Summary Issues SWS Observations It is difficult to distinguish between cloudy and cloud- free air in remote sensing observations. This transition zone is neither precisely clear nor precisely cloudy. This problem has major climatic consequences, in particular on aerosol indirect effect studies, which demand a precise separation of clear and cloudy air. Using spectral measurements of the ARM shortwave spectrometer (SWS) we test the following hypothesis: Zenith radiance spectrum in the transition zone is a linear combination of cloudy and clear sky spectra with a wavelength-independent weighting function: Motivation and Major Hypothesis Results and Discussion Cloud Cloudy to clear Clear Clear to cloudy a)Total sky images on 2007/05/18; SZA=45 o; ; b)Raw spectral data from 150 s to 250 s with 10 s interval c)Time series of normalized radiance spectrum d)Same as in b) but for normalized zenith radiance 0 sec120 sec240 sec a)Total sky images on 2007/09/05; SZA=65 o; ; b)Raw spectral data from 45 s to 85 s with 5 s interval c)Time series of normalized radiance spectrum d)Same as in b) but for normalized zenith radiance (c) (a) (b) (a) Spectral invariant behavior of clear sky near clouds found in the ARM SWS measurements Alexander Marshak 1, Yuri Knyazikhin 2, Christine Chiu 3, Warren Wiscombe 1,4, and Peter Pilewskie 5 1 NASA/Goddard Space Flight Center 2 Boston University 3 Joint Center for Earth Systems Technology/UMBC 4 Brookhaven national Laboratory 5 University of Colorado There is a wavelength-independent (invariant) function a ( time ) or a ( space ) that characterizes the transition zone between cloudy and clear areas; The transition zone spectrum is fully determined by a wavelength- independent function a ( t ) and zenith radiances at only two wavelengths (at least for shorter wavelengths); High temporal resolution zenith radiance measurements in the transition zone can be approximated by low temporal resolution measurements using linear interpolation. 0 1 a(t)a(t) t from cloudy to clear Blocked around 400 sec Unblocked from 0 to 380 sec Contamination by the SWS Direct Irradiance Problem Nonlinearity seen for dominantly clear sky areas (b) (c) (d) 120 sec 0 sec60 sec (d) Hypothesis to test: We plot: Check: ( i ) linearity and ( ii ) a+b=1 a+b a+b Spectral-invariant function a (t) for the case. Normalized zenith radiances for several wavelengths are added for comparison. Spectral-invariant function a (t) characterizes the transition zone between cloudy and cloud-free regions. It carries information about the internal cloud structure and is independent of wavelength. Knowledge of a (t) allows us to retrieve the transition zone spectra and approximate high resolution measurements by low resolution ones using linear interpolation Blocked (in shadow) Unblocked (illuminated) The direct irradiance contamination of observed radiance has been corrected as where s( ) is the difference between the nearest block and unblocked regions (assumed to be the same for the whole time interval). Blocked (in shadow) Unblocked (illuminated) Blocked (in shadow) For the cloud-free dominated regions for shorter wavelength, the nonlinearity may result from strongly non-linear molecular scattering ??? Uncertainty in data ??? a+b Look at the color of the background Schematic representation of function a (t) The measured function a (t) is shown at the bottom of the middle column zoom