Marine stratocumulus clouds

Transcription

1 Marine stratocumulus clouds E. Pacific E. Atlantic Subtropical high-p regions Typical cold sea surface temperatures, in conjunc6on with the subsidence associated with the subtropical high, are responsible for the forma6on of a shallow cool moist marine boundary layer capped by a strong inversion. Marine stratus clouds form in this marine boundary layer. h=p:// /euromet/courses/english/nwp/n9l00/n9l00009.htm

2 How do these clouds form? Over the (warm) ocean, sensible and latent heat fluxes into the BL create buoyant plumes BL grows by entraining drier air Eventually deepens above LCL cloud forms shift from dynamics of a dry BL to that of moist BL (From Houze, 1993)

3 (From Houze, 1993) The well- mixed boundary layer h is the depth of the BL This jump at cloud top is maintained by subsidence, which maintains a high potential temperature and low dew point just above the BL q T = total water = q V + q L (conserved if BL is not precipitating) θ e = equivalent potential temperature, also conserved in dry adiabatic motion By conservation of mass, dh (entrainment velocity, typically > 0, dt = w + w(h) e Typically, w(h) < 0 (divergent) + mean vertical velocity at h, i.e. the rate of change of height associated with net horizontal convergence or divergence in the mixed layer)

4 Asides: Equivalent poten6al temperature / mixing The poten6al temperature that a parcel of air would have, if all its water vapor was condensed, and the latent heat converted into sensible heat: where

5 Cloud- topped boundary later Entrainment (deepening h) works against the sinking by large-scale subsidence In dry BL, surface fluxes of heat and moisture are the main source of kinetic energy In MOIST BL, negative buoyancy occurs at the top of the cloud radiation cooling and evaporative cooling due to entrainment of dry air drive the motion in the CTBL Notice importance of radiation cooling at cloud top that has been introduced in this figure! Radiation continuously DEstabilizes the laps rate to maintain the cloud as an unstable, turbulent layer (Cotton, Bryan, van den Heever)

6 Cloud water contents Consider the cloud liquid water content, star6ng at cloud base and ignoring the role of the kine6cs of CCN ac6va6on and drop growth. The adiaba6c liquid water content should be condensed out at any height z (i.e., enough water to bring the environment down to satura6on at the temperature T(z)). This yields the classic linear profile of liquid water (g water / m 3 air). What does ver6cal profile of CCN or drop number concentra6on look like? Mean drop size? As h increases, LWP increases Stevens, 2006)

7 Aerosol Indirect Effects on Climate [Chapter 22, Seinfeld and Pandis; IPCC] The indirect aerosol effects are hypothesized to be caused by anthropogenic emissions of aerosols and their precursor gases, and their links to cloud forma6on

8 Postulated indirect effects First indirect aerosol effect ( Twomey effect ; Twomey, 1977) for a constant cloud water content, more aerosols lead to more and smaller cloud droplets larger op6cal depth more reflec6on of solar radia6on MODIS true-color satellite image (04/03/2009)! (From:

9 Postulated indirect effects Second indirect aerosol effect ( Cloud lifecme effect ; Albrecht, 1989) the more and smaller cloud droplets do not collide as efficiently decrease drizzle forma6on increase cloud life6me more reflec6on of solar radia6on MODIS true-color satellite image (04/03/2009)! (From:

10 Postulated indirect effects Semi- Direct aerosol effect (Hansen et al., 1997) absorp6on of solar radia6on by black carbon within a cloud increases the temperature decreases rela6ve humidity evapora6on of cloud droplets more absorp6on of solar radia6on (opposite sign as other indirect effects) NASA scien6sts announced a giant, smoggy atmospheric brown cloud, which forms over South Asia and the Indian Ocean, has intercon6nental reach. The scien6sts discussed the massive cloud's sources, global movement and its implica6ons. The brown cloud is a moving, persistent air mass characterized by a mixed- par6cle haze. It also contains other pollu6on, such as ozone. (AGU, 2004)

11 What is the basis for these hypothesized linkages? Aerosol cloud condensa6on nuclei (CCN) rela6onships: CCN concentra6ons are observed to be higher in con6nental airmasses than in the marine atmosphere (e.g., order of 5000 cm - 3 vs. 10 cm - 3 ) Remember [CCN] is reported for a par6cular supersatura6on Soluble par6cles are easier to ac6vate to cloud drops Larger par6cles are easier to ac6vate to cloud drops Aerosol cloud drop number concentra6ons (CDNC) rela6onships: CDNC are observed to be higher in con6nental clouds than in marine clouds

