Development and Application of HECORA Cloud Retrieval Algorithm Based On the O2-O2 477 nm Absorption Band

Shuntian Wang,Chengxin Zhang,Cheng Liu,Wenjing Su,Chengzhi Xing,Nan Hao,Wenqiang Zhang,Sebastián Gimeno García,Jianguo Liu

doi:10.3390/rs12183039

Abstract

In this paper, we present the Hefei EMI Cloud Retrieval Algorithm (HECORA), which uses information from the O2-O2 absorption band around 477 nm to retrieve effective cloud fraction and effective cloud pressure from satellite observations. The retrieved cloud information intends to improve the atmospheric trace gas products based on the Environment Monitoring Instrument (EMI) spectrometer. The HECORA method builds on OMCLDO2 and presents some evolutions. The Vector Linearized Discrete Ordinate Radiative Transfer (VLIDORT) model has been used to produce the Top of the Atmosphere (TOA) reflectance Look-up Tables (LUT) as a function of the cloud fraction and cloud pressure. Applying the Differential Optical Absorