Effects of radiation on clouds

Atmospheric Research, 23 (1989) 259-286

259

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Effects of Radiation on Clouds PETER R. JONAS .1

Meteorological Office, London Road, Bracknell, Berkshire RG12 2SZ (Great Britain) (Received August 23, 1988; accepted after revision April 27, 1989)

ABSTRACT Jonas, P.R., 1989. Effects of radiation on clouds. Atmos. Res., 23: 259-286. The effects of radiation on the growth of individual cloud particles and on the structure of clouds are reviewed. It is shown that the rate of growth of both water drops and ice crystals may be changed significantly by effects of radiation either near the edges of cloud sheets or below cloud, although the overall effect on particle size depends on the period spent in such regions. Sub-cloud evaporation of ice particles may be reduced by radiative cooling of the crystals giving rise to persistent fall streaks. The role of radiative transfer in the formation of radiation fog is well established. It is shown, however, that the nature of the radiative process is crucial in explaining the observed distribution of liquid water in fogs. The way in which radiation fogs may be dissipated due to changes in the radiation balance are also described. Layer clouds contain regions of radiative flux divergence which give rise to localised heating or cooling. In low-level water clouds and in some higher-level ice clouds this gives rise to destabilisation of the cloud layer leading to the generation of turbulent kinetic energy. This may be enhanced by evaporation of cloud particles into air entrained into the cloud layer. The sensitivity of the cloud structure to the radiative processes may give rise to a pronounced diurnal variation in the structure and to a dependence of the structure on latitude. The turbulent motions generated, in part, by radiative processes feed back on the cloud particle size distribution either by increasing the time spent in the cloud by a small fraction of the particles or by modifying the coupling between those cloud regions in which particles form with those regions where growth occurs. The review brings together theoretical studies and observations. It is concluded that while the effects of radiation on the evolution of stratocumulus cloud sheets are comparatively well understood, some details remain unresolved. However, cirrus clouds are less fully understood owing to the limited observations and the greater coupling between radiative, dynamical and microphysical processes. RESUME On passe en revue les effets du rayonnement sur la croissance des particules de nuages et sur la *1Present address: Meteorological Office, Meteorological Research Flight, RAE Farnborough, Great Britain. ©British Crown Copyright 1989

0169-8095/89/$03.50

© 1989 Elsevier Science Publishers B.V.

260 structure des nuages. On montre que le taux de croissance tant des gouttes d'eau que des cristaux de glace peut ~tre modifi6 de faqon significative par les effets du rayonnement prbs des bords des couches de nuages ou en-dessous du nuage, bien que l'effet global sur la taille des particules d6pende du temps pass6 dans ces rdgions. L'dvaporation sous-nuage des particules de glace peut btre r6duite par le refroidissement radiatif donnant lieu h des colonnes persistentes de cristaux. Le rSle du transfert radiatif dans la formation du brouillard de rayonnement est bien 6tabli. On montre n6anmois que la nature du processus radiatif est crucial pour expliquer la distribution observ6e d'eau liquide dans les brouillards. On d6crit la fa~on dont les brouillards de rayonnement peuvent ~tre dissip6s par suite des variations du bilan radiatif. Les nuages stratif6s contiennent des rdgions de divergence du flux radiatif qui donne lieu hun 6chauffement ou refroidissement localis& Dans les nuages bas d'eau et dans certains nuages dlevds de glace, ceci donne lieu h une destabilisation de la couche nuageuse, conduisant h la g6n6ration d'6nergie cin6tique turbulente. Ceci peut 6tre amplifid par 6vaporation des particules du nuage dans l'air entraind dans la couche de nuage. La sensibilit6 de la structure du nuage au processus radiatif peut donner naissance h une variation diurne prononc6e de la structure et h une d6pendance de la structure en latitude. Les mouvements turbulents g6n6r6s, en partie par les processus radiatifs, r6agissent sur la distribution en dimension des particules, soit en augmentant le temps pass6 dans le nuage par une petite fraction des particules, soit en modifiant le couplage entre les r6gions du nuage oh les particules se forment avec celles oh la croissance se produit. Cette revue r6unit des 6tudes thdoriques et des observations. La conclusion est que, si les effets du rayonnment sur l'dvolution des couches de stratocumulus sont comparativement bien compris, certains d6tails restent h r6soudre. N6anmoins, les cirrus sont moins complbtement compris, par suite du nombre limit6 d'observations et du plus grand couplage entre processus radiatifs, dynamiques et microphysiques.

1 INTRODUCTION

The fact that clouds affect the radiative flux is obvious; it is less obvious that clouds are also influenced by radiative processes. This influence takes two forms, a direct influence of radiative processes on the growth or evaporation of cloud particles and an influence, through the creation of localised regions of heating and cooling in a cloud layer, on the structure of the clouds. The processes are not independent as cloud particle growth is related to the structure of the cloud in which the particles grow and the radiative flux divergence depends on the size distribution of the cloud particles. In this review the problem of the growth of cloud particles, allowing for the effects of radiation, is considered first. While the impact of radiation on the growth of water drops and ice crystals can be treated in a similar manner, it will be shown that radiative processes may be of particular importance in explaining the persistence of ice crystal fall streaks. In section 3 the role of radiative processes on the evolution of radiation fog are outlined. The formation of the fog is reasonably well understood but it is shown, by reference to numerical studies, that the detailed vertical structure of the fog can only be understood by reference to the interaction between the microphysical processes and the radiative processes. The complex factors lead-

261 ing to the dissipation of fog following a change in the radiative balance, are also outlined. Effects of the radiative balance on cloud structure are described in sections 4 and 5. The effects of the flux divergence or convergence in clouds are of crucial importance in modelling the general circulation of the atmosphere but, of course, such localised sources of heating or cooling can have an effect on cloud structure. As with radiation fog many layer clouds result from a balance between radiative and turbulent transfers so that changes in the radiation balance give rise to changes in cloud structure which are demonstrated by reference to observations and numerical simulations. Cirrus clouds are more complex than lower-level water clouds both due to the large particle size and small ice content and to the sensitivity of radiation transfer in these clouds to the presence of clouds at other levels. However, some progress has been made in understanding the complex radiative-microphysical-turbulentinteractions and this is also described. Section 6 summarises the present status of the work on effects of radiation on clouds and suggests areas for future work. In many cases the work is hampered by a lack of suitable observations but field experiments, either in progress or planned, should significantly improve our understanding of these processes. 2 GROWTHOF CLOUDPARTICLES The growth of individual cloud particles by condensation or sublimation is determined by the rate at which vapour diffuses to the particle and the rate at which the latent heat of condensation is removed. Rapid growth, for example in a high supersaturation leads to an increased particle temperature which acts to reduce the vapour flux to the particle.

Dropletgrowth Roach (1976) recognised that earlier theories of particle growth had neglected the role of radiative processes in the heat balance equation. He derived an equation for growth of a droplet from the heat and water flux equations: 4

3 dTr

4

dr 3

2 dT

2

~~r pc-~= L~~p-~-47~r K~r +47crR

(1)

and: 4

dr 3

2 [dpw \

~~p--~=47~rD~-~ )~ In these equations,

r= droplet

(2) radius,

p= droplet

density, c = specific heat of

262

T=

water, Tr = droplet temperature, air temperature, L = latent heat of vaporisation, pw=water vapour density in air, diffusion coefficients for heat and water vapour, R = radiative heat input per unit surface area of the droplet and suffix's'denotes a value at the droplet surface. Manipulation of eqs. 1 and 2 yields the equilibrium growth equation:

dr (a_Rf(T) )/g(T) dt

K,D=molecular

(3)

where [ and g are slowly varying functions of temperature and a is the supersaturation. The above analysis has neglected the effects of surface curvature and dissolved solutes on the saturation vapour pressure at the droplet surface. The temperature dependent terms are positive. In the absence of radiative effects eq. 3 shows that drop growth only occurs in a supersaturated environment ( a > 0) while evaporation is inevitable in a subsaturated environment. However, depending on the sign of the radiative term in eq. 3 drop growth may occur even in subsaturated air if the drop is cooled by radiative effects (R < 0) while a radiatively warmed drop may evaporate even into supersaturated air. To determine whether the radiative term has any significant effect on drop growth, it is necessary to consider its magnitude. The magnitude of R was estimated by Roach (1976) using typical values of atmospheric parameters. He showed that, near the top of a layer of fog or low cloud, longwave radiative cooling of the drops occurred with a contribution to R of around - 30 W m - 2. Absorption of solar radiation by droplets is a small fraction ( < 10% ) of the incident solar radiation except for wavelengths below 2.5/~m. However, the total short wave flux (the total of direct solar flux and the flux reflected from the surface) may reach 1.5.103 W m -2 near cloud top in subtropical latitudes during the day. The solar heating may therefore be in excess of the longwave cooling during some parts of the day and typical values of R lie in the range +_30 W m -2. The effect of such radiative heat fluxes on the growth of droplets is illustrated by Fig. 1, from Roach (1976), which shows the growth of droplets assumed to contain 10-'e g of ammonium sulphate, at a fixed supersaturation of 0.05%. It can be seen that, at night when R is negative, growth may be enhanced provided that the drop spends a sufficiently large part of its life near the top of a cloud layer in a region where there is significant longwave cooling. In contrast, during the day, growth may be inhibited. In many clouds growth may be limited by the availability of water vapour, leading to a transfer of mass between droplets even when the supersaturation remains almost constant, a problem investigated by Barrett and Clement (1988). The analysis of the growth of isolated particles is complicated by the fact that cloud particles do not behave as spheres of pure water. Observations of

263

J J J

2O

j

J

J J J J J

E"

15

f/

rr 10 j

7

.....................................

.

. ..........

5 I

I

1

I

500

1000

1500

2000

Time (s)

Fig. 1. Droplet growth on a nucleus of 10-12 g of ammonium sulphate at 0.05% supersaturation. Solid curve, no radiative effects; dashed curve, cooling of 30 W m-2; dotted curve, heating of 30 W m -2. (From Roach, 1976.)

the radiative properties of water clouds by Foot (1988) are difficult to reconcile with theory unless allowance is made for the effects of aerosol which can increase the absorption of solar radiation. Twomey (1987) has demonstrated that internal scattering may also give rise to enhanced absorption by a weakly absorbing particle. However, the analysis only applies if the extinction coefficient of aerosol in water is comparable to the liquid water absorbtion coefficient. This is not true in the near infra-red so that the analysis may not explain the observed enhanced infra-red absorption.

Ice crystalgrowth The growth of ice crystals can be treated in a similar manner to that of water drops but additional factors result from the shape of the particle (Mason, 1971, chapt. 5). The growth equation (3) is replaced by: dridt=C~ V( ai ri -C2R[i( T) )/gi(

(4)

The suffix 'i' denotes ice, V is a ventilation factor and C~,2 are shape factors, ri denotes the radius of a sphere with volume equal to that of the crystal. C1 is the ratio of the electrostatic capacity of an object with the ice crystal shape to that of a sphere of the same volume while C2 is the ratio of the ice crystal surface area to that of the sphere. The shape factors can be estimated for certain idealised forms of crystal habit. Observations of particles in cirrus clouds suggest that they are usually large

264 compared with thermal wavelengths. Longwave radiative properties are assumed to be those of spherical particles. The value of R depends crucially on the presence of upper cloud layers. With a thin layer of cirrus with clear sky above and a thick layer of cloud below then there will be a maximum cooling with R ~ - 5 0 W m -~. However, the growth of ice crystals in fall streaks from thick ice cloud layers with no lower-level cloud may be influenced by longwave heating with R ~ +50 W m -2. Ice is more absorbing of solar radiation than is water. In addition to weak absorbing bands around 1.55 and 2.0 zm there is strong absorption at wavelengths longer than 2,7/~m. A typical ice crystal of dimension 100 zm (ri= 50 zm) will will absorb 4% of the incident solar flux so that with a strong downward solar flux ( ~ 1.2-10:3 W m - 2) and reflected upward flux ( ~ 800 W m - ~) the heating may reach R ~ 80 W m -2 offsetting infra-red cooling. As crystal size increases the dominance of solar heating also increases. Substitution of typical values into eq. 4 suggests that with radiative cooling around 50 W m-2 the critical supersaturation between evaporation and growth by sublimation is around - 5 % for needle-shaped crystals of equivalent radius 200/~m and around - 10 to - 20% for similar sized plate-like crystals. It seems probable therefore that ice crystals can settle through significant regions of subsaturated air without evaporation. In contrast, under strong solar fluxes supersaturations of 20% may be necessary to prevent evaporation.

Persistence of fall streaks The order of magnitude estimates above suggest that ice crystals may persist in subsaturated air if they are subject to infra-red cooling thus providing an explanation of the observations of Braham and Spyers-Duran (1967) of crystals falling 5 km in relatively dry air. Hall and Pruppacher (1976) using a model of the effect of radiative transfer similar to that described above showed that while some increase in the time for evaporation might result, this was less than about 10% if the air was at less than about 70% relative humidity. In thin ice clouds with ice content less than 0.1 g m -3 the response time for changes in supersaturation is greater than that for changes in ice particle size and hence significant supersaturations, or subsaturations, could be sustained for sufficient times to change particle growth. However, for visible clouds the ice content is normally greater than 0.1 g m -'~ and it is to be expected that the relative humidity would be close to 100% except following a sudden change in the vertical velocity of an air parcel. W.T. Roach (private commun., 1988) has carried out an analysis in which a monodispersed population of ice crystals with a concentration around 103 m - :~is assumed to fall through air initially substantially subsaturated. Evaporation of the first crystals results in increases in the relative humidity so that subsequent ice crystals evaporate more slowly and the level of total evapora-

265 tion propagates downwards. The air within fall streaks has little turbulence so that the humidity is only decreased slowly by mixing with air outside which permits the humidity to increase steadily within the fall streak. The process will occur even in the absence of radiative effects and is often observed at lower levels when rain evaporating below cloud base results in increases in the relative humidity. At present therefore the contribution of radiative cooling to the persistence of fall streaks is rather uncertain, detailed measurements of humidity within the fall streaks are required in addition to microphysics and radiation observations to supplement the early observations (e.g. Kerley, 1961 ) of relative humidities of less than 70%. 3 RADIATIONFOG It is well known that the fog which forms under clear skies at night results from the cooling of layers of air close to the ground as a consequence of radiative cooling of the underlying surface. In this respect such fog is fundamentally different from many clouds in which the cooling results from free or forced ascent of the air. The alternative method of forming fog, mixing of air masses having significantly different temperatures, cannot act under these conditions although it is the dominant process in forming sea fog or advection fog. Models of the formation of radiation fog (e.g. Brown and Roach, 1976; Brown, 1980; Turton and Brown, 1987) have demonstrated the crucial balances between radiative and turbulent transfer of heat and the roles played by momentum transport and conduction of heat through the soil. The main results from numerical simulations, which are supported by detailed observations, are the following. (1) Fog formation results from, largely, radiative cooling of the air to the colder ground surface following radiative cooling of the surface by radiation to space. Turbulent mixing is not necessary to generate the fog and may result in delayed fog formation. This is because turbulence may enhance the deposition of dew while mixing lengths are too small to permit fog formation by the mixing mechanism. Turbulence may also increase the mixing of potentially warmer air from above the nocturnal inversion downwards through the cloud. This will influence the relative humidity and hence fog formation. (2) Development of fog on realistic timescales can only be obtained if the bulk radiative effects of water vapour and carbon dioxide are included. Once fog has formed the cooling is dominated by radiative cooling as shown in Fig. 2; most of the cooling is due to the direct radiative cooling of the droplets. (3) The observed vertical distribution of liquid water in the fog can only be obtained if allowance is made for gravitational settling of the fog droplets. The mean time taken for fog droplets of around 5-10 ]~m radius to settle through a 50 m deep fog is about 2 h, much shorter than the lifetime of many fogs. Fig. 3 shows how droplet-settling results in much decreased values of the liquid water

266

~4 ~+2

Model J

,4 -2

f

~_

4

--4

Observed . .

"

L....

T -2 0400

0600

0800 Time

I

1000

(GMT)

Fig. 2. Model and observed heating rates in fog showingthe total heating rate (solid), radiative heating rate (dashed) and non-radiative heating rate (dotted). Observations are averaged over 2-9 m and model results are at 4 m. (From Brown and Roach, 1976.)

/ l°2 ~T) .....

~-~ s

1°°[I," 10-2

fi ~ 0

I

I

I

1

I 2

Water content (g kg-')

Fig. 3, Liquidwaterprofiles froma model of radiation fog after 5 h integration. The dashed curve shows the effect of omitting droplet settling. Solid and dash-dot lines include different parametrizations of settling velocity as a function of water content. (FromBrown and Roach, 1976. ) content. Detailed observations of the vertical profile of liquid water are not available but the model predictions with settling of maximum water content around 0.2 to 0.3 g kg -1 are in better agreement with observed values (e.g. Roach et al., 1976) than the value of 1 g kg- 1 obtained when settling is neglected. The sensitivity of the fog properties to microphysical/radiative interactions can be explained by feedback between the liquid water content and the radiative properties of the fog. Droplet-settling reduces the liquid water content which reduces the radiative cooling due to the droplets, thereby reducing the condensation rate. The reduced liquid water content and condensation rate imply a reduced mean droplet radius which reduces the gravitational flux. Eventually a balance may be achieved between the condensation and settling. The direct effects of radiation on the growth of fog droplets were considered by Brown (1980). It was shown that the effect of removing this term was to reduce the mean drop radius by about 30% when the condensation nucleus concentration was low while increasing the liquid water content and the drop-

