Aerosol-cloud-precipitation interaction in ultraclean layers and optically thin veil cloud system in the stratocumulus to cumulus transition: remote sensing measurements and cloud resolving model results

Abstract

Recent observational studies have shown that ultraclean layers (UCLs) and optically thin veil clouds associated with precipitating deep cumulus are common features of the marine boundary layer in the stratocumulus to cumulus transition. The very low number concentration of cloud droplet and cloud condensation nuclei in UCLs, strong precipitation in the associated cumulus, together with the low optical thickness of optically thin veil clouds, make such a system particularly appealing for the study of aerosol-cloud-precipitation interactions. More importantly, low cloud radiative properties biases (i.e., too few, too bright low cloud bias) in the current generation of global climate models (GCMs) seems strongly associated with the uncertainty in representing optically thin veil clouds, and these clouds may serve as an important constraint on the too few, too bright problem. However, systematic investigation of (1) global contribution and seasonal variability of optically thin veil clouds and (2) aerosol-cloud-precipitation interactions in UCLs and optically thin veil clouds is still lacking. We aim to investigate these problems with aircraft remote sensing, satellite measurements and a cloud resolving model. The dissertation is organized into the following three sections: • Using high resolution aircraft remote sensing measurement to characterize optically thin veil clouds in the stratocumulus to cumulus transition (SCT): Aircraft remote sensing measurements (i.e., lidar and radar) taken abroad NSF/NCAR GV-HIAPER research flights flown during the Cloud System Evolution in the Trades field campaign (CSET) sampled marine air masses between Sacramento, California (38.68N, 121.58W), and Kona (19.68N, 156.08W) are used in our study. Optically thin veil clouds, defined as the subset of low clouds with cloud bases > 1 km that do not fully attenuate high-spectral-resolution lidar signal (HSRL) (i.e., indicating optical depths <3), comprise considerable cover of low clouds (~ 40 %) over the SCT. It is found that optically thin veil clouds are also geometrically thin with cloud thickness ~ 200 m, and commonly reside in the upper boundary layer with average cloud base > 1.5 km. • Investigating deeper, precipitating PBLs associated with optically thin veil clouds in the Sc-Cu Transition using spaceborne satellite measurements: Variability and vertical structure of optically thin veil clouds over SCT regions around the globe are investigated using both passive and active satellite observations. These observations reveal pronounced relationships between optically thin veil clouds, strong precipitation, deep planetary boundary layer (PBL) height and low cloud droplet number concentration (CDNC). The results are in agreement with the hypothesis that the low optical thickness of veil clouds over the SCT is contingent on the low CDNC caused by strong precipitation scavenging occurring in active cumuli, a process whose efficiency is strongly dependent on maximum condensate amount in updrafts and thus is highly constrained by PBL height. • Exploring aerosol-cloud-precipitation processes in UCLs and optically thin veil clouds system using a cloud resolving model: Characteristics of UCLs and optically thin veil clouds are investigated in the cloud resolving model (CRM). The domain mean cloud and aerosol properties in UCLs and optically thin veil clouds from CRM simulations agree with recent observational studies in general. The simulation results show that the detrainment from active precipitating cumulus produces the stratiform veil clouds, which are strongly depleted in particle concentration due to very efficient coalescence-scavenging process in ascending parcels passing through cumulus towers. The simulation shows a median CDNC in thin veil clouds of 5.8 cm-3, implying that majority of thin veil clouds are UCLs as well and confirming the strong connection between veil clouds and UCLs. In addition, there is a strong correlation between surface precipitation and the fraction of low clouds that are UCLs, and such correlation implies the importance of precipitation scavenging for the formation of UCLs. A cloud resolving model coupled with a prognostic aerosol scheme is used in our study, enabling characterization of the spatiotemporal variability of aerosol in the boundary layer. The results show that depletion of aerosol concentration starts first in the upper boundary layer that is associated with in-cloud coalescence scavenging process. The evaporation of veil clouds leaves very low CCN number concentration (Na <10 cm-3) in the upper MBL, resulting in higher Na in the subcloud layer than in the cloud layer of the upper MBL. Slow vertical mixing in the decoupled MBL gradually dilutes the near-surface Na, and between upper MBL and surface mixed layer, there is a lag in Na of approximately one day, implying a mixing timescale between the upper and lower MBL of approximately this duration. This strong vertical stratification of the aerosol profile in the SCT is consistent with the observations in the recent field campaigns (i.e., CSET). Susceptibility of cloud fields to aerosol forcing in the SCT regime is investigated using a set of simulations in which initial aerosol concentrations are varied from 25 to 400 cm-3. It is found that the change in cloud cover caused by aerosol is fairly small, suggesting a weak second indirect effect on cloud cover. In the high initial aerosol concentration run, precipitation is weaker and LWP is thicker, but the analysis shows that condensate change only has minor effect on domain mean cloud optical depth. Instead, the increase of cloud optical depth is primarily driven by the increased cloud droplet number concentration (i.e., the Twomey effect), which drastically increases the magnitude of the short-wave cloud radiative effect. Therefore, we conclude that the radiative effect of cloud fields in the SCT regime could be perturbed by aerosol mostly through the first indirect effect. The increase in cloud albedo from the first indirect effect is expected to be stronger in low Na conditions (Platnick and Twomey 1994). However, in our simulations, the aerosol susceptibility of clouds in the SCT by the Twomey effect is found to be largely independent of the initial aerosol concentration. It is proposed that coalescence scavenging decreases the degree of perturbation to cloud Nd from the Twomey effect by depleting added CCN and droplets in a fairly short time scale in the SCT regime we investigate. Precipitation scavenging scales as Nd*P (where Nd is droplet concentration and P is precipitation). Nd increases with aerosol loading while P decreases with aerosol loading, which results in the maximum precipitation scavenging rate for the intermediate aerosol loading case. Such behaviors change the fundamental nature of the Nd v.s Na (aerosol loading) relationship, and this plays into the finding that the first aerosol indirect effect response to Na less nonlinear than be expected from the susceptibility construct of albedo.