The importance of vertical velocity variability for estimates of the indirect aerosol effects

Abstract

Abstract. The activation of aerosols to form cloud droplets is dependent upon vertical velocities whose local variability is not typically resolved at the GCM grid scale. Consequently, it is necessary to represent the subgrid-scale variability of vertical velocity in the calculation of cloud droplet number concentration.This study uses the UK Chemistry and Aerosols community model (UKCA) within the Hadley Centre Global Environmental Model (HadGEM3), coupled for the first time to an explicit aerosol activation parameterisation, and hence known as UKCA-Activate. We explore the range of uncertainty in estimates of the indirect aerosol effects attributable to the choice of parameterisation of the subgrid-scale variability of vertical velocity in HadGEM-UKCA. Results of simulations demonstrate that the use of a characteristic vertical velocity cannot replicate results derived with a distribution of vertical velocities, and is to be discouraged in GCMs.This study focuses on the effect of the variance (σ 2 w ) of a Gaussian pdf (probability density function) of vertical velocity. Fixed values of σ w (spanning the range measured in situ by nine flight campaigns found in the literature) and a configuration in which σ w depends on turbulent kinetic energy are tested. Results from the mid-range fixed σ w and TKE-based configurations both compare well with observed vertical velocity distributions and cloud droplet number concentrations.The radiative flux perturbation due to the total effects of anthropogenic aerosol is estimated at −1.9 W m −2 with σ w = 0.1 m s −1 , −2.1 W m −2 with σ w derived from TKE, −2.25 W m −2 with σ w = 0.4 m s −1 , and −2.3 W m −2 with σ w = 0.7 m s −1 . The breadth of this range is 0.4 W m −2 , which is comparable to a substantial fraction of the total diversity of current aerosol forcing estimates. Reducing the uncertainty in the parameterisation of σ w would therefore be an important step towards reducing the uncertainty in estimates of the indirect aerosol effects.Detailed examination of regional radiative flux perturbations reveals that aerosol microphysics can be responsible for some climate-relevant radiative effects, highlighting the importance of including microphysical aerosol processes in GCMs.

Effects of atmospheric dynamics and aerosols on the fraction of supercooled water clouds

“…Bower et al (1996) found that the vigorous in-cloud updrafts in convective clouds do not leave enough time for supercooled droplets to transform into ice crystals, thus suppressing ice formation or pushing supercooled liquid water to a colder cloud top height. West et al (2014) concluded that the sub-grid vertical velocity enhancing leads to an increase of the liquid water path. Some studies also verified the importance of in-cloud vertical motions for supporting the growth of liquid water in Arctic mixed-phase clouds (Shupe et al, 2006(Shupe et al, , 2008.…”

The Met Office Unified Model Global Atmosphere 7.0/7.1 and JULES Global Land 7.0 configurations

“…2.3. Cloud condensation nuclei are activated into cloud droplets using the UKCA-Activate aerosol activation scheme (West et al, 2014). In addition to the GLOMAP-mode species listed in Table 3, we continue to model mineral dust separately using the CLASSIC dust scheme (Woodward, 2011), although a modal framework for the emission of mineral dust is being developed for future implementation.…”

“…Padé fits are used for the variation with effective radius, which is computed from the number of cloud droplets. In configurations using prognostic aerosol, cloud droplet number concentrations are not calculated within the radiation scheme itself but are calculated by the UKCA-Activate scheme (West et al, 2014), which is based on the activation scheme of Abdul-Razzak and Ghan (2000). Note that in simulations using 25 climatological rather than prognostic aerosol, the approach described here is not yet available and instead we use CLASSIC (Coupled Large-scale Aerosol Simulator for Studies in Climate, Bellouin et al (2011)) aerosol climatologies and the calculation of optical properties and cloud droplet concentrations described in Sect.…”

“…We also make use of multiple sub-time steps of the precipitation scheme, as in Posselt and Lohmann (2008) with one sub-time step for every two minutes of the model time step to achieve a realistic treatment of in-column evaporation. With prognostic aerosol, we use the UKCA-Activate aerosol activation scheme (West et al, 2014) to provide the cloud droplet number for autoconversion, where only soluble aerosol species (which can be composed of sulphate,…”

Coupling spectral‐bin cloud microphysics with the MOSAIC aerosol model in WRF‐Chem: Methodology and results for marine stratocumulus clouds

“…When r d > r [Kogan, 1991]. If aerosol activation occurs in a downdraft, the newly formed droplet radius is set to be 2.0 mm, since S is usually small in downdrafts and cannot be sustained [West et al, 2014]. The original calculation of sizes of fresh nucleated droplets in SBM follows Ivanova et al [1977] and offers another option: when r d < 0.03 mm, the radius of a nucleated droplet is the equilibrium size of its critical supersaturation.…”