What can be learned from ARM shortwave hyperspectral observations?

Poster PDF

Description

The transition zone between cloudy and clear air is a region of strong aerosol-cloud interactions where aerosol particles humidify and swell while cloud drops evaporate and shrink, and vice versa. In an effort to improve our understanding of the transition zone, we study the radiative signature of transmitted shortwave radiation using radiative transfer models and observations of the recently renovated shortwave spectrometer (SWS) and the recently deployed shortwave array spectrometer-zenith (SASZE). This work serves as a step in closing the surface shortwave radiation gap that exists between models and observations at the surface under fully 3D cloud scenes. The transition zone also plays a vital role in climate questions surrounding aerosol-cloud interactions, or “aerosol indirect effects”. It was discovered that the spectra in the transition zone could be represented by a linear combination of the spectra taken in the contiguous cloudy and cloudless regions. The linear relationship results in a spectrally invariant slope and intercept. The slope, fit over a range of visible wavelengths, is mostly dependent on the cloud optical thickness, and the intercept, fit over wavelengths in the near-infrared, is dependent on both the optical thickness and the particle size. Due to normalization to the cloud-free spectra, both the slope and the intercept only weakly depend on the surrounding aerosols and surface properties. We explore the use of this relationship as a means for understanding the spectral response to the transition zone and changes in the particle size there. An algorithm is proposed, and the limitations of the algorithm are presented.