Abstract
Abstract. Cloud top height (CTH) affects the radiative properties of clouds. Improved CTH observations will allow for improved parameterizations in large-scale models and accurate information on CTH is also important when studying variations in freezing point and cloud microphysics. NASA's airborne Research Scanning Polarimeter (RSP) is able to measure cloud top height using a novel multi-angular contrast approach. For the determination of CTH, a set of consecutive nadir reflectances is selected and the cross correlations between this set and collocated sets at other viewing angles are calculated for a range of assumed cloud top heights, yielding a correlation profile. Under the assumption that cloud reflectances are isotropic, local peaks in the correlation profile indicate cloud layers. This technique can be applied to every RSP footprint and we demonstrate that detection of multiple peaks in the correlation profile allows retrieval of heights of multiple cloud layers within single RSP footprints. This paper provides an in-depth description of the architecture and performance of the RSP's CTH retrieval technique using data obtained during the Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC4RS) campaign. RSP-retrieved cloud heights are evaluated using collocated data from the Cloud Physics Lidar (CPL). The method's accuracy associated with the magnitude of correlation, optical thickness, cloud thickness and cloud height are explored. The technique is applied to measurements at a wavelength of 670 and 1880 nm and their combination. The 1880 nm band is virtually insensitive to the lower troposphere due to strong water vapor absorption. It is found that each band is well suitable for retrieving heights of cloud layers with optical thicknesses above about 0.1 and that RSP cloud layer height retrievals more accurately correspond to CPL cloud middle than cloud top. It is also found that the 1880 nm band yields the most accurate results for clouds at middle and high altitudes (4.0 to 17 km), while the 670 nm band is most accurate at low and middle altitudes (1.0–13.0 km). The dual band performs best over the broadest range and is suitable for accurately retrieving cloud layer heights between 1.0 and 16.0 km. Generally, the accuracy of the retrieved cloud top heights increases with increasing correlation value. Improved accuracy is achieved by using customized filtering techniques for each band with the most significant improvements occurring in the primary layer retrievals. RSP is able to measure a primary layer CTH with a median error of about 0.5 km when compared to CPL. For multilayered scenes, the second and third layer heights are determined median errors of about 1.5 and 2.0–2.5 km, respectively.
Highlights
Clouds cover roughly two thirds of the globe (Mace et al, 2009) and act as an important regulator of the Earth’s radiation budget (Boucher et al, 2013)
This paper provides an in-depth description and performance analysis of the Research Scanning Polarimeter (RSP)’s cloud top height (CTH) retrieval technique using data obtained during the Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC4RS; Toon et al, 2016) campaign
In order to investigate to which depths within the cloud layers the retrieved layer heights pertain, we present here a comparison of the RSP cloud layer heights using the 1880 nm, 670 nm and dual bands with the Cloud Physics Lidar (CPL)’s cloud top and cloud middle heights
Summary
Clouds cover roughly two thirds of the globe (Mace et al, 2009) and act as an important regulator of the Earth’s radiation budget (Boucher et al, 2013). K. Sinclair et al.: Remote sensing of multiple cloud layer heights tical structure (location of cloud top and base, number and thickness of layers) affect the radiative properties of clouds (Boucher et al, 2013) and can have significant effects on climate (Collins et al, 1994). In addition to global studies, detailed regional observations are crucial to improve our physical understanding of the relationships between cloud top height (CTH), environmental conditions and other cloud properties. Additional observations of cloud top height will lead to a better understanding of its relationship to cloud thermodynamic phase, atmospheric dynamics, relative humidity and aerosol concentrations that is needed for improved sub-grid parameterizations in largescale models
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