Indirect warming effect from dispersion forcing

Yangang Liu, Peter H. Daum

Anthropogenic aerosols enhance cloud reflectivity by increasing the number concentration of cloud droplets, leading to a cooling effect on climate that is referred to as the Twomey effect1,2. Here we show that anthropogenic aerosols exert an additional effect on cloud properties that is derived from changes in the spectral shape of the size distribution of cloud droplets in polluted air and acts to diminish this cooling. This finding could help to improve our understanding of the indirect aerosol effect and its treatment in climate modelling.

An equation commonly used for examination of the indirect aerosol effect is

（1）

Where THORN; is the water density, re is the effective radius, L is the cloud liquid-water content and N is the number concentration of cloud droplets. The parameter beta; is an increasing function of the relative dispersion(ε) of the cloud droplet size distribution (ratio of the standard deviation to the mean radius), which is well described3,4 by

（2）

An increase (decrease) in effective radius causes a decrease (increase) in cloud reflectivity1,2. A prevailing assumption implicit in the evaluation of the indirect aerosol effect is that increasing aerosol loading does not

alter εor beta;hence . However, examination of data from field studies of the indirect aerosol effect shows that marine clouds classified as being polluted or having a continental origin generally have not only a larger N, but also a larger relative to unaffected marine clouds. Figure 1 shows the dependence of and beta;on N. The points connected by lines represent cases identified by different investigators as evidence for the indirect effect. In each case, the points with lower N were characterized as background clouds and the higher points were characterized as similar clouds that had been perturbed by anthropogenic aerosols. Eleven of the 13 cases show an increase in
εthat is concurrent with an increase in N, with negligible change to slight decreases in the other two; there is also a general increase in εwith N, as implied previously5–7.

One explanation for the simultaneous increase in εand N is that anthropogenic aerosols have a more complex chemical composition and a broader size distribution than marine aerosols, and that the more numerous small droplets formed in a polluted cloud compete for water vapour and broaden the droplet size distribution

Figure 1 Relation between the relative dispersion of cloud droplet size distribution,, and the number concentration of cloud droplets, N. Symbols indicate programs and/or references from which the data points were derived. Connected points represent cases previously identified as evidence for an indirect aerosol effect. The parameter is defined by equation (2). Green symbols (from ref. 8): triangle, FIRE, northeastern Pacific; crossed circles, SOCEX, Southern Ocean; filled circle, ACE1, Southern Ocean. Blue symbols: filled circles, ASTEX8, northeastern Atlantic; diamonds, SCMS8, Florida coast; filled triangles, Sounding9, ASTEX; filled squares, horizontal9, ASTEX; open inverted triangles, level 1; open upright triangles, level 2; open circles, level 3 — all from southwest of San Diego10; open diamonds, SCMS11; stars, vertical, ASTEX12; plus signs, horizontal, ASTEX12; multiplication signs, ASTEX13; squares, INDOEX, Indian Ocean (G. M. McFarquhar, personal communication). Red circles, MAST6,14,15, California coast.

compared with clean clouds that have fewer droplets and less competition. According to equations (1) and (2), an increase in εacts to negate the effect of increased N on effective radius and cloud reflectivity. Because this effect has been largely neglected in estimates of the indirect aerosol effect, cooling by an indirect aerosol effect is likely to have been overestimated. From the data presented in Fig. 1, we estimate that a 15% increase in N at N=100 cm^-3 causes a total forcing that ranges between 0.19 and 0.93 W m^-2 , which corresponds to a factor that is 10–80% lower than the 1.03 W m^-2 calculated for the Twomey effect alone2. The effect of the enhancement inε is evidently large enough to be considered in assessing the indirect aerosol effect, and understanding the relation between εand N will help to reduce the large uncertainty inherent in this effect.

Brookhaven National Laboratory, Upton,

New York 11973, USA

e-mail: lyg@bnl.gov

1. Twomey, S. Atmos. Environ. 8, 1251–1256 (1974).

2. Charlson, R. J. et al. Science 255, 423–430 (1992).

3. Liu, Y. amp; Daum, P. H. Geophys. Res. Lett. 27, 1903–1906 (2000).

4. Liu, Y. amp; Daum, P. H. Proc. 13th Int. Conf. On Clouds and

Precipitation, Reno, USA 586–589 (2000).

5. Martin, G. M., Johnson, D. W. amp; Spice, A. J. Atmos. Sci. 51,

1823–1842 (1994).

6. Ackerman, A. S. et al. J. Atmos. Sci. 57, 2684–2695 (2000).

7. McFarquhar, G. M. amp; Heymsfield, A. J. J. Geophys. Res. D 106,

28675–28698 (2001).

8. Yum, S. S. amp; Hudson, J. G. Atmos. Res. 57, 81–104 (2001).

9. Hudson, J. G. amp; Yum, S. S. J. Atmos. Sci. 54, 2642–2654 (1997).

10.Noonkester, V. R. J. Atmos. Sci. 41, 829–845 (1984).

