Effect of the droplet activation process on microphysical properties of warm clouds (original) (raw)
Related papers
Journal of the Atmospheric Sciences, 2010
This study investigates the correlation patterns between cloud droplet effective radius (CDR) and cloud optical thickness (COT) of warm clouds with a nonhydrostatic spectral bin microphysics cloud model. Numerical experiments are performed with the model to simulate low-level warm clouds. The results show a positive and negative correlation pattern between CDR and COT for nondrizzling and drizzling stages of cloud development, respectively, consistent with findings of previous observational studies. Only a positive correlation is simulated when the collection process is switched off in the experiment, whereas both the positive and negative correlations are reproduced in the simulation with collection as well as condensation processes. The positive and negative correlations can also be explained in terms of an evolution pattern of the size distribution function due to condensation and collection processes, respectively. Sensitivity experiments are also performed to examine how the CD...
Microphysical relationships in warm clouds
Atmospheric Research, 2001
Relationships among cloud microphysical parameters from six aircraft projects carried out over different parts of the globe in different seasons are presented. Comparisons of project averages Ž . showed that projects with higher droplet 2-50 mm diameter concentrations generally had smaller Ž . droplets and less drizzle diameter ) 50 mm . Hence, when all projects were combined there were negative correlations between droplet concentrations and sizes. However, this was not the case for individual cloud penetrations where only 24% had negative droplet concentration-size correlations. For the stratocumulus clouds, these negative correlations were more likely to be found in clouds with greater droplet spectral broadening and drizzle. The small cumulus clouds in this study were unique in that 95% of the droplet concentration-size correlations were positive regardless of spectral broadening or drizzle. Thus, for the majority of clouds, droplets were smaller in parcels with lower droplet concentrations. Entrainment in these clouds was then consistent with homogeneous mixing that occurs locally and intermittently. The dearth of observations of larger droplets in diluted cloud parcels suggested that mixing of entrained air did not promote droplet growth. q
Journal of Geophysical Research, 2011
This study presents a global assessment of the sensitivity of droplet number to diabatic activation (i.e., including effects from entrainment of dry air) and its first order tendency on indirect forcing and autoconversion. Simulations were carried out with 5 the NASA Global Modeling Initiative (GMI) atmospheric and transport model using climatological metereorological fields derived from the former NASA Data Assimilation Office (DAO), the NASA Finite volume GCM (FVGCM) and the Goddard Institute for Space Studies version II' (GISS) GCM. Cloud droplet number concentration (CDNC) is calculated using a physically-based prognostic parameterization that explicitly includes 10 entrainment effects on droplet formation. Diabatic activation results in lower CDNC, compared to adiabatic treatment of the process. The largest decrease in CDNC (by up to 75%) was found in the Tropics and in zones of moderate CCN concentration. This leads to a global mean effective radius increase between 0.2 − 0.5 µm (up to 3.5 µm over the Tropics), a global mean autoconversion rate increase by a factor of 1.1 to 1.7 15 (up to a factor of 4 in the Tropics) and a 0.2-0.4 W m −2 decrease in indirect forcing. The spatial patterns of entrainment effects on droplet activation tend to reduce biases in effective radius (particularly in the Tropics) when compared to satellite retrievals. Considering the diabatic nature of ambient clouds, entrainment effects on CDNC need to be considered in GCM studies of the aerosol indirect effect.
Atmospheric Chemistry and Physics, 2011
Cloud-aerosol interaction is a key issue in the climate system, affecting the water cycle, the weather, and the total energy balance including the spatial and temporal distribution of latent heat release. Information on the vertical distribution of cloud droplet microphysics and thermodynamic phase as a function of temperature or height, can be correlated with details of the aerosol field to provide insight on how these particles are affecting cloud properties and their consequences to cloud lifetime, precipitation, water cycle, and general energy balance. Unfortunately, today's experimental methods still lack the observational tools that can characterize the true evolution of the cloud microphysical, spatial and temporal structure in the cloud droplet scale, and then link these characteristics to environmental factors and properties of the cloud condensation nuclei. Here we propose and demonstrate a new experimental approach (the cloud scanner instrument) that provides the microphysical information missed in current experiments and remote sensing options. Cloud scanner measurements can be performed from aircraft, ground, or satellite by scanning the side of the clouds from the base to the top, providing us with the unique opportunity of obtaining snapshots of the cloud droplet microphysical and thermodynamic states as a function of height and brightness temperature in clouds at several development stages. The brightness temperature profile
The effect of physical and chemical aerosol properties on warm cloud droplet activation
Atmospheric Chemistry and Physics, 2006
The effects of atmospheric aerosol on climate forcing may be very substantial but are quantified poorly at present; in particular, the effects of aerosols on cloud radiative properties, or the "indirect effects" are credited with the greatest range of uncertainty amongst the known causes of radiative forcing. This manuscript explores the effects that the composition and properties of atmospheric aerosol can have on the activation of droplets in warm clouds, so potentially influencing the magnitude of the indirect effect. The effects of size, composition, mixing state and various derived properties are assessed and a range of these properties provided by atmospheric measurements in a variety of locations is briefly reviewed. The suitability of a range of process-level descriptions to capture these aerosol effects is investigated by assessment of their sensitivities to uncertainties in aerosol properties and by their performance in closure studies. The treatment of these effects within global models is reviewed and suggestions for future investigations are made.
