Abstract
Abstract. Clouds exhibit distinct horizontal inhomogeneities of their optical and microphysical properties, which complicate their realistic representation in weather and climate models. In order to investigate the horizontal structure of cloud inhomogeneities, 2-D horizontal fields of optical thickness (τ) of subtropical cirrus and Arctic stratus are investigated with a spatial resolution of less than 10 m. The 2-D τ-fields are derived from (a) downward (transmitted) solar spectral radiance measurements from the ground beneath four subtropical cirrus and (b) upward (reflected) radiances measured from aircraft above 10 Arctic stratus. The data were collected during two field campaigns: (a) Clouds, Aerosol, Radiation, and tuRbulence in the trade wind regime over BArbados (CARRIBA) and (b) VERtical Distribution of Ice in Arctic clouds (VERDI). One-dimensional and 2-D autocorrelation functions, as well as power spectral densities, are derived from the retrieved τ-fields. The typical spatial scale of cloud inhomogeneities is quantified for each cloud case. Similarly, the scales at which 3-D radiative effects influence the radiance field are identified. In most of the investigated cloud cases considerable cloud inhomogeneities with a prevailing directional structure are found. In these cases, the cloud inhomogeneities favour a specific horizontal direction, while across this direction the cloud is of homogeneous character. The investigations reveal that it is not sufficient to quantify horizontal cloud inhomogeneities using 1-D inhomogeneity parameters; 2-D parameters are necessary.
Highlights
The globally and annually averaged cloud cover is in the range of about 70 % (Rossow and Schiffer, 1999)
In order to investigate the horizontal structure of cloud inhomogeneities, 2-D horizontal fields of optical thickness (τ ) of subtropical cirrus and Arctic stratus are investigated with a spatial resolution of less than 10 m
Four subtropical cirrus cases collected during CARRIBA and 10 Arctic stratus cases sampled during VERtical Distribution of Ice in Arctic clouds (VERDI) were studied in detail
Summary
The globally and annually averaged cloud cover is in the range of about 70 % (Rossow and Schiffer, 1999). High ice clouds (cirrus) and Arctic stratus exhibit horizontal inhomogeneities at different horizontal scales Both cloud types can either warm or cool Earth’s climate system, depending on their optical and microphysical properties and the meteorological conditions. Three-dimensional Monte Carlo radiative transfer simulations account for horizontal photon transport (Barlakas et al, 2016) They are costly in terms of computational time and memory (Huang and Liu, 2014). To reduce uncertainties associated with the 1D plane-parallel assumption, Huang and Liu (2014) applied spatial autocorrelation functions of cloud extinction coefficients to capture the net effects of sub-grid cloud interactions with radiation.
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