Dependence of the effect of aerosols on cirrus clouds on background vertical velocity
Highlights
► This study identifies mechanisms of aerosol-cloud interactions in cirrus clouds. ► Dependence of the mechanisms on background vertical velocity is examined. ► Aerosol-cloud interactions in cirrus clouds should consider background conditions.
Introduction
Cirrus clouds cover approximately 20–25% of the globe and as much as 70% over the tropics and, thus, can act as one of major modulators of global radiation budget (Liou, 1986, Liou, 2005). Hence, the effect of aerosols on cirrus clouds may have contributed to changing global radiation budget and to climate change since industrialization.
Lee et al. (2009) showed that aerosols can lead to significant changes at the top of the atmosphere (TOA) longwave radiation as well as shortwave radiation through their effects on cirrus clouds which have their origins in strong deep-convective vertical motions. About half of the large-scale cirrus clouds have their origins in the upper layers detrained from deep, precipitation cloud systems (Houze, 1993). The other half of cirrus clouds has their origins in large-scale vertical motion, much lower than vertical motion in deep convective clouds. Different dynamic intensities are likely to lead to different interactions between microphysics and dynamics by affecting the magnitude of deposition and sublimation. Hence, the properties of cirrus clouds and thus the effect of aerosols on them with the weak large-scale motion are likely to be different from those coupled with the strong deep-convective motion.
Starr and Cox (1985) investigated cirrus clouds developing with a large-scale vertical motion which was not associated with deep convective clouds. They reported that large-scale vertical motion significantly affected cirrus-cloud radiative and microphysical properties. This report implies that aerosol–cloud interactions are likely to depend on large-scale vertical motion. That a varying large-scale vertical motion is likely to accompany changing supersaturation and thus nucleation rate supports this implication (Starr and Cox, 1985). Supersaturation and nucleation rate basically determine an initial variation of cloud properties such as CINC and IWP (two most important estimates of the effect of aerosols on clouds) induced by an aerosol variation. Thus, different large-scale vertical motions are likely to lead to the different effects of aerosols on clouds.
In this study, the effect of aerosols on cirrus clouds developing with large-scale vertical motion (not associated with deep convective motions) is examined using a cloud-system resolving model (CSRM). Then, the dependence of this effect on large-scale vertical motion is examined using the CSRM.
Section snippets
Dynamics and turbulence
For numerical experiments, the Goddard Cumulus Ensemble (GCE) model (Tao et al., 2003) is used as a three-dimensional nonhydrostatic compressible model. The detailed equations of the dynamical core of the GCE model are described by Tao and Simpson (1993) and Simpson and Tao (1993).
The subgrid-scale turbulence used in the GCE model is based on work by Klemp and Wilhelmson (1978) and Soong and Ogura (1980). In their approach, one prognostic equation is solved for the subgrid-scale kinetic energy,
Case descriptions
A case of cirrus clouds located at (18° N, 30° W) off the coast of Western Africa is simulated here. A pair of simulations from 6 LST (local solar time) on July 1st to 18 LST on July 1st in 2002 is performed in which the aerosol concentration is varied from the PI level to the PD level. Henceforth, this case is referred to as “CONTROL” and the simulation with the PD (PI) level is referred to as the PD-aerosol (PI-aerosol) run, henceforth.
Reanalysis data from the European Centre for Medium-Range
Idealized cases
This study aims to isolate the varying role of aerosol–cloud interactions in the cloud-mass response to aerosols with varying large-scale vertical velocity. For the isolation, the simulations in CONTROL are repeated with differences only in large-scale vertical velocity. For the first of these idealized cases, the initial large-scale vertical velocity between 12 and 15 km in altitude in CONTROL, multiplied by a factor of 2, is applied. This idealized case is referred to as “W-M2”. For the other
Cloud properties
Fig. 4 shows the temporal evolution of the domain-averaged IWP. Clouds form earlier with larger background vertical velocity as shown in comparison among Fig. 4a, b, c, and d. Fig. 4 shows that the IWP in the PD-aerosol run is generally higher than that in the PI-aerosol run by 0.1–1.0 g m− 2 during the time integration in CONTROL and W-M2. The time- and domain-averaged IWPs are 6.26 (6.01) and 8.32 (8.03) g m− 2 for the PD (PI) aerosol case in CONTROL and W-M2, respectively. Unless otherwise
Summary and conclusion
Aerosol–cloud interactions in cirrus clouds developing with the large-scale vertical motion off the coast of Western Africa in the summer in 2002 were simulated using a CSRM coupled with a double-moment microphysics. Also, the dependence of these interactions on large-scale vertical motion is examined by varying the intensity of large-scale vertical motion.
Simulations showed that increasing aerosols increased IWP with a relatively low large-scale vertical motion (as shown in CONTROL and W-M2).
Acknowledgments
The authors wish to thank Dr. Wei-Kuo Tao and Derek Posselt for providing the GCE coupled with the double-moment microphysics used here and for valuable discussions.
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