The Marine Stratocumulus Yi Lu Aerosol, Cloud, and Climate. Class Paper April 8th, 2013
Aerosols Climate Clouds
Dust Wildfire Volcano Bk/Brn Carbon SOA Aerosols Climate Clouds
Dust Wildfire Volcano Bk/Brn Carbon SOA Aerosols Climate Clouds Cirrus Altostratus Cumulonimbus Stratocumulus
Outline The characterisic of the stratocumulus in the marine boundary layer What s the effect of stratocumulus on global scale radiaion budget? How s the stratocumulus interacts with the large scale circulaion? What s the connecion between aerosols and the stratocumulus?
The characterisic of the stratocumulus in the marine boundary layer Stratocumulus- Topped Boundary Layer (STBL) LCL
The characterisic of the stratocumulus in the marine boundary layer The stratocumulus- topped boundary layer (STBL) Decoupled STBL
The characterisic of the stratocumulus in the marine boundary layer Annual mean Shallow STBL Decoupled STBL
The characterisic of the stratocumulus in the marine boundary layer Temporal varia=on Diurnal Maxima tend to be during the early morning SynopIc Associated with ridging condiion Seasonal Maxima in the spring and early summerime (Klein and Hartmann 1993; Wood and Bretherton 2006; Richter and Mechoso 2004, 2006) Interannual Correlated with LTS (lower- troposphere stability), which is related to SST
Outline The characterisic of the stratocumulus in the marine boundary layer What s the effect of stratocumulus on global scale radiaion budget? How s the stratocumulus interacts with the large scale circulaion? What s the connecion between aerosols and the stratocumulus?
The effect of stratocumulus on global scale radiaion budget Longwave radia=on The leading term in the STBL energy budget The volume absorpion coefficient increases approximately linearly with liquid water mixing raio. (Plaa 1976; Pinnick et al. 1979) The mass absorpion coefficient is independent of cloud droplet size for droplet effecive radius smaller than the wavelength, but depends upon effecive radius when a significant fracion of the mass is contained in droplets larger than the wavelength. (Stephens 1978a; Chylek et al. 1992) => Considering the indirect effect of aerosols!!
The effect of stratocumulus on global scale radiaion budget Solar absorp=on The primary driver of the diurnal cycle The fracion of incident solar radiaion is of the order of a few percent up to around 15%. Absorbing species: - water vapor - Droplets containing nonsoluble absorbing material - intersiial aerosols Even with strong dayime solar insolaion the net effect of radiaion is, in most cases, to destabilize the stratocumulus layer.
The effect of stratocumulus on global scale radiaion budget Sca@ering of solar radia=on ReflecIon >> AbsorpIon Cloud albedo is governed by the cloud opical thickness, the single sca@ering albedo, the asymmetry parameter, and the solar zenith angle. The impacts of microphysical variability The microphysical suscepibility (dα/dn d ) is largest for α~50%, corresponding to LWP in the range 50-200 g/m 2.
The effect of stratocumulus on global scale radiaion budget Radia=ve driving of stratocumulus Longwave radiaion Cooling at the cloud top Solar absorpion AbsorpIon of solar radiaion at day Ime Scaaering of solar radiaion Significant across the visible and infrared
Outline The characterisic of the stratocumulus in the marine boundary layer What s the effect of stratocumulus on global scale radiaion budget? How s the stratocumulus interacts with the large scale circulaion? What s the connecion between aerosols and the stratocumulus?
InteracIons with the large- scale circulaion Large- scale divergence Subsidence large- scale divergence determines the subsidence rate Eq. boundary layer depth week divergence: decouple with MBL strong divergence: top of MBL below LCL Local meteorological forcing mean low- level divergence 2~4 X 10-6 s - 1 subsidence rates 2~4 mm/s relaxaion Ime scale 3~6 days => generally in equilibrium with local mesoscale forcing
InteracIons with the large- scale circulaion Lower- tropospheric stability DefiniIon of LTS potenial temperature in 700 and 1000 hpa Good correlaion between seasonal mean LTS and low- cloud amounts Stable - > strong inversion - > shallow MBL - > strong couple - > high cloud cover
InteracIons with the large- scale circulaion Free- tropospheric moisture Dry free- troposphere more elevated cloud base - > liqing LCL - > decreasing cloud thickness increasing evaporaive enhancement of entrainment increasing longwave cooling => stronger entrainment => higher cloud top TransiIon of cloud type strong entrainment - > dissipate/break up - > decouple - > forming cumulus below the stratocumulus in terms of deepening warming mechanism (Bretherton and Wyant 1997)
InteracIons with the large- scale circulaion Remaining issue Despite this top down approach, what is the role of stratocumulus in the large scale circulaion?
Outline The characterisic of the stratocumulus in the marine boundary layer What s the effect of stratocumulus on global scale radiaion budget? How s the stratocumulus interacts with the large scale circulaion? What s the connecion between aerosols and the stratocumulus?
The connecion to the aerosols Aerosols? Stratocumulus
The connecion to the aerosols Aerosols Droplet size Stratocumulus OpIcal depth PrecipitaIon RadiaIve cooling
The connecion to the aerosols Controlling factors of cloud droplet concentra=on Updraq strength Available CCN Twomey (1959) derived an analyical formulaion for the number of droplets acivated as a funcion of the CCN spectrum and the updraq speed. Extensions to Twomey s formulaion have been derived that account for more realisic variability in aerosol size distribuions, kineic effects, and more accurate treatments of the condensaion rate integral. (Cohard et al. 1998; Abdul- Razzak et al. 1998; Abdul- Razzak and Ghan 2000; Nenes and Seinfeld 2003) The aerosol size and the verical wind speed w as primary variables determine the frac=on of aerosols that ac=vate, f act.
The connecion to the aerosols Controlling factors of cloud droplet concentra=on Ø Updraq strength Available CCN Strong ascent - > raise the peak of supersaturaion - > reduce the min size of CCN acivated - > higher f act For high f act, the sensiivity to w weakens.
The connecion to the aerosols Controlling factors of cloud droplet concentra=on Updraq strength Ø Available CCN f act increases with the mean radius of the aerosols increases. (Abdul- Razzak et al. 1998; McFiggans et al. 2006) In clean condiions, the number of acivated droplets in stratocumuli approaches the accumulaion mode aerosol concentraion N a. (Gultepe and Isaac 2004; Twohy et al. 2005; Lu et al. 2007) Within a paricular stratocumulus cloud system where N a does not strongly vary, there is evidence that variability in N d is primarily controlled by variability in updraq speed.
Summary Stratocumulus are the most common cloud type covered the globe. Nearly ¼ of them are located over ocean region. The complexity of the stratocumulus study is due to the huge range of the scale, including the radiaion, turbulence, micro/macro- physical processes, from millimeters to tens of kilometers.
Summary (cont.) From the dynamics view, the large- scale divergence, free- tropospheric stability and moisture controls the stratocumulus. But its feedback to the climate dynamics is not included in this review. Cloud droplet size is the most important variable linking the stratocumulus to the aerosols. For which the indirect effect is dominated and controlled by the updraq and available CCN.
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