Comparing parameterized versus measured microphysical properties of tropical convective cloud bases during the ACRIDICON–CHUVA campaign

Abstract

Abstract. The objective of this study is to validate parameterizations that were recently developed for satellite retrievals of cloud condensation nuclei supersaturation spectra, N CCN (S), at cloud base alongside more traditional parameterizations connecting N CCN (S) with cloud base updrafts and drop concentrations. This was based on the HALO aircraft measurements during the ACRIDICON-CHUVA campaign over the Amazon region, which took place in September 2014. The properties of convective clouds were measured with a cloud combination probe (CCP), a cloud and aerosol spectrometer (CAS-DPOL), and a CCN counter onboard the HALO aircraft. An intercomparison of the cloud drop size distributions (DSDs) and the cloud water content (CWC) derived from the different instruments generally shows good agreement within the instrumental uncertainties. To this end, the directly measured cloud drop concentrations (N d ) near cloud base were compared with inferred values based on the measured cloud base updraft velocity (W b ) and N CCN (S) spectra. The measurements of N d at cloud base were also compared with drop concentrations (N a ) derived on the basis of an adiabatic assumption and obtained from the vertical evolution of cloud drop effective radius (r e ) above cloud base. The measurements of N CCN (S) and W b reproduced the observed N d within the measurements uncertainties when the old (1959) Twomey's parameterization was used. The agreement between the measured and calculated N d was only within a factor of 2 with attempts to use cloud base S, as obtained from the measured W b , N d , and N CCN (S). This underscores the yet unresolved challenge of aircraft measurements of S in clouds. Importantly, the vertical evolution of r e with height reproduced the observation-based nearly adiabatic cloud base drop concentrations, N a . The combination of these results provides aircraft observational support for the various components of the satellite-retrieved methodology that was recently developed to retrieve N CCN (S) under the base of convective clouds. This parameterization can now be applied with the proper qualifications to cloud simulations and satellite retrievals.

Overview: Precipitation characteristics and sensitivities to environmental conditions during GoAmazon2014/5 and ACRIDICON-CHUVA

“…As noted by Molleker et al (2014), the CDP uncertainty is approximately 10 %. Braga et al (2017) performed a comparison between the CDP and the other HALO cloud probes and concluded that they agree within instrumental uncertainties. The vertical wind component (w) was measured by the Basic HALO Measurement and Sensor System (BAHAMAS) located at the nose of the aircraft and calibrated according to Mallaun et al (2015).…”

“…Cecchini et al (2017a) have also demonstrated that a 100 % increase in aerosol concentrations led to an 84 % increase in the concentration of droplets, but the same relative increase in vertical velocity corresponded only to a 43 % change. Braga et al (2017) have compared HALO microphysical probe measurements of cloud droplet concentrations with a parameterization based on CCN and updraft at the cloud base. Jäkel et al (2017) have presented a new methodology to retrieve the vertical distribution of the hydrometeor phase using cloud-side reflected solar radiation measurements and have discussed the mixed-phase layer as a function of aerosol loading.…”

Long-term observations of cloud condensation nuclei in the Amazon rain forest – Part 2: Variability and characteristic differences under near-pristine, biomass burning, and long-range transport conditions

“…In terms of sources and conditions, the ATTO and ZF2 sites are comparable (C. Pöhlker et al, 2017). The cCO data include ATTO site measurements from 2012 to 2017.…”

“…Briefly: The BT analysis is based on the hybrid single particle Lagrangian integrated trajectory model (HYSPLIT, NOAA-ARL) with meteorological input data from the global data assimilation system (GDAS1) (Draxler and Hess, 1998 in reddish colors; and (iv) a group of 'inland' trajectories in east-southeasterly (ESE) directions including clusters ESE1, ESE2, and ESE3 as well as one cluster towards the south-west (i.e., SW1). For a detailed characterization of the region, which is covered by the BTs and represents the ATTO site footprint with the trace gas, aerosol, and CCN sources that are presumably most relevant for the long-term observation at the ATTO site, we refer the reader to C. Pöhlker et al (2017).…”

“…These fires are localized along the Cuiabá-Santarém highway (BR-163). This 5 highway corridor is known as a region of intense logging and burning of primary forest and its conversion to cattle pasture (Nepstad et al, 2002;Fearnside, 2007; C. Pöhlker et al, 2017). The transport time of the smoke from the fires to the ATTO site is about 2-3 days (given by the BTs), which provides a reference for the residence time and aging of the aerosol particles in the atmosphere.…”