12 A signal has been looked for in satellite and in- situ data other than the specialized case of ship tracks The remote sensing studies show that polluted clouds are more reflec6ve, and show correlacons between aerosol op6cal depth and cloud op6cal depth (and nega6ve correla6on between AOD and cloud drop effec6ve radius). The in- situ measurements find rela6onships between [CCN] and drizzle and effec6ve radius in marine stratocumulus. An example of observa6ons linking cloud reflec6vity to pollu6on is shown below (Brenguier et al., 2000). IPCC TAR concludes that these studies leave li=le doubt that anthropogenic aerosols have a non- zero impact on warm cloud radia6ve forcing. This forcing is es6mated to be 0.7 to 1.7 W m - 2 over oceans.

13 The CLAW Hypothesis R. Charlson, J. Lovelock, M. Andreae and S. Warren (1987). Oceanic phytoplankton, atmospheric sulphur, cloud albedo and climate. Nature, 326,

14 The CLAW Hypothesis we now have evidence that some of the steps within the CLAW hypothesis are correct but we s6ll don't know whether the system really operates as a nega6ve feedback loop. This makes it very difficult to represent the process in climate models and so we are s6ll unsure quite how important DMS is to the cooling of our planet.

15 Modeling indirect effects Anthropogenic emissions (of SO 2, combus6on and biomass burning aerosols, and mineral dust aerosols) CCN CCN CDNC CDNC cloud op6cal depth cloud op6cal depth cloud albedo cloud albedo changes in short- wave forcing

16 To relate CDNC to cloud opccal depth: First, consider a spa6ally uniform cloud of depth h with a drop number concentra6on distribu6on n(r) based on drop radius, r. Ex6nc6on of radia6on by the cloud is given by, The op6cal depth of the cloud, τ c, is the product b ext h. If the cloud drop size distribu6on is monodisperse, with number concentra6on N and radius r e (effec6ve radius), then since for large size parameters (sa6sfied by 10 micron drops with visible wavelengths), Q approaches 2.

17 (con6nued) The cloud liquid water content, in g water per cubic meter air, is (where rho is the density of water), Combining, So we can see that the cloud op6cal depth depends on the liquid water content, physical thickness, and mean radius of the cloud drops.

18 (con6nued) We could also express the op6cal depth in terms of N, by replacing effec6ve radius with its defini6on in terms of L (see above): So cloud op6cal depth is propor6onal to liquid water content to the 2/3 power, physical thickness, and number concentracon of cloud drops to the 1/3 power.

19 Rela6ng cloud albedo (reflec6vity) to op6cal depth: (Seinfeld and Pandis) cloud becomes completely reflec6ve if it has an op6cal depth much bigger than 7.7

20 Sample calculated rela6onships For a costant liquid water content and constant cloud thickness (isolines), cloud albedo increases with increasing CDNC. For example, for a 50 m thick cloud, changing CDNC from 100 cm- 3 to 1000 cm- 3 (like a change from clean marine to con6nental), the cloud albedo doubles.

21 Cloud Susceptibility to Meteorology versus Aerosol FIRE July 2009 case study θ = 287 K, qt = 10 g/kg zt=600 m, zb=200 m, H=400 m, W=220 gm-2, dzb/dqt=150 m/gkg-1 A CDNC doubling is balanced by a decrease of the cloud thickness of ΔH=H/ 5=80 m, i.e. Δqt=-0.5 g/kg, equivalent ΔT=0.8 K. (Δqt=-0.12 g/kg or ΔT=0.2 K for a 100 m thick cloud layer). J. L. Brenguier, Météo-France CNRS CNRM/GAME AAAR Orlando, 5 October 2011

22 Cloud Susceptibility to Meteorology versus Aerosol In MBL clouds, the susceptibility to the meteorology is two orders of magnitude greater than to the aerosols. Assuming we observe a pristine and a polluted cloud system, and detect different trends in LWP, it is difficult to attribute these differences to the aerosols because the small differences in meteorological forcings that could also explain the LWP changes are not measurable! Statistical approaches are necessary to filter out the variability of the meteorology (same methodology as in weather modification assessments) J. L. Brenguier, Météo-France CNRS CNRM/GAME AAAR Orlando, 5 October 2011