267

let concentration as shown in Fig. 4. Additional nuclei are activated because of the effect of radiative processes on the critical supersaturation as indicated in Table I which is based on an extension of the theory in section 2 to include effects of dissolved material. Large nuclei are most affected by the radiative processes and their rapid growth initially, when radiative terms are included, results in rapid removal of water vapour and, later in the integration, to a lower supersaturation which reduces the number of smaller nuclei which are activated. The reduced drop growth in the absence of the radiative term results in decreased settling and hence to increased water content. The radiative term has little effect at higher nucleus concentrations as can be seen in Fig. 4. This is largely due to the influence of the nucleus concentra-

80

-

F--]

I

60

40

20

SPECTRUM B

i

I

i

i

I

I i I I I [---- J

i i I___

1 I

I

I I I I I

I I I

.__.J 100

F-o L~

I---3 ] I I

I I

i t

I

L---

I I I

I

SPECTRUM E

80

60

40

| 20

- -- 7

I 4

8

12

16

20

Radius (pm)

Fig. 4. Droplet spectra 2 m above ground from a fog simulation with different nucleus concentrations; spectrum B contains 2.32"103 cm -a and spectrum E contains 1.16-104 cm -a. Radiative terms are included in the drop growth equation (solid) and excluded (dashed). (From Brown, 1980. )

268 TABLE I

Critical supersaturation a for ammonium sulphate nuclei with and without radiative loss (from Brown, 1980 ) Nucleus mass

Critical supersaturation ( % )

(g)

R=0

R=-25

10 11 10 1~ 10 -l:~ 10 -l~ 10 -15

2.0.10-:/ 6.3"10 :* 2.0-10 2 6.3.10 -2

-0.11 -4.0-10 _7.0.10-3 6.1.10 -2

01

0.2

-

0.2

lm

-

W m -'2

9m

_~ >,

'~

64

0.01

16 ~"

....,

U-

0.001 - i i

4 1800

2000

2200

0000

Time(GMT) Fig. 5. C h a n g e s in fog top (dotted) and opacity at 1, 9 a n d 27 m ( solid ) observed during a radiation fog. A sheet of stratocumulus spread over the fog around 2 2 h 0 0 G M T . (From Roach et al., 1976.)

tion on the optical depth of the fog. The high nucleus concentration gives rise to much decreased values of the radiative loss (2 W m -2 at 35 m below fog top compared with 27 W m -2 for nucleus concentrations of 1.16.104 and 2.32-103 cm -3, respectively). In addition the wavelength-averaged droplet absorption efficiency factor is reduced for the smaller drops as given in eq. 8 of Roach (1976). This has the effect of reducing the effect of radiative transfer on drop growth as well as on the critical supersaturation. It can be seen, on the basis of the numerical experiments that radiation fog not only forms as a consequence of radiative effects but that the detailed structure of radiation fog is also dependent on the radiative and microphysical processes. Radiation fog is often dissipated as a result of radiative processes. In particular, increased solar insolation after sunrise may cause the fog to disperse as may the advection of cloud sheets over a layer of fog. Observations of the effect of cloud advection on fog development were reported by Roach et al. (1976) and typical observations are shown in Fig. 5. Model simulations (Brown and

269 Water content (g kg 1) 02 I

I

_ ~ ,

I

/

!

~

I

:t+l -t

10 2

I

J]

102 ~'t+2 "-'4"~. . . . . . . . . . .

~- 10°

04

|

tI

t

t÷2

I

~s~ I

I

5

I 5

Temperature (°C)

Fig. 6. Liquid water (dashed) and temperature (solid) profiles in a simulated fog at t + x hours following advection of cloud cover at time t. (From Brown and Roach, 1976. )

Roach, 1976) suggest that the direct effect of the reduction in the net radiation at the surface, due to cloud layer advection, is not important since the fog is dominated by radiative cooling in its upper region. It is probable that this cooling is sufficiently reduced by the cloud cover that the heat input from the lower surface cannot be balanced so that the fog is dissipated by the effect of heating from below. Fig. 6 shows how the liquid water and temperature profiles vary following the advection of cloud cover in the numerical simulations. The essential difference between dispersion of a fog due to cloud cover advection and due to solar insolation is that, in the former case, longwave cooling from the fog is reduced immediately. With increased solar insolation the cooling at upper levels continues until the fog begins to thin due to a combination of radiative and turbulent processes. In the example which is simulated in Fig. 7 the fog continues to increase in depth after sunrise and the peak liquid water content is not achieved until more than one hour after sunrise. The effect of continued cooling is responsible for the thickening of the fog, only slightly reinforced by increased evaporation at the surface due to the increased solar heating. Fog clearance, in this example, takes nearly 4 h which is consistent with observations. The clearance is generally associated with the erosion of the inversion, due eventually to warming of the layers close to the ground, which leads to increased downward m o m e n t u m flux causing the fog to thin rapidly from above. The numerical simulations show that radiation is not only the dominant factor in the formation of fog during clear nights but that changes in the radiative balance can give rise to fog dissipation. This is because of the balance between radiative and turbulent processes in a mature radiation fog. Furthermore the radiative processes influence the growth of the fog droplets especially

270

Water content (g kg -~) o

10 a

O5 I

l

~ ".'~

i •

.~..-.

~

1 I

I

10o .

