BACKGROUND ON CLOUD FORCING

CF = <F> - <Fclear>	(1)

where F is the net flux and Fclear is the flux expected if clouds are removed from the column. Traditionally only radiative fluxes are considered, but here we will also consider turbulent fluxes.

Cloud forcing can be computed at the top of the atmosphere (TOA) or at the surface (S).

Maximum cloud forcing is defined as the actual difference in net flux observed when the sky is overcast versus when it is clear.

MCF = Ff=1 - Ff=0 = [F f=1 - F f=1 ]- [F f=0 + F f=0]	(2)

where f is cloud fraction. We can approximate CF as

CF f* MCF	(3)

SHEBA DATA

Radiative fluxes: upward/downward Pyranometers/Pyrgeometers

located at the SHEBA surface flux group site at metcity

Clouds:

Prior to JD 530 :ETL DABUL lidar

f (1 hr) = mean of 6 10-min samples

Up to six cloud layers: base height and top height

After JD 530:	ARM Vaisala CT-25k ceilometer

f (1hr) = fraction of 240 15-sec samples with cloud

Note these are not true instantaneous fractional coverage of the sky, but are time-averaged probability of a cloud being directly overhead.

Turbulent heat fluxes:

sensible - Sonic anemometers, eddy covariance

latent - bulk

For and example of cloud and downward IR flux data, see Fig. 1 where the 1-hr cloud fraction is shown with the flux for November-December. The blue dots are the IR flux when f=0; the x's when f>0. The bimodal nature of cloud fraction (i.e., near 0 or 1) is obvious in the time series.

Clear Sky Models

Shortwave:

from M. Iqbal in "Physical Climatology for Solar and Wind Energy", 1988, World Scientific, pp. 196-242.

Inputs:

latitude, longitude, GMT

w = integrated water vapor (g/cm^2 ; used qa /4)

p == surface pressure in hPa (used SPO data)

k1 == aerosol optical thickness at 380 nm (used = 0.01)

k2 == aerosol optical thickness at 500 nm (used = 0.01)

l == ozone-layer thickness in cm (NTP) (used = 0.3)

See sample time series from a short period in May (Fig. 2)

Longwave:

Rl = [A +B* qa1/2]	Ta4

qa = specific humidity at 10-m height;

Ta = air temperature at 10-m height

A=0.70	B=0.03 tuned to f=0 data

A time series of the clearsky model and measurements when f=0 is shown in Fig. 3; modeled vs measured is shown in Fig. 4. Periods when clouds are present but f=0 from the vertically pointing sensors are apparent as outliers well above the line values. These are cases when clouds passed close enough to influence IR, but did not pass directly over the sensors. The ARM whole-sky imager may provide a better sample of cloud fraction.

Fig. 1	Time series of cloud fraction (red line) and downward IR flux (symbols: blue dots are for cloud fraction = 0; the green x's otherwise).

Fig. 2	Downward solar flux for several days in May: line - measured; dots - model clear sky.

Fig. 3	Time series of downward IR flux: dots - measurements for cloud fraction=0.0; line - clear sky model.

Fig. 4	Downward IR flux: clear sky model vs measurements when cloud fraction=0.0.

Fig. 5	Annual cycle of downward IR flux: solid line - measured when f=1.0; circles - measured when f=0.0; x's - clear sky model when f=0.0; dashed line - clear sky model, all data.

Fig. 6	As in Fig. 5, but for upward IR flux.

Fig. 7.	Annual cycle of net IR maximum cloud forcing: circles - using measured flux when f-
0.0; x's - using model clear sky flux when f=0.0.

Fig. 8	Annual cycle of net IR cloud forcing using the model clear sky. The circles are using (1) and the x's are using (2). A positive value means clouds warm the surface.

Fig. 9	Annual cycle of downward solar flux: solid line - measured when f=1.0; circles - measured when f=0.0; diamond's - clear sky model, all data.

Fig. 10 As in Fig. 5, but for upward solar flux.

Fig. 11 Annual cycle of net solar maximum cloud forcing: circles - using measured flux when f-
0.0; x's - using model clear sky flux when f=0.0.

Fig. 12 Annual cycle of net solar cloud forcing using the model clear sky.

Fig. 13. Annual cycle of net radiative cloud forcing: circles - measured clear sky; x's - model clear sky.

Fig. 14 Annual cycle of sensible heat flux for clear skies (green line) and cloud fraction = 1.0 (blue line).

Fig. 15 As in Fig. 14, but for latent heat flux.

Fig. 16 Annual cycle of surface heat budget cloud forcing component: cloud fraction (see labels on graph). The turbulent flux is the sum of sensible and latent heat fluxes.

(a) Using measurement clear sky values

(b) Using model clear sky values

Fig. 17 As in Fig. 13, but for net total cloud forcing (sum of components).

Cloudbase	Time series of mean hourly cloud base (cirlces) and cloud top (x's) heights from ETL DABUL lidar (prior to day 530) and the ARM ceilometer (after day 530).

Albedo.	Time series of albedo (x's) from the surface flux group's site at metcity and cloud fraction (circles) for SHEBA.