Stratocumulus clouds – A summary

Robert Wood (University of Washington, Seattle, Washington) has written a very nice overview article on Stratocumulus clouds referring to their climatology, their dynamics and microphysics. The paper was published in Vol. 140 (p. 2373-2423) of Monthly Weather Review. The paper can be downloaded at here: http://www.atmos.washington.edu/~robwood/papers/reviews/MWR-D-11-00121.1.pdf

In the following i want to sum up some of the most interesting facts Robert Wood lists about Stratocumulus clouds. I reordered the information in a slightly new way. All the references can be found below.

Definition


Stratocumulus cloud. Photo by en.wikipedia.org


Stratocumulus clouds are low-level convective clouds. They are in contrast to clouds like stratus driven by convective instability caused mainly by radiative cooling of the cloud top. In contrast to Cumulus clouds they are vertically constrained by a inversion on top of a well-mixed boundary layer.
The Stratocumulus cloud system is the product of a tight coupling between radiation, turbulence, and cloud microphysical processes occurring over a wide range of scales.

Occurrence

Sc clouds exist over both land and ocean, but are most regularly found over cold parts of the ocean (temperature being one of the main reasons for strong lower-tropospheric stability). Sc are associated with stable conditions at ridges. Frequency of occurrence is maximal at the east of the midtropospheric ridge line with cold-air advection and large-scale subsidence (Norris and Klein 2000).
Over continents the occurrence of Sc clouds is less frequent. There they are mainly concentrated in postfrontal cold air masses.
For 97% of Earth’s surface, stratocumulus clouds constitute 25% or more of the low-cloud cover.

Diurnal cycle

Stratocumulus clouds exhibit strong diurnal modulation largely due to the diurnal cycle of solar insolation the maximum coverage of stratocumulus tends to be during the early morning hours before sunrise (Rozendaal et al. 1995; Bergman and Salby 1996), diurnal maxima in cloud thickness and LWP also typically occur in the early morning hours. Drizzle too has a strong diurnal cycle (Leon et al. 2008), typically peaking during the early morning hours (although the peak time is more variable over land than over ocean).

Climate effect

Sc clouds have a negative net radiative effect. This is due to their behaviour in the longwave and solar spectrum. Sc clouds strongly reflect the incoming solar radiation but alter the outgoing longwave radiation emitted from the surface and lower atmospheric layers only slightly.
It was found that an increase in the overall area coverage by Sc cloud of 3.5% to 5% would be sufficient to offset the warming induced by a doubling of CO2 (Randall et al., 1984 and Slingo ,1990).
Cloud droplet number concentration is directly influenced by aerosol concentration. Therefore anthropogenic aerosol increase (e.g. by ships) increases the cloud droplet number concentration (CDNC). As found by Twomey this results in the first indirect aerosol effect and alters the cloud micropyhsics in further ways:

  • decreasing effective radius
  • increased cloud albedo
  • reduced droplet surface area
  • precipitation suppression
  • changes in evaporation and condensation rates (decreased droplet radius)
  • enhancement of entrainment through cloud droplet sedimentation suppression

Dynamics

Sc clouds typically form under statically stable atmospheric conditions. Buoyancy flux is mainly caused by the cloud top radiative cooling. But also latent heat release in the cloud help to maintain the convective structure (condensation in upward branch and evaporative cooling in downward branch) of the cloud with a sharp temperature inversion on top (for the latter 10-20K within a few meters are possible). Latent heat release results in a sharp increase in buoyancy flux directly above the cloud base. The main source for latent heat release by condensation of moisture is the surface.

Buoyancy flux is defined as

  \overline{w'b'} = (g/\theta_v) \overline{w' \theta_v'}

with g the gravity in [m/s^{-2}], \theta the virtual temperature and w the vertical velocity. Primed values define fluctuations of the value.

Fluctuations in the vertical wind speed mainly influence mixing within in the inversion layer. Fluctuations in the horizontal wind speed can cause local wind shear near the top of the Stratocumulus topped boundary layer even when mean shear is close to zero influencing cloud-top mixing indirectly.