,.t ~'

/,J

~

:1 ,

~ ~: -! i : I

0

I

5

m~ •. 1 'l

I 10

Temperature (°C)

Fig. 7. Liquid water a n d t e m p e r a t u r e profiles in fog s i m u l a t e d following sunrise. D a s h e d line shows c o n d i t i o n s a t sunrise a n d long dashed, solid, d o t t e d a n d d o t - d a s h lines c o n d i t i o n s 1, 2, 3 a n d 3.5 h later. ( F r o m B r o w n a n d R o a c h 1976.)

in clean air masses. Feedback processes operate in which the growth of droplets influences the distribution of liquid water because of changes in the gravitational flux. Changes in the liquid water distribution lead in turn to changes in the radiative fluxes. 4 STRATOCUMULUS

DYNAMICS

Layer clouds influence the radiative flux and this often leads to the formation of regions of heating and cooling within a cloud layer. We consider here the effect of such processes on low-level stratocumulus clouds. Lilly (1968), using a simplified steady state model, was among the first to show that radiation processes were essential to the dynamics of the stratocumulus layer and that their inclusion was essential if observed vertical velocities were to be reproduced. However, the model was not capable of predicting the detailed radiative flux profiles in the cloud layer. Using observed distributions of droplet size obtained from extensive measurements of marine stratocumulus, Nicholls (1984) demonstrated that there was significant longwave cooling at the top of the cloud with some longwave warming at the cloud base. Considering the cloud layer as a whole, however, the net longwave cooling was balanced by shortwave heating which was only slightly dependent on the position within the cloud layer. The net effect of longwave and shortwave radiation is therefore to destabilise the cloud layer as shown in Fig. 8; cooling is confined to a shallow layer, normally less than 50 m deep, at the cloud top. The shortwave absorption by the cloud layer is diurnally varying. The vari-

271

1000

r

[

q

900

800

Z

700

"i600

500

4

200

0

150

0

~

-100

-50

0

+50

Net heating rate (K d -1)

Fig. 8. Net heating rate in a mid-latitude maritime stratocumulus sheet based on observed droplet spectra. The vertical bar shows the extent of the cloud. 1.0

I

[

0.8

0.6

0 0.2

0

10

I

I

100

1000

Liquid water path ( g m -2)

Fig. 9. Calculated cloud albedo for stratocumulus as a function of liquid water path and solar zenith angle. (From Stephens and Webster, 1981. )

ation of the scattering by the droplets with solar zenith angle is seen in the variation of albedo with zenith angle presented in Fig. 9 from results by Stephens and Webster (1981). Since the liquid water path in stratocumulus is often in the range 10-100 g m -2 the dependence of zenith angle of albedo is very marked. The diurnal variations in shortwave effects might be expected to give rise to a diurnal variation in the structure of layer clouds and such variations have been seen in satellite data by Minnis and Harrison (1984). Representative observations are shown in Fig. 10, which suggest that diurnal changes occur in both cloud cover and cloud depth. The structure of a layer of stratocumulus represents a balance between tur-

272

285

100 Crnax

f

zc ax

80

284

60

283

{3

'E

E o

\

E

40

L.)

\

/

/

282

-~ .Q

Tcmin

281

20

LU

l 6

I 12

r 18

280 24

Local time (hours)

Fig. 10. Mean cloud amount ( % ) and cloud top temperature (K) as functions of time of day during November 1978. Data are for an area of 250)<250 km 2 around 21°S 86°W. (From Minnis and Harrison, 1984. )

bulent transport processes, entrainment and radiative effects. For this reason the cloud structure is particularly sensitive to the cloud top entrainment and to the depth of the region over which longwave cooling occurs. Randall (1980a,b) demonstrated that the effect of increasing the depth of the region throughout which longwave cooling occurred was to lower the base and top of a maritime cloud layer towards the sea surface. It was also shown that the sensitivity of the cloud structure to the representation of the cloud top entrainment mechanism increased with increases in the depth of the cooled layer, demonstrating the need for model studies in which all of the interactions are specified adequately. The mechanisms which overturn the structure of stratocumulus layers, and which lead to the diurnal variation in structure, have been modelled by Turton and Nicholls (1987). The absorption of solar radiation at cloud top, entrainment at cloud top and small surface buoyancy fluxes tend to stabilise the cloud layer while cloud top longwave cooling and the release of latent heat near the base of the cloud lead to destabilisation of the layer. However, the regions of generation and consumption of buoyancy are localised in the vertical. In a steady state system, or one which is only slowly varying in time, turbulent transport redistributes the buoyancy to achieve a well mixed layer. Under certain conditions balance requires negative buoyancy fluxes in the sub-cloud layer but observations (e.g. Nicholls and Leighton, 1986) suggest that in maritime cloud layers in daytime with weak surface buoyancy fluxes little turbulent kinetic energy is exported from generation regions near cloud top so that only

273

(°C) 0 5

5 z(m)

Inversion layer I

/

. . . .

I

'

'

I ....

i ....

1

s~:;,oo

I

~

1

'

I

I

I

I

I

r

I

I

,

,

I

T

T

I

I

r

~--Z T

.......... ~--- --'" ~ :'iI - --- ~r~'q- _' I7 -

8®

?i

600 . . . . . . . . . . . . . . . 400

Surfaceor I Ekrnan layer

~

1400

i 1200Icl°ud Ilayerdi 1000 .... . . ql~-i . -. --~L . . Mixed ,.yer

~

(gkg-~) 0 05

•e

~ T

200 300

305 (K)

310

2

4

6 (gkg')

8

6 3 0 (ms'}

Fig. l l . Profiles of equivalent potential temperature, temperature, total water, liquid water and horizontal velocity components observed in a layer of marine stratocumulus (Nicholls and Leighton, 1986).