Atmospheric Chemistry and Physics
A new scheme of droplet nucleation at cloud base is implemented into the Hebrew University Cloud Model (HUCM) with spectral (bin) microphysics. In this scheme, supersaturation maximum <i>S</i><sub><mo>max</mo></sub> near cloud base is calculated using theoretical results according to which <i>S</i><sub><mo>max</mo></sub> ∼ <i>w</i><sup>3∕4</sup><i>N</i><sub>d</sub><sup>−1∕2</sup>, where <i>w</i> is the vertical velocity at cloud base and <i>N</i><sub>d</sub> is droplet concentration. Microphysical cloud structure obtained in the simulations of a midlatitude hail storm using the new scheme is compared with that obtained in the standard approach, in which droplet nucleation is calculated using supersaturation calculated in grid points. The simulations were performed with different concentrations of cloud condensational ...
Effects of in-cloud nucleation and turbulence on droplet spectrum formation in cumulus clouds
Quarterly Journal of the Royal Meteorological Society, 2002
Drop spectrum evolution is investigated using a moving mass grid microphysical cloud parcel model containing 2000 mass bins and allowing turbulent effects on droplet collisions. Utilization of precise methods of diffusion and collision drop growth eliminates any arti cial droplet spectrum broadening. Simulation of continental, intermediate and maritime clouds is conducted using different concentrations of cloud condensation nuclei and different vertical velocities at the cloud base.
Aircraft observations of cloud droplet number concentration: Implications for climate studies
Quarterly Journal of the Royal Meteorological Society, 2004
Droplet number concentration (N d) is a major parameter affecting cloud physical processes and cloud optical characteristics. In most climate models, N d is usually assumed to be constant or a function of the droplet and aerosol number concentration (N a). Three types of cloud systems over Canada, namely Arctic clouds, maritime boundary-layer clouds, and winter storms, were studied to obtain values of N d as a function of temperature (T). The probability density function of N d was also calculated to show the variability of this parameter. The results show that N d reaches a maximum at about 10 • C (200 cm −3) and then decreases gradually to a minimum (∼1-3 cm −3) at about −35 • C. A comparison of relationships between N d and N a indicates that estimates of N d from N a can have an uncertainty of about 30-50 cm −3 , resulting in up to a 42% uncertainty in cloud shortwave radiative forcing. This study concludes that the typical fixed values of N d , which are ∼100 cm −3 and ∼200 cm −3 for maritime and continental clouds, respectively, and the present relationships of N d to N a , could result in a large uncertainty in the heat and moisture budgets of the earth's atmosphere. It is suggested that the use of relationships between N d and T can improve climate simulations.
Adiabatic predictions and observations of cloud droplet spectral broadness
Atmospheric Research, 2005
The evolution of cloud droplet size spectra is calculated using an adiabatic condensational growth model. Broadness (e.g., standard deviation of diameter) of cloud droplet spectra in adiabatic cloud parcels was determined to be critically dependent on cloud supersaturation. Although droplet spectra become narrower as growth continues, the rate of narrowing is slower when cloud supersaturation is lower. This actually leads to broader droplet spectra for more continental clouds or for weaker updrafts because both of these conditions are associated with lower cloud supersaturations. More continental type clouds, which have higher concentrations of smaller droplets, were indeed found to have larger dispersions (standard deviation of diameter/mean diameter of cloud droplets). Some of these results were consistent with observations, but the larger dispersions that were much more commonly observed for continental compared to maritime clouds were due almost exclusively to smaller droplets rather than broader droplet distributions. Contrary to the model calculations, typical observations show that cleaner clouds usually have broader droplet spectra. The gaps in magnitude between theory and observations of broadness are significant in all clouds. When cloud parcels that had ascended under different updraft conditions were compared at a constant cloud altitude, parcels with lower updrafts were predicted to have broader droplet spectra with larger mean diameters. This trend of apparent spectral broadening was consistent with observations for some near-adiabatic cloud parcels.
Geophysical Research Letters, 2006
1] Using a large amount of aircraft measurements of cloud droplet size distributions, the relationship between cloud spectral relative dispersion (e) and cloud droplet number concentration (N c ) is studied. The results indicate that the value of e varies between 0.2 to 0.8 when the cloud droplet number concentration is low (about 50 cm À3 ), and converges toward a narrow range of 0.4 to 0.5 when the cloud number concentration is higher. Because the distribution of the cloud droplet size is an important parameter in estimating the first indirect radiative effect of aerosols on the climate system, the uncertainty in the corresponding radiative forcing can be reduced by 10-40% (depending on cloud droplet number density) under high aerosol loading.. This finding is important for improving climate change projections, especially for the regions where aerosol loading is high and continues to increase. Citation: Zhao, C., and et al. (2006), Aircraft measurements of cloud droplet spectral dispersion and implications for indirect aerosol radiative forcing, Geophys.