Drizzle

Drizzle has direct influence on the dynamics of the Sc cloud system as it warms the cloud layer and thereby stabilizes the layer resulting in more stratification due to reduced turbulent mixing.
Maximum drizzle is found in early morning hours when also the greatest thickness and LWP were found (the latter is due to the fact that at night the bad solar radiation effect on the cloud is switched of (for details see next section)). Drizzle tend to evaporate when falling out of the cloud because of the small cloud droplet size.
Initial formation of drizzle requires coalescence of cloud droplets because growth by condensation to sizes larger than about 20 mm takes too long to explain observed precipitation growth in warm clouds (Jonas 1996).

The combined influence of LWP, droplet number concentration N_d and cloud thickness on precipitation rate is as follows:
For fixed LWP the precipitation rate is reduced with increasing N_d.
Precipitation rate increases with cloud thickness (Pawlowska and Brenguier 2003; vanZanten et al. 2005) or LWP (Wood et al. 2009b) or mean radius of the droplets (e.g., Gerber 1996).

Radiation

During daytime solar radiation results in a warming of the cloud due to absorption and therefore alters the longwave cooling at cloud-top which is one of the main drivers for the cloud dynamics. This explains why the manifestation of the cloud is most prominent during early morning hours.

The interacting of radiation with the cloud can be divided into the shortwave and longwave spectrum. Scattering is important at all wavelengths but absorption is most dominant in the thermal infrared. Scattering and absorption behaviour depend not only on wavelength but also on the cloud droplet size.

Longwave

Most of the longwave cooling is found in the uppermost few meters of the Sc cloud layer and is practically independent of droplet size. Only for those clouds with a very low droplet number concentration or low liquid water content (LWC) [e.g. Artic clouds, very clean subtopic clouds] the droplet radius must be taken into account for the longwave emissivity (potential to exhibit indirect aerosol effects in the infrared (Garrett et al. 2002)).

The volume absorption coefficient increases approximately linearly with liquid water mixing ratio q_l (Platt 1976; Pinnick et al. 1979) and can be expressed as \beta_l =\kappa_{\lambda} \rho q_l, with \rho the density of air, \kappa_l the spectrally dependent mass absorption coefficient.

Solar

As pointed out before the solar absorption has direct influence on the diurnal cycle of Sc clouds. With nonabsorbing aerosol clouds the solar absorption takes place mostly in the near-infrared (absorption band between 1.2 and 4 mm [Ramaswamy and Freidenreich 1991]). Nearly half of absorption is due to water vapour (increasing with temperature and thickness of the cloud). Solar heating due to absorption is biggest at cloud-top through to strong scattering that limits absorption further down.
Reflection is of bigger importance for Sc than absorption. The high cloud albedo is governed by optical thickness, the single-scattering albedo, the asymmetry parameter g, and the solar zenith angle (Liou 1992).

Microphysics

Typical climatological values of microphysical parameters for Sc clouds are:

  • LWP: marine stratocumulus 40 to 150 g m^{-2}
  • thickness: 200 to 500 m with a median of 320 m and tendency for thicker clouds (median=429 m) in mid-/high-latitudes
  • N_d: aerosol-rare conditions: 10 cm^{-3}; high-aerosol concentrations: 500 cm^{-3}; big ocean-continent contrasts

Liquid water mixing ratios q_l typically increase quasi-linear with height in Sc layers (adiabatic liquid water profile). The adiabatic rate of increase of q_l with height increases primarily with temperature and exhibits a weaker dependence upon pressure (Albrecht et al. 1990). Deviations from this adiabatic profile can be found at cloud top where entrainment causes evaporation. But also drizzle can result in subadiabtic layer or for heavy drizzle even in a nonlinear profile.

Optical thickness vary even for completely overcast Sc cloud fields (e.g., Roach 1961).

Impact of cloud droplet size on cloud optical thickness and precipitation. For a given cloud liquid water content the droplet radius is determined primarily by the cloud droplet concentration N_d, which is the key variable linking aerosol and cloud microphysical properties.

Aerosol size (acting as cloud condensation nuclei) and the vertical wind speed (typically < 1 m s^{-1} for Sc clouds) are the primary variables that determine the fraction of aerosols that activate.