weak negative fluxes can be supported. Under such conditions a single mixed layer cannot always be supported and stable layers may develop below cloud base decoupling the cloud layer from the surface layer. Such a structure is shown in Fig. 11 where part of the layer between the surface and cloud base has a stable profile of temperature and is only weakly turbulent. Turton and Nicholls (1987) used a multiple mixed layer model to examine the structure and evolution of stratocumulus taking account of the diurnal variation of the incoming radiation and making use of observations of droplet size distributions in the radiative calculations. The model predicted a diurnal variation in the stratocumulus layer thickness only when separation of the cloud and surface layers was allowed; a stable layer formed after sunrise and persisted for some time until around local noon, when it was replaced by a more persistent inversion. Typical results for midlatitude and subtropical stratocumulus are shown in Fig. 12. Although the model could not predict cloud cover it is reasonable to suppose that in reality the thin layer of cloud during the afternoon might break up leading to a diurnal variation in cloud cover similar to that shown in Fig. 10. It is interesting to note that this radiatively forced diurnal variation of cloud thickness has a significant effect on the radiative balance at the surface. In the subtropical case the average net surface energy input which was 7 W m-2 in the absence of separation and the diurnal variation in cloud thickness increased to 102 W m -2 when separation was allowed. The model, which shows similar features to those predicted by more complex models (e.g. Bougeault, 1985), reproduces many of the observed features of

274

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Fig. 12. Modelled evolution of mid-latitude (left) and subtropical (right) stratocumulus shown stippled allowing for the formation of a stable layer (hatched). Net surface long and short wave fluxes are shown for the subtropical case. (From Turton and Nicholls, 1987. )

stratocumulus. It also predicts that cloud layers in different regions of the world may behave in slightly different ways with decoupling being more common in summer than in winter at mid- or high latitudes. Decoupling is also likely to be related to large-scale dynamical constraints. The model predictions are being examined using data from the First ISCCP Regional Experiment, the field phase of which was conducted in summer 1987 (Albrecht et al., 1988). A factor in the evolution of low-level layer clouds, which as will be shown later is more important in cirrus clouds, is the settling of the cloud particles. It can be shown that feedback processes tend to hold the temperature and liquid water content near equilibrium conditions with a time constant of several hours. This equilibrium may be modified by the diurnal variation of solar heating or destroyed by synoptic-scale variation at mid-latitudes. These factors have been shown to be important in the evolution of a particular form of sea fog, 'haar' (see Findlater et al., 1989 ), and it would be of interest to examine their importance for the evolution of stratiform cloud sheets. Although the work described above has included the generation of turbulence as a result of radiatively driven destabilisation of the cloud layer, the

275

details of the generation were not considered. Nicholls (1989) has shown, using detailed observations of marine stratocumulus, that the destabilisation results in the formation of negatively buoyant downdraughts occupying rather more than one third of the cloud area at cloud top. The downdraught thermodynamic properties show that they result from air which has been radiatively cooled together with air entrained from above cloud top which is cooled and moistened by evaporation of droplets into it. Radiative cooling is however the dominant process driving the turbulence. The downdraughts increase in width and decrease in number as distance below cloud top increases. Between the downdraughts are larger regions of updraughts. The effects of radiative transfer on stratocumulus layers can therefore be seen to influence both the large-scale structure and temporal evolution of the

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276

clouds as well as the detailed turbulent structure. Space does not permit a detailed discussion but the radiatively driven turbulent structure within the cloud is a dominant factor influencing the growth of drizzle droplets within such layers (Nicholls, 1987), as shown in Fig. 13. However, it should be noted that direct effects of radiation on drop growth are small because of the mixing throughout cloud depth which limits the time spent by drops in regions of flux divergence near cloud top. 5 CIRRUS LAYERS

It has been noted earlier that the growth of ice crystals may be more sensitive to the influence of radiation than the growth of water droplets. In addition, clouds composed of ice crystals contain, in general, an ice content which is much lower than the water content of lower-level clouds. The low water content and consequent reduction in optical path imply that radiative effects will be spread through a greater depth of the cloud than is the case with water clouds. These factors together with the fact that the radiative transfer in highlevel clouds may be influenced by the presence of clouds at lower levels make the interaction between radiation and cloud structure more complex in cirrus cloud than with lower-level water cloud layers. Observations of cirrus clouds, some of which are summarised by Heymsfield and Platt (1984) from which Fig. 14 was obtained, show that while the mean ice particle size spectra often contain a range of particle sizes and shapes ranging from submicron to several millimetres in concentrations which may be around 1 cm -3, there are usually significant variations within a cloud on a scale of 100-500 m. There are also significant differences between clouds which form under, apparently, similar conditions. This might be expected if there is a complex interaction between the microphysical and dynamical processes in the clouds. The observations reported by Heymsfield and Platt also demonstrate the variability of the ice content in such clouds together with the generally rather low values at cirrus cloud temperatures as shown in Table II. The effects of radiative processes on the development and structure of cirrus clouds are best studied using numerical models which incorporate both microphysical processes and radiative processes. Detailed studies have been made by Starr and Cox (1985a, b) and by Starr (1987) who concentrated largely on the effect of radiation on cloud structure, and by Ramaswamy and Detwiler (1986) who were concerned more with the interaction between radiation and microphysical properties. As was seen in the case of stratocumulus clouds, radiative transfer may tend to destabilise cloud layers. To study this Starr and Cox (1985a, b) therefore used a two-dimensional numerical model which permits the explicit treatment of the formation of downdraught regions within the clouds although the clouds were assumed to be forced by uniform large-scale motions. A typical model

277

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simulation was of a layer of cirrus formed at a height of around 7 km with uniform ascent of up to 10 cm s - 1 with no higher or lower cloud. The overall effects of radiative processes can be examined by comparing integrations using midday or night-time conditions or with an integration where radiative heat sources or sinks are ignored. The effect of longwave cooling is evident in the horizontal mean ice content shown in Fig. 15. The changes in the vertical distribution of heat sources and sinks due to radiative processes is also seen in the mean turbulent kinetic energy. The importance of the radiative term in the heat budget can be seen in Fig. 16 where radiative effects are compared with those due to phase changes. The effects of radiation on the mean cloud properties are significant but the

278 TABLE II Ice content in cirrus clouds observed at different temperatures (from Heymsfield and Platt, 1984) Temperature (:C)

Ice content (gm -3)

