Stephen Noble

CCN and Vertical Velocity Influences on Droplet Concentrations and Supersaturations in Clean and Polluted Stratus Clouds

AbstractCloud microphysics and cloud condensation nuclei (CCN) measurements from two marine stratus cloud projects are presented and analyzed. Results show that the increase of cloud droplet concentrations Nc with CCN concentrations NCCN rolls off for NCCN at 1% supersaturation (S)N1% above 400 cm−3. Moreover, at such high concentrations Nc was not so well correlated with NCCN but tended to be more closely related to vertical velocity W or variations of W (σw). This changeover from predominate Nc dependence on NCCN to Nc dependence on W or σw is due to the higher slope k of CCN spectra at lower S, which is made more relevant by the lower cloud S that is forced by higher NCCN. Higher k makes greater influence of W or σw variations than NCCN variations on Nc. This changeover at high NCCN thus seems to limit the indirect aerosol effect (IAE).On the other hand, in clean-air stratus cloud S often exceeded 1% and decreased to slightly less than 0.1% in polluted conditions. This means that smaller CCN [those wit...

Cloud droplet spectral width relationship to CCN spectra and vertical velocity

[1] Cloud droplet spectral width (standard deviation; sigma; s) was inversely related to cloud condensation nuclei (CCN) concentrations (NCCN) in the Rain in Cumulus over the Ocean (RICO) project. This was determined from thorough comparisons between flight-averaged NCCN and microphysics of the lowest altitude cloud passes of 17 RICO flights. Adiabatic model predictions of droplet spectra based on complete below cloud CCN spectra and within-cloud vertical velocity (W) showed good agreement with measured droplet concentrations and mean diameter (MD) but s predictions were only weakly correlated with measured s. Significantly better s predictions were obtained for mixtures of droplet spectra for distributions of W that were measured in each flight. Adiabatic model predictions for various W applied uniformly to all RICO flights displayed a trend of correlation coefficients (R) for s-NCCN plotted against W that changed from positive to negative with increasing W. The s-NCCN positive R range at low W corresponds to previous results in stratus clouds where it has been suggested that albedo calculations that include s reduce the indirect aerosol effect (IAE). The s-NCCN negative R range of higher W corresponds to more convective clouds such as RICO where albedo calculations that include s thus might seem to augment IAE.

Cloud supersaturations from CCN spectra Hoppel minima: Hoppel Minima Cloud Supersaturations

High-resolution cloud condensation nucleus (CCN) spectral measurements in two aircraft field projects, Marine Stratus/Stratocumulus Experiment (MASE) and Ice in Clouds Experiment-Tropical (ICE-T), often showed bimodality that had previously been observed in submicrometer aerosol size distributions obtained by differential mobility analyzers. However, a great deal of spectral shape variability from very bimodal to very monomodal was observed in close proximity. Cloud supersaturation (S) estimates based on critical S, Sc, at minimal CCN concentrations between two modes (Hoppel minima) were ascertained for 63% of 325 measured spectra. These cloud S were lower than effective S (Seff) determined by comparing ambient CCN spectra with nearby cloud droplet concentrations (Nc). Averages for the polluted MASE stratus were 0.15 and 0.23% and for the cumulus clouds of ICE-T 0.44 and 1.03%. This cloud S disagreement between the two methods might in part be due to the fact that Hoppel minima include the effects of cloud processing, which push CCN spectra toward lower S. Furthermore, there is less cloud processing by the smaller cloud droplets, which might be related to smaller droplets evaporating more readily. Significantly lower concentrations within the more bimodal spectra compared with the monomodal spectra indicated active physical processes: Brownian capture of interstitial CCN and droplet coalescence. Chemical cloud processing also contributed to bimodality, especially in MASE.

