Simulation of Aerosol Indirect Effects on Cloud Streets Over the Northwestern Pacific Ocean

Abstract

We simulated aerosol effects on cloud streets formed during continental cold‐air outbreaks over the Northwestern Pacific using an aerosol‐sensitive cloud microphysical scheme coupled with the Weather Research and Forecast model. The micro‐ and macro‐scale responses were examined under a wide range of aerosol concentrations. The cloud streets generally remain in a transient state, indicating that the aerosol particles may significantly affect the cloud system's development. The simulation results revealed a clear Twomey effect, but they were contrary to some of the Albrecht effect, especially under relatively clean conditions. Such reversed Albrecht effects stem mainly from dynamic response to changes in atmospheric stability associated with drizzle evaporation in the sub‐cloud layer. The dynamic response works against thermal forcing from the large air‐sea temperature gradient in a cold‐air outbreak. As the aerosol increases, the drizzle mechanisms weaken, and the boundary layer becomes more convective, leading to stronger dry‐air entrainment from the free troposphere. In individual clouds, the effect of entrainment on cloud water content may be compensated by more substantial vertical vapor flux. These mechanisms lead to a reduction in the cloud amount. Furthermore, the reduced moisture flux from drizzle evaporation in the sub‐cloud layer, as well as the reduced sedimentation speed of the smaller cloud drops, tend to lessen cloud thickness. The overall reduction in cloud dimensions may offset the Twomey effect by up to 20% in cloud albedo.

Global evidence of aerosol-induced invigoration in marine cumulus clouds

“…While increased mixing with the free atmosphere may lead to cloud deepening, when the boundary layer unstable, it may also lead to an early onset of cloud breakup processes through evaporation-entrainment (Small et al, 2009). In some cases, increased entrainment and evaporation at the cloud top could lead to reduced cloud top heights, which is the opposite of an invigoration effect (Xue and Feingold, 2006). Whether the growth of a particular cloud is enhanced or inhibited may depend on the distribution of liquid water near the cloud top and the ability of the cloud to penetrate the free atmosphere.…”

“…Below-cloud evaporation and its associated cooling destabilizes the boundary layer, which could then further invigorate the cloud layer through amplified turbulence (Xue and Feingold, 2006). The effects of below-cloud evaporation on the stability are sensitive to the location of the cooling and drop size of the cloud; it is possible in some circumstances that cooling can act to stabilize the boundary layer.…”

Observing severe precipitation near complex topography during the Yilan Experiment of Severe Rainfall in 2020 (YESR2020)

“…Previous investigations of precipitation processes in marine stratocumulus have been mainly focused on the formation of drizzle and light rain associated with the changes in cloud microphysics and dynamics, predominantly for the persisting low cloud decks over the northeastern and southeastern Pacific (Wood, 2012). Over the northwestern Pacific, where the East Asian winter monsoon is active, transient marine stratocumulus clouds are formed in the regions where the near-surface northeasterly monsoonal wind is enhanced by the synoptic systems (Koike et al, 2012;Koike et al, 2016;Liu et al, 2016;Wu and Chen, 2021), as shown in the example in Figure 1a. With the presence of the marine stratocumulus, the boundary layer is moistened through enhanced vertical mixing (Lilly, 1968).…”