-2O -25 -30 -35 -40 --45 --50 --55 --60

0.001 -0.063 0.001 -0.066 0.008 -0.043 0.009 -0.025 0.0004-0.008 0.0002-0.008 0.0002-0.004 0.0002-0.0018

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integrations reveal an even more significant effect on horizontal structure. Fig. 17 shows contour plots of ice mixing ratio obtained from the model under nighttime and midday conditions. The midday case has a more cellular pattern with significantly higher maximum values although, at this time, the horizontal averages are around 20% smaller. Although not apparent from the results at one time the convective elements were more persistent in the midday simulations while having a lifetime of only 20 min in the night-time case. This difference in structure is a direct result of the different radiative fluxes during the day and at night. Detailed analysis of the radiative balances shows that one of the factors leading to the formation of a cellular structure, rather than to the more random structure observed in stratocumulus, is the low infra-red absorption coefficient in the ice clouds which provides a feedback mechanism by which radiative processes reinforce any cellular motions. In water clouds the radiative effects

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280

are more important near cloud edge and hence cellular motions within the cloud do not influence local radiative processes. The results presented by Starr and Cox (1985b) are derived from a model with a bulk representation of microphysical properties. It is important to note however that the radiative properties do depend on the size and shape of the ice crystals present. As noted by Ramaswamy and Detwiler (1986) variations in ice crystal length may be as important as changes in ice content in determining the radiative properties as illustrated in Fig. 18. In order to study properly the formation of cirrus clouds, models of the detailed microphysical processes are necessary to support the bulk models of cloud dynamics. The radiative properties of cirrus cloud layers are very dependent on the level of formation because of the effect of temperature on ice content and on the presence of other cloud layers. In an attempt to shed some light on the differences between cloud formed under different conditions, Starr (1987) carried out further simulations using the model of Starr and Cox (1985a). Daytime and night-time simulations were made of low-level cirrus with a cloud top :LOUD TOP

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281

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282

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Fig. 20 shows horizontal profiles of ice mixing ratio (averaged through the cloud depth) for the day and night cases of sub-tropopause and lower-level clouds. The variability of these profiles reflects the degree of organised motions within the clouds. The upper-level cloud, apart from the reduced mean ice content, shows little difference in structure between day and night while the lower-level cloud shows greater cellular structure during the day. Inspection of the profile of latent heating shows that ice crystals are growing towards the top of the higher-level cloud with a weaker maximum growth around mid-level while in the lower-level cloud maximum growth is around the mid-level with some growth throughout the upper regions of the cloud. Evaporative cooling occurs in the lower parts of the clouds. The effect on the potential temperature

283 profile due to radiative processes is compared with that due to condensation/ evaporation in Fig. 21. It is of note that infra-red cooling extends throughout much of the cloud depth; it is not confined to cloud top as in the case of water clouds. Solar heating is, relatively, more concentrated at cloud top level especially in the sub-tropopause case where the ice content reaches its maximum value near the top of the cloud. Ice production is dominated by the adiabatic ascent although this production rate is enhanced by 10-20% at night due to radiative cooling. This does not completely explain the day-night differences obtained in simulations of weakly stratified clouds in which the dynamical structure is influenced by radiative processes. These differences are enhanced by the effect of the radiatively driven convective cells within the clouds. Due to the small optical depth infra-red processes are spread through a considerably larger part of the cloud and tend to reduce convective cellular motions while solar heating has the opposite effect. Changes in the radiative processes therefore modulate the coupling between the region of cloud particle growth and the lower regions where particle evaporation may occur with more rapid cycling of water associated with nighttime disorganised convection. From the calculations described above it can be seen that the interaction between microphysics, radiation and dynamics in cirrus clouds is very complex and a full solution of the problem of the formation and maintenance of cirrus awaits analysis of the comprehensive data obtained during the first ISCCP Regional Experiment (Albrecht et al., 1988; Starr, 1988). A feature of particular concern is the maintenance of the optically thin tropical cirrus which is subject to net radiative heating during the day and night. Under such conditions the clouds would be expected to dissipate (Ackerman, et al., 1987). 6CONCLUSIONS In this review two effects of radiation on clouds have been considered. It has been shown that radiation directly influences the growth of individual cloud particles and may either increase their growth rate or persistence or lead to evaporation depending on the sign of the radiative flux to the particle. In general, however, such effects are reduced in clouds since individual particles spend only a small portion of their lifetime in regions of significant radiative flux owing to the optical thickness of the clouds and the turbulent motions which act to overturn the microphysical structure. An exception to this is in radiation fog where turbulence levels are low. In some cirrus clouds flux divergence extends through a significant part of the cloud and it is possible that direct radiative effects may also be important here. The persistence of fall streaks appears to be partly due to radiative processes but is dominated by premoistening of air by evaporating crystals. The structure of layer clouds is also influenced by radiative processes. In

284

general the effects of radiation are seen in the turbulent structure of both stratocumulus and cirrus layers. The effect on cloud structure feeds back into the microphysics both through modification to the water profile and to the growth of cloud particles. In cloud layers the obvious influence of radiative processes is seen by the diurnal variation in their structure but this often represents a change in the balance of several processes of which radiation is but one. The interaction between microphysical and radiative processes has been shown to be crucial to the evolution and structure of radiation fog where radiative processes led to the formation and, less directly, to the dissipation of the fog. A number of problems remain unresolved. Among these is the direct role of radiative processes on particle growth in thin clouds and clarification of the relative-humidity evolution in fall streaks. The testing of multiple mixed layer models of stratocumulus under a wide range of conditions has not been completed due to the limited availability of suitable comprehensive data sets. Persistence of thin cirrus remains an important problem which has climatological importance. Mixed clouds have also received relatively little attention; in such layer clouds the microphysics and radiative properties are more difficult to model, and observations are very limited compared with lower-level stratocumulus layers. It is towards the resolution of some of these problems that two major field programmes are being undertaken: the first ISCCP Regional Experiment, the field phase of which has been completed, and the International Cirrus Experiment due to take place in 1989. These experiments include a detailed study of radiative and microphysical properties of the clouds from which the interactions between them may be better understood. From the point of view of incorporating radiation into models of the large-scale circulation it is probably correct to say that the radiative properties of clouds can be predicted (at least in principle ) to an acceptable accuracy provided that the microphysical properties of the clouds can be established. However, since the formation and evolution of the clouds as well as their microphysical properties are strongly influenced by radiative processes, it may be necessary to incorporate much more detailed cloud parametrizations in climatological models before the treatment of radiative transfer can be improved. ACKNOWLEDGEMENTS

The author is grateful for the advice received from Drs S. Nicholls and W.T. Roach during the preparation of this review.

