a new parcel model, that combined consideration of droplet number concentration (Nc) and relative dispersion (e, ratio of standard deviation to mean radius of the cloud droplet size distribution) better characterizes the regime dependence of aerosol-cloud interactions than considering Nc alone. Given updraft velocity (w), e increases with increasing aerosol number concentration (Na) in the aerosol-limited regime, peaks in the transitional regime, and decreases with further increasing Na in the updraft-limited regime. This new finding further reconciles contrasting observations in literature and reinforces the compensating role of dispersion effect. The nonmonotonic behavior of e further quantifies the relationship between the transitional Na and w that separates the aerosol- and updraft-limited regimes.

Accounting for dust aerosol size distribution in radiative transfer

The impact of size distribution of mineral dust aerosol on radiative transfer was investigated using the Aerosol Robotic Network-retrieved aerosol size distributions. Three methods for determining the aerosol optical properties using size distributions were discussed. The first is referred to as a bin method in which the aerosol optical properties are determined for each bin of the size distribution. The second is named as an assembly mean method in which the aerosol optical properties are determined with an integration of the aerosol optical parameters over the observed size distribution. The third is a normal parameterization method based on an assumed size distribution. The bin method was used to generate the benchmark results in the radiation calculations against the methods of the assembly mean, and parameterizations based on two size distribution functions, namely, lognormal and gamma were examined. It is seen that the assembly mean method can produce aerosol radiative forcing with accuracy of better than 1%. The accuracies of the parameterizations based on lognormal and gamma size distributions are about 25% and 5%, respectively. Both the lognormal and gamma size distributions can be determined by two parameters, the effective radius and effective variance. The better results from the gamma size distribution can be explained by a third parameter of skewness which is found to be useful for judging how close the assumed distribution is to the observation result. The parameterizations based on the two assumed size distributions are also evaluated in a climate model. The results show that the reflected solar fluxes over the desert areas determined by the scheme based on the gamma size distribution are about 1 W m−2 less than those from the scheme based on the lognormal size distribution, bringing the model results closer to the observations.

Height Dependency of Aerosol-Cloud Interaction Regimes: Height Dependency of ACI Regime

This study investigates the height dependency of aerosol-cloud interaction regimes in terms of the joint dependence of the key cloud microphysical properties (e.g., cloud droplet number concentration and cloud droplet relative dispersion) on aerosol number concentration (Na) and vertical velocity (w). The three distinct regimes with differentmicrophysical features are the aerosol-limited regime, the updraft-limited regime, and the transitional regime. The results reveal two new phenomena in updraft-limited regime: (1) the “condensational broadening” of cloud droplet size distribution in contrast to the well-known “condensational narrowing” in the aerosol-limited regime and (2) above the level of maximum supersaturation; some cloud droplets are deactivated into interstitial aerosols in the updraft-limited regime, whereas all droplets remain activated in the aerosol-limited regime. Further analysis shows that the particle equilibrium supersaturation plays important role in understanding these unique features. Also examined is the height of warm rain initiation and its dependence on Na and w. The rain initiation height is found to depend primarily on either Na or w or both in different Na-w regimes, suggesting a strong regime dependence of the second aerosol indirect effect.

Role of microphysical parameterizations with droplet relative dispersion in IAP AGCM 4.1

Previous studies have shown that accurate descriptions of the cloud droplet effective radius (Re) and the autoconversion process of cloud droplets to raindrops (Ar) can effectively improve simulated clouds and surface precipitation, and reduce the uncertainty of aerosol indirect effects in GCMs. In this paper, we implement cloud microphysical schemes including two-moment Ar and Re considering relative dispersion of the cloud droplet size distribution into version 4.1 of the Institute of Atmospheric Physics’s atmospheric GCM (IAP AGCM 4.1), which is the atmospheric component of the Chinese Academy of Sciences’ Earth System Model. Analysis of the effects of different schemes shows that the newly implemented schemes can improve both the simulated shortwave and longwave cloud radiative forcings, as compared to the standard scheme, in IAP AGCM 4.1. The new schemes also effectively enhance the large-scale precipitation, especially over low latitudes, although the influences of total precipitation are insignificant for different schemes. Further studies show that similar results can be found with the Community Atmosphere Model, version 5.1.摘 要前人的研究结果指出云滴有效半径和云水自动转化过程的精确参数化可以有效的提高云和降水的模拟, 同时也可以减少模式给出的气溶胶间接效应的不确定性. 本研究在 IAP AGCM 4.1 中耦合了考虑云滴谱离散度的云滴有效半径和双参数云水自动转化过程的参数化方案. 研究结果显示, 该新云微物理方案可以明显的提高云的短波辐射和长波辐射的模拟. 另外, 新方案可以有效的增加模式的大尺度降水, 特别是低纬度大尺度降水. 进一步的结果表明, 耦合新方案的 CAM5.1 同样也可以更好模拟云的辐射强迫.