Uncertainties in cloud phase and optical thickness retrievals from the Earth Polychromatic Imaging Camera (EPIC).

Kerry Meyer,Yuekui Yang,Steven Platnick

doi:10.5194/amt-9-1785-2016

Abstract

This paper presents an investigation of the expected uncertainties of a single channel cloud optical thickness (COT) retrieval technique, as well as a simple cloud temperature threshold based thermodynamic phase approach, in support of the Deep Space Climate Observatory (DSCOVR) mission. DSCOVR cloud products will be derived from Earth Polychromatic Imaging Camera (EPIC) observations in the ultraviolet and visible spectra. Since EPIC is not equipped with a spectral channel in the shortwave or mid-wave infrared that is sensitive to cloud effective radius (CER), COT will be inferred from a single visible channel with the assumption of appropriate CER values for liquid and ice phase clouds. One month of Aqua MODIS daytime granules from April 2005 is selected for investigating cloud phase sensitivity, and a subset of these granules that has similar EPIC sun-view geometry is selected for investigating COT uncertainties. EPIC COT retrievals are simulated with the same algorithm as the operational MODIS cloud products (MOD06), except using fixed phase-dependent CER values. Uncertainty estimates are derived by comparing the single channel COT retrievals with the baseline bi-spectral MODIS retrievals. Results show that a single channel COT retrieval is feasible for EPIC. For ice clouds, single channel retrieval errors are minimal (< 2%) due to the particle size insensitivity of the assumed ice crystal (i.e., severely roughened aggregate of hexagonal columns) scattering properties at visible wavelengths, while for liquid clouds the error is mostly limited to within 10%, although for thin clouds (COT < 2) the error can be higher. Potential uncertainties in EPIC cloud masking and cloud temperature retrievals are not considered in this study.

Highlights

The Deep Space Climate Observatory (DSCOVR) satellite was launched on 11 February 2015 and in June 2015 began making observations of both the Earth and the Sun from its Lissajous orbit about the Earth’s L1 Lagrangian point, a gravity-neutral position 1.5 million km from the Earth
We note again that this paper focuses on the uncertainties in cloud optical thickness (COT) retrievals resulting from a fixed cloud effective radius (CER) assumption and the uncertainties in cloud thermodynamic phase determination using only cloud temperature thresholds; uncertainties in cloud temperature due to effective height retrieval errors are beyond the scope of this paper and are left for future investigations
Cloud thermodynamic phase results from the operational MOD06 cloud optical properties phase product (c) and the proxy Earth Polychromatic Imaging Camera (EPIC) algorithm (d) that uses dual cloud top temperature (CTT) thresholds (Sect. 3) are shown for an Aqua MODerate-resolution Imaging Spectroradiometer (MODIS) granule obtained over the eastern Pacific Ocean on 9 April 2005 (22:15 UTC)

Summary

Introduction

The Deep Space Climate Observatory (DSCOVR) satellite was launched on 11 February 2015 and in June 2015 began making observations of both the Earth and the Sun from its Lissajous orbit about the Earth’s L1 Lagrangian point, a gravity-neutral position 1.5 million km from the Earth. At the L1 point, DSCOVR will remain near the Sun–Earth line, viewing the entire sunlit half of the Earth continuously at nearbackscatter directions with scattering angles ranging from roughly 164 to 176◦. NISTAR observes the Earth as a single pixel and provides measurements of the solar radiation reflected by the Earth, as well as the Earth’s total radiant power, in four broadband channels. EPIC, with its 2048 × 2048 CCD (charge-coupled device) array, provides higher-spatialresolution details of the daytime Earth that can be used to interpret the single-pixel NISTAR data. Note that at the L1 point the CCD array resolution yields roughly 8 km pixel sampling at the Earth’s surface at nadir, though aggregating the native radiances to coarser resolution (e.g., 1024 × 1024) remains a possibility

Methods

Results

Conclusion