Response of Cloud Supersaturation to Radiative Forcing

Davies, R.

doi:10.1175/1520-0469(1985)042<2820:rocstr>2.0.co;2

Cited by 24 publications

(32 citation statements)

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“…If the updraft velocity and/or the flux divergence are steady on timescales longer than a few seconds, it can be shown that the supersaturation calculated by the integration of (4) -(6) relaxes to a quasi-equilibrium value Seq [Roach, 1976;Davies, 1985]. We use the approach of Davies [1985] to derive an expression for Seq in the Appendix; in a closed parcel it is determined predominately by the vertical velocity, the net flux divergence and the integral radius, I: where I -f rn(r)dr, al, a2 and a3 are slowly varying functions of the pressure and temperature and rs -1/(47rpla2GI) is the relaxation time (see, for example, Cooper [1989]). The overbars in (8) represent an average weighted by the integral radius, while the angle brackets represent an average weighted by the droplet flux divergence (see the Appendix).…”

Section: Es(t) Es(t)(1 + E/wv)mentioning

confidence: 99%

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Constraints on droplet growth in radiatively cooled stratocumulus clouds

Austin¹,

Siems²,

Wang³

1995

J. Geophys. Res.

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Radiative cooling near the top of a layer cloud plays a dominant role in droplet condensation growth. The impact of this cooling on the evolution of small droplets and the formation of precipitation‐sized drops is calculated using a microphysical model that includes radiatively driven condensation and coalescence. The cloud top radiative environment used for these calculations is determined using a mixed‐layer model of a marine stratocumulus cloud with a subsiding, radiatively cooled inversion. Calculations of the radiatively driven equilibrium supersaturation show that net long wave emission by cloud droplets produces supersaturations below 0.04% for typical nocturnal conditions. While supersaturations as low as this will force evaporation for droplets smaller than ≈ 5 μm, radiatively enhanced growth for larger droplets can reduce the time required to produce precipitation‐sized particles by a factor of 2–4, compared with droplets in a quiescent cloud without flux divergence. The impact of this radiative enhancement on the acceleration of coalescence is equivalent to that produced in updrafts of 0.1–0.5 ms−1, and varies linearly with the total emitted flux (the “radiative exchange”).

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Section: Es(t) Es(t)(1 + E/wv)mentioning

confidence: 99%

“…• p-• •ZZ total pla2•Cpm T where F -(dEn/dZ)drops/(dEn/dz)total is the fraction of the total flux divergence due to the droplets [Davies, 1985]. We will follow Fukuta and Walter [1970] and use c• -1, /• -0.04 in (8)-(10), which produces as • 0.1 /•m, az m 4 pm.…”

Section: Seq • Ckmentioning

confidence: 99%

Constraints on droplet growth in radiatively cooled stratocumulus clouds

Austin¹,

Siems²,

Wang³

1995

J. Geophys. Res.

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“…For further details on the iteration procedure, see also B90 and Hall (1980). As already mentioned in B90, the droplet growth equation has been extended to include radiational effects (Davies, 1985):…”

Section: Aerosolusedmentioning

confidence: 99%

On the influence of the physico-chemical properties of aerosols on the life cycle of radiation fogs

Bott

1991

Boundary-Layer Meteorol

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A one-dimensional model of radiation fog with detailed microphysics is presented. Aerosols and cloud droplets are treated in a joint two-dimensional size distribution. Radiative fluxes are calculated as functions of the radiative properties of the time-dependent particle spectra. The droplet growth equation is solved by considering radiative effects. Turbulence is treated by means of a higher order closure model. The interaction between the atmosphere and the earth's surface is explicitly simulated.Three numerical sensitivity studies are performed to investigate the impact of the different physicochemical properties of urban, rural and maritime aerosols on fog formation. Numerical results elucidate that depending on the aerosol type used, the resulting fog events are completely different. This is particularly true for the times of fog formation and dissipation as well as for the liquid water content and supersaturations within the fogs. In the activated part of the particle spectra, the aerosol mass is very inhomogeneously distributed. The maxima of the curves do not coincide with the maxima of the corresponding liquid water distributions.

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“…Though the influence of LW cooling on drop growth has been studied somewhat extensively over the past 20 years, few studies have examined the influence of SW heating on the growth of drops. (The two notable exceptions are Davies [1985], who examined how LW and SW radiation alters the equilibrium value of supersaturation, and Ackerman et al [1995], who included LW and SW heating influences on drop growth in a one‐dimensional model of boundary layer stratiform clouds.) That this is the case is notable because, unlike LW cooling, SW heating occurs throughout most, or all, of a stratocumulus cloud deck.…”

Section: Introductionmentioning

confidence: 99%

Radiative influences on drop and cloud condensation nuclei equilibrium in stratocumulus

Marquis

Harrington

2005

J. Geophys. Res.

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[1] Radiative heating and cooling occurs throughout most of a stratocumulus cloud layer. Shortwave (SW) heating of individual drops can be strong enough that drop temperatures at equilibrium deviate by as much as 6°C from the surrounding environment. These large temperature differences lead to substantial errors when the classical equation for vapor growth is used. A new form of the vapor growth equation is derived for cases of strong radiative heating. The new equation is used to assess the equilibrium supersaturation (s eq ) state of drops and cloud condensation nuclei (CCN) within a stratocumulus cloud. Longwave (LW) cloud top cooling has two primary effects on s eq . It tends to reduce s eq for large drops to values as low as À6%, providing for drop growth at subsaturations. It also tends to reduce the critical supersaturation and size at which larger CCN activate to become growing cloud drops. In contrast, LW heating of cloud base suppresses the growth of cloud drops and CCN. Larger CCN (sizes between 0.5 and 5 mm) lose their Köhler curve maximum, indicating restricted CCN growth. Solar heating produces s eq up to 20% for drops with sizes between 500 and 1000 mm, indicating strong evaporation of drizzle drops. Since solar absorption increases more slowly with drop size than LW emission, a minimum appears in the Köhler curves at sizes between 20 and 100 mm, suggesting a preferred size range for vapor growth in SW heated clouds. Like LW heating, SW heating causes suppression of growth for larger CCN.

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Response of Cloud Supersaturation to Radiative Forcing

Cited by 24 publications

References 0 publications

Constraints on droplet growth in radiatively cooled stratocumulus clouds

Constraints on droplet growth in radiatively cooled stratocumulus clouds

On the influence of the physico-chemical properties of aerosols on the life cycle of radiation fogs

Radiative influences on drop and cloud condensation nuclei equilibrium in stratocumulus

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