REFERENCES Ackerman, T.P., Valero, F.P.J. and Liou, K-N., 1987. Heating rates in tropical anvils. VI Conf. Atmospheric Radiation, Williamsburg, Va., USA, 13-16 May 1987, pp. 26-29.

285 Albrecht, B.A., Randall, D.A. and Nicholls, S., 1988. Observations of marine stratocumulus clouds during FIRE. Bull. Am. Meteorol. Soc., 69: 618-626. Barrett, J,C. and Clement, C.F., 1988. Growth rates for liquid drops. J. Aerosol Sci., 19: 223-242. Bougeault, P., 1985. The diurnal cycle of the marine stratocumulus layer: A higher order study. J. Atmos. Sci., 42: 2826-2843. Braham, R.R. and Spyers-Duran, P., 1967. Survival of cirrus crystals in clear air. J. Appl. Meteorol., 6: 1053-1061. Brown, R., 1980. A numerical study of radiation fog with an explicit formulation of the microphysics. Q. J. R. Meteorol. Soc., 106: 781-802. Brown, R. and Roach, W.T., 1976. The physics of radiation fog, II. A numerical study. Q. J. R. Meteorol. Soc., 102: 335-354. Findlater, J., Roach, W.T. and McHugh, B., 1989. The haar of north-east Scotland. Q.J.R. Meteorol. Soc., 115: 581-608. Foot, J.S., 1988. Some observations of the optical properties of clouds, II. Cirrus. Q. J. R. Meteorol. Soc., 114: 145-164. Hall, W.D. and Pruppacher, H.R., 1976. The survival of ice particles falling from cirrus clouds in subsaturated air. J. Atmos. Sci., 33: 1995-2006. Heymsfield, A.J. and Platt, C.M.R., 1984. A parametrisation of the particle size spectrum in terms of the ambient temperature and the ice water content. J. Atmos. Sci., 41: 846-855. Kerley, M.J., 1961. High-altitude observations between the United Kingdom and Nairobi. Meteorol. Mag., 90: 3-18. Lilly, D.K., 1968. Models of cloud-topped mixed layers under a strong inversion. Q. J. R. Meteorol. Soc., 94: 292-309. Mason, B.J., 1971. The Physics of Clouds. Oxford Univ. Press., 671 pp. Minnis, P. and Harrison, E.F., 1984. Diurnal variability of regional cloud and clear sky radiative parameters derived from GOES data, Part II. November 1978 cloud distributions. J. Climate Appl. Meteorol., 23: 1012-1031. Nicholls, S., 1984. The dynamics of stratocumulus: aircraft observations and comparisons with a mixed layer model. Q. J. R. Meteorol. Soc., 110:783-820. Nicholls, S., 1987. A model of drizzle growth in warm, turbulent, stratiform clouds. Q. J. R. Meteorol. Soc., 113: 1141-1170. Nicholls, S., 1989. The structure of radiatively driven convection in stratocumulus. Q. J. R. Meteorol. Soc., 115: 487-511. Nicholls, S. and Leighton, J., 1986. An observational study of the structure of stratiform cloud sheets, Part I. Structure. Q. J. R. Meteorol. Soc., 112: 431-460. Ramaswamy, V. and Detwiler, A., 1986. Interdependence of radiation and microphysics in cirrus clouds. J. Atmos. Sci., 43: 2289-2301. Randall, D.A., 1980a. Conditional instability of the first kind upside down. J. Atmos. Sci., 37: 125130. Randall, D.A., 1980b. Entrainment into a stratocumulus layer with distributed radiative cooling. J. Atmos. Sci., 37: 148-159. Roach, W,T., 1976. On the effect of radiative exchange on the growth by condensation of a cloud or fog droplet. Q. J. R. Meteorol. Soc., 102: 361-372. Roach, W.T., Brown, R., Caughey, S.J., Garland, J.A. and Readings, C.J., 1976. The physics of radiation fog, I. A field study. Q. J. R. Meteorol. Soc., 102: 313-333. Starr, D.O'C., 1987. Effects of radiative processes in thin cirrus. J. Geophys. Res., 92: 3973-3978. Starr, D.O'C., 1988. A cirrus cloud experiment: intensive field observations planned for FIRE. Bull. Am. Meteorol. Soc., 68: 119-124. Starr, D.O'C. and Cox, S.K., 1985a. Cirrus clouds, Part I. A cirrus cloud model. J. Atmos. Sci., 42:2663-2681.

286 Starr, D.O'C. and Cox, S.K., 1985b. Cirrus clouds, Part II. Numerical experiments on the formation and maintenance of cirrus. J. Atmos. Sci., 42: 2682-2694. Stephens, G.L. and Webster, P.J., 1981. Clouds and climate: sensitivity of simple systems. J. Atmos. Sci., 38: 235-247. Turton, J.D. and Brown, R., 1987. A comparison of a numerical model of radiation fog with detailed observations. Q. J. R. Meteorol. Soc., 113: 37-54. Turton, J.D. and Nicholls, S., 1987. A study of the diurnal variation of stratocumulus using a multiple mixed layer model. Q. J. R. Meteorol. Soc., 113: 969-1009. Twomey, S., 1987. Influence of internal scattering on the optical properties of particles and drops in the near infrared. Appl. Optics, 26: 1342-1347.