Abstract
Abstract. The study attempts to evaluate the aerosol indirect effects over tropical oceans in regions of deep convection applying a three-dimensional cloud-resolving model run over a doubly-periodic domain. The Tropics are modelled using a radiative-convective equilibrium idealisation when the radiation, turbulence, cloud microphysics and surface fluxes are explicitly represented while the effects of large-scale circulation are ignored. The aerosol effects are modelled by varying the number concentration of cloud condensation nuclei (CCN) at 1% supersaturation, which serves as a proxy for the aerosol amount in the environment, over a wide range, from pristine maritime (50 cm−3) to polluted (1000 cm−3) conditions. No direct effects of aerosol on radiation are included. Two sets of simulations have been run: fixed (non-interactive) sea surface temperature (SST) and interactive SST as predicted by a simple slab-ocean model responding to the surface radiative fluxes and surface enthalpy flux. Both sets of experiments agree on the tendency of increased aerosol concentrations to make the shortwave cloud forcing more negative and reduce the longwave cloud forcing in response to increasing CCN concentration. These, in turn, tend to cool the SST in interactive-SST case. It is interesting that the absolute change of the SST and most other bulk quantities depends only on relative change of CCN concentration; that is, same SST change can be the result of doubling CCN concentration regardless of clean or polluted conditions. It is found that the 10-fold increase of CCN concentration can cool the SST by as much as 1.5 K. This is quite comparable to 2.1–2.3 K SST warming obtained in a simulation for clean maritime conditions, but doubled CO2 concentration. Assuming the aerosol concentration has increased from preindustrial time by 30%, the radiative forcing due to indirect aerosol effects is estimated to be −0.3 W m−2. It is found that the indirect aerosol effect is dominated by the first (Twomey) effect. Qualitative differences between the interactive and fixed SST cases have been found in sensitivity of the hydrological cycle to the increase in CCN concentration; namely, the precipitation rate shows some tendency to increase in fixed SST case, but robust tendency to decrease in interactive SST case.
Highlights
The aerosol amount in the environment, over a wide range, from pristine maritime (50 cm−3) to polluted (1000 cm−3)
The source of cloud droplet number concentration is described assuming that the number of activated cloud condensation nuclei (CCN) depends on the supersaturation according to the power-law N = CCCNSk, where CCCN is the prescribed concentration of CCN active at 1 % supersaturation S, and k is a constant set in this study to 0.4, which is a typical value for maritime conditions (Pruppacher and Klett, 1997)
It is apparent that the sea surface temperature (SST) change in response to the same absolute change of CCN count is substantially higher for clean conditions than polluted conditions
Summary
The CRM used in this study is the System for Atmospheric Modeling (SAM; Khairoutdinov and Randall, 2003), version 6.8. The source of cloud droplet number concentration is described assuming that the number of activated cloud condensation nuclei (CCN) depends on the supersaturation according to the power-law N = CCCNSk, where CCCN is the prescribed concentration of CCN active at 1 % supersaturation S, and k is a constant set in this study to 0.4, which is a typical value for maritime conditions (Pruppacher and Klett, 1997). The SST Ts can be specified or calculated using a simple slab-ocean model. In this model, the ocean mixed layer with prescribed depth h and heat capacity of water cw can change its heat content per unit area cwhTs through the surface radiation fluxes, enthalpy fluxes and prescribed ocean-transport flux (so called q-flux). The surface fluxes can vary horizontally depending on the atmospheric conditions above; the SST is horizontally uniform
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