  \tau = \frac{3}{2\rho_w} \int_0^h \frac{\rho q_l}{r_e} dz   \tau = \frac{9}{5\rho_w r_e(h)}

with

  L =\int_0^h \rho q_l dz

the LWP.

Expressed in terms of CDNC

  \tau = A_v \frac{(N_d k)^{\frac{1}{3}} L^{\frac{5}{6}}}{\rho_w^\frac{2}{3} \Gamma^{\frac{1}{6}}}

with A_v = (9/5)(8\pi^2/9)^{1/6} = 2.585 and k=(\frac{r_{vol}}{r_e})^3 with r_{vol} the mean volume radius.


References

  • Norris and Klein 2000: Low cloud type over the ocean from surface observations. Part III: Relationship to vertical motion and the regional synoptic environment. J. Climate, 13, 245–256.
  • Rozendaal et al. 1995: An observational study of the diurnal cycle of marine stratiform cloud. J. Climate, 8, 1795–1809.
  • Bergman and Salby 1996: Diurnal variations of cloud cover and their relationship to climatological conditions. J. Climate, 9, 2802–2820.
  • Leon et al. 2008: Climatology of drizzle in marine boundary layer clouds based on 1 year of data from CloudSat and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO). J. Geophys. Res., 113, D00A14, doi:10.1029/2008JD009835.
  • Slingo ,1990: Sensitivity of the Earth’s radiation budget to changes is low clouds. Nature, 343, 49–51.
  • Jonas 1996: Turbulence and cloud microphysics. Atmos. Res., 40, 283–306.
  • Pawlowska and Brenguier 2003: An observational study of drizzle formation in stratocumulus clouds for general circulation model (GCM) parameterizations. J. Geophys. Res., 108, 8630, doi:10.1029/2002JD002679.
  • vanZanten et al. 2005: Observations of the structure of heavily precipitating marine stratocumulus. J. Atmos. Sci., 62, 4327–4342.
  • Wood et al. 2009b: Understanding the importance of microphysics and macrophysics for warm rain in marine low clouds. Part II: Heuristic models of rain formation. J. Atmos. Sci., 66, 2973–2990.
  • Gerber 1996: Microphysics of marine stratocumulus with two drizzle modes. J. Atmos. Sci., 53, 1649–1662.
  • Garrett et al. 2002: Aerosol effectson cloud emissivity and surface longwave heating in the Arctic. J. Atmos. Sci., 59, 769–778.
  • Platt 1976: Infrared absorption and liquid water content in stratocumulus clouds. Quart. J. Roy. Meteor. Soc., 102, 553–556.
  • Pinnick et al. 1979: Verfication of a linear relation between IR extinction, absorption and liquid water-content of fogs. J. Atmos. Sci., 36, 1577–1586.
  • Ramaswamy and Freidenreich 1991: Solar radiative line-by-line determination of water vapor absorption and water cloud extinction in inhomogeneous atmospheres. J. Geophys. Res., 96 (D5), 9133–9157.
  • Liou 1992: Radiation and Cloud Processes in the Atmosphere: Theory, Observation and Modeling. Oxford University Press, 504 pp.
  • Albrecht et al. 1990: Surface-based remote-sensing of the observed and the adiabatic liquid water content of stratocumulus clouds. Geophys. Res. Lett., 17, 89–92.
  • Roach 1961: Some aircraft observations of fluxes of solar radiation in the atmosphere. Quart. J. Roy. Meteor. Soc., 87, 346–363.


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2 Comments

  1. Anibal Rendon, October 4, 2012:

    Excellent sumary, I will use SOME oF IT it to prepare a PDF. I am a Peruvian Aviation Weather forecaster who is interested in using most of Mr. Woods findings to elavorate an operative forecasting method to forecast summer time precipitation in LIMA-PERU, Mr. WooDS Researchs ARE A RICH SOURCE OF KNOWLEDGE READY TO BE TRANSFERRED TO THE OPERATIONAL COMMUNITY, AND I WILL USE THIS PUBLICATION AS A FORECASTING BIBLE.tHANKS ANIBAL


  2. larrynewman, October 5, 2012:

    Totally agree about the rich source of knowledge. I’m still busy reading trough some more papers from Robert Wood.


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