11.Hudson, J. G. amp; Yum, S. S. J. Atmos. Sci. 58, 915–926 (2001)

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分散强迫引起间接变暖效应

Yangang Liu, Peter H. Daum

通过人为增加云滴的个数和浓度，来提高云层反射率，从而导致气候变冷的现象简称为图梅效应^[1,2]。这表明人为改变气溶胶可以对云的特性施以附加影响，在被污染的空气里云滴的分布是通过人为改变气溶胶在被污染的空气中尺寸分布的频谱形状变化而得，其作用是减少这种冷却云特性的附加效果。结果表明，人为改变气溶胶对云特性产生的附加效果。污染的空气中云的大小分布随着光谱的变化而变化。这一发现可能有助于提高我们间接的了解气溶胶效应及其在气候模型中的预测。

常用的间接气溶胶效应检验的方程是 ：

（1）

其中p是水的密度，r_e是有效半径，L是云液体水含量,N是云滴的个数浓度。参数beta;是相对分散的云滴尺寸分布的增函数（标准偏差与平均半径的比率），下面的方程进行了更好的描述^[1,2]：

（2）

有效半径的增加（减少）导致云层反射率^[1,2]的减少（增加）。同时可以假设隐含在间接气溶胶效应的条件是气溶胶加载不改变ε或增加beta;。然而，从间接气溶胶效应实地研究数据表明，被污染的海洋云和大陆的起源通常不仅有较大的N，ε是一个较大的相对于未受影响的海洋云。

图1所示，ε和beta;的变化取决于N。由线连接的点代表的是不同的研究者把以间接影响为证明的例子。在每一种情况下，用低位的N点分别表征为背景云，而高点被定性为已被人为扰动的气溶胶云相似。这13种情形的第11条所示，ε的增加并发与N的增加，其他两种情况可以忽略不计。正如前面所说的，ε与N也有普遍增加^[5-7]。

对于ε和N同时增加有一种解释是，人为气溶胶有更复杂的化学成分，比海洋气溶胶粒

图一：云滴大小分布ε和云滴浓度N的相对色散关系。符号显示程序和/或引用的数据点。连接的点代表之前确认的间接气溶胶效应的一种情况的证据。beta;定义为参数方程（2）。绿色代表（来自于ref .8）三角形，FIRE,和东北太平洋。十字圈，SOCEX,南大洋;实心圆圈代表ACE1,和南大洋。蓝色代表：实心圆圈 ASTEX^[8]，东北大西洋；小钻石SCMS^[8]代表佛罗里达海岸;实心三角形，探测⁹，ASTEX^[8]，倒转的空心三角，一级；向上的空心三角，二级；空心圆圈，三级；都来自于译圣地牙哥西南^[10]，正三角，SCMS^[11];小星星，垂直，ASTEX,加号，水平^[9]，ASTEX^[10]；乘号，ASTEX ^[13];正方形表示INDOEX印度洋(G. M. McFarquhar, personal communication).红色圆圈代表，MAST ^[6,14,15]，加利福尼亚海岸。

分布更广泛，在被污染的云中形成的更多小水滴和干净的云里形成的小水滴的分布和大小进行相比，污染的空气里的水滴和水蒸气有着更强的竞争力。

根据方程（1）和（2），ε的增加在云反射率和有效的半径上对N产生抵消作用。因为这种效应已经在很大的程度上被忽略了要估计的间接气溶胶效应。这样，间接地气溶胶冷却效应可能会被高估。从图1中给出的数据我们能估计N增长15%，N=100cm 3 引起总强迫在0.19 和0.93 W·m^-2之间，它对应于单独计算出来的图梅效应因子数值1.03 W·m^-2相比低于10-80%。

ε增强的效果显然足以达到被评估为间接气溶胶效应，并且认为ε和N之间的关系将会有助于减少这种效应中固有的不确定性。

Yangang Liu, Peter H. Daum

Brookhaven National Laboratory, Upton,

New York 11973, USA

e-mail: lyg@bnl.gov

1. Twomey, S. Atmos. Environ. 8, 1251–1256 (1974).

2. Charlson, R. J. et al. Science 255, 423–430 (1992).

3. Liu, Y. amp; Daum, P. H. Geophys. Res. Lett. 27, 1903–1906 (2000).

4. Liu, Y. amp; Daum, P. H. Proc. 13th Int. Conf. On Clouds and Precipitation, Reno, USA 586–589 (2000).

5. Martin, G. M., Johnson, D. W. amp; Spice, A. J. Atmos. Sci. 51, 1823–1842 (1994).

6. Ackerman, A. S. et al. J. Atmos. Sci. 57, 2684–2695 (2000).

7. McFarquhar, G. M. amp; Heymsfield, A. J. J. Geophys. Res. D 106, 28675–28698 (2001).

8. Yum, S. S. amp; Hudson, J. G. Atmos. Res. 57, 81–104 (2001).

9. Hudson, J. G. amp; Yum, S. S. J. Atmos. Sci. 54, 2642–2654 (1997).

10.Noonkester, V. R. J. Atmos. Sci. 41, 829–845 (1984).

11.Hudson, J. G. amp; Yum, S. S. J. Atmos. Sci. 58, 915–926 (2001).

12.Garrett, T. J. amp; Hobbs, P. V. J. Atmos. Sci. 52, 2977–2984 (1995).

13.Hudson, J. G. amp; Li, H. J. Atmos. Sci. 52, 3031–3040 (1995).

14.Noone, K. J. et al. J. Atmos. Sci. 57, 2729–2747 (2000).

15.Noone, K. J. et al. J. Atmos. Sci. 57, 2748–2764 (2000).

Competing financial interests: declared none.

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