Comparisons of CCN with Supercooled Clouds

Abstract More than 140 supercooled clouds were compared with corresponding out-of-cloud cloud condensation nuclei (CCN) measurements. In spite of significant differences in altitude, temperature, distances from cloud base, updraft velocity (W), entrainment, and so on, the correlation coefficients (R) between droplet and CCN concentrations were substantial although not as high as those obtained in warm clouds with less variability of nonaerosol influences. CCN at slightly lower altitudes than the clouds had higher R values than CCN measured at the same altitude. Ice particle concentrations appeared to reduce droplet concentrations and reduce R between CCN and droplet concentrations, but only above 6-km altitude and for temperatures below −20°C. Although higher CCN concentrations generally resulted in higher droplet concentrations, increases in droplet concentrations were generally less than the increases in CCN concentrations. This was apparently due to the expected lower cloud supersaturations (S) when CC...

MODIS comparisons with northeastern Pacific in situ stratocumulus microphysics

Abstract Vertical sounding measurements within stratocumuli during two aircraft field campaigns, Marine Stratus/stratocumulus Experiment (MASE) and Physics of Stratocumulus Top (POST), are used to validate Moderate Resolution Imaging Spectroradiometer (MODIS) cloud optical thickness (COT), cloud liquid water path (LWP), and cloud effective radius (r e). In situ COT, LWP, and r e were calculated using 5 m vertically averaged droplet probe measurements of complete vertical cloud penetrations. MODIS COT, LWP, and r e 1 km pixels were averaged along these penetrations. COT comparisons in POST showed strong correlations and a near 1:1 relationship. In MASE, comparisons showed strong correlations; however, MODIS COT exceeded in situ COT, likely due to larger temporal differences between MODIS and in situ measurements. LWP comparisons between two cloud probes show good agreement for POST but not MASE, giving confidence to POST data. Both projects provided strong LWP correlations but MODIS exceeded in situ by 14–36%. MODIS in situ r e correlations were strong, but MODIS 2.1 µm r e exceeded in situ r e, which contributed to LWP bias; in POST, MODIS r e was 20–30% greater than in situ r e. Maximum in situ r e near cloud top showed comparisons nearer 1:1. Other MODIS r e bands (3.7 µm and 1.6 µm) showed similar comparisons. Temporal differences between MODIS and in situ measurements, airplane speed differences, and cloud probe artifacts were likely causes of weaker MASE correlations. POST COT comparison was best for temporal differences under 20 min. POST data validate MODIS COT but it also implies a positive MODIS r e bias that propagates to LWP while still capturing variability.

Low‐altitude summer/winter microphysics, dynamics, and CCN spectra of northeastern Caribbean small cumuli, and comparisons with stratus

Low-altitude cloud microphysics and cloud condensation nuclei (CCN) spectra are presented and analyzed from two cumulus cloud aircraft field campaigns, Ice in Clouds Experiment-Tropical (ICE-T) and Rain in Cumulus over the Ocean (RICO), in the same eastern Caribbean location at opposite seasons. These are further compared with two previously analyzed stratus cloud aircraft campaigns off the central California coast, Physics of Stratocumulus Tops (POST) and Marine Stratus/Stratocumulus Experiment (MASE). Multiple regression analysis predictions of cloud droplet concentrations (Nc) based on CCN spectra, NCCN(S) where S is supersaturation, compared much better with measured Nc than single NCCN-Nc regressions in both cumulus projects. The addition of vertical velocity (W) to the single and multiple regressions showed small improvements. For RICO the multiple regression correlations were also superior to previous adiabatic model predictions of Nc also based on NCCN(S) and mean W. More adiabatic cloud parcels with considerably higher Nc, liquid water contents, and W showed only slightly better correlations than flight-averaged Nc of all low clouds. Results show the value of more extensive CCN spectra and the relative unimportance of W variations for determining Nc, in these Caribbean cumuli. The fact that flight-averaged Nc of all low cloud data was almost as well correlated with NCCN(S) as were Nc of more adiabatic cloud parcels indicates that entrainment did not significantly perturb CCN-Nc relationships. As should be expected higher cloud S were determined for the cumulus clouds than for the stratus clouds. Suppression of cloud S by higher NCCN that had previously been observed in stratus was observed in ICE-T but not in RICO where the NCCN range may have been too low for cloud S suppression. But ICE-T and POST even showed this S suppression over the same limited NCCN range as RICO (< 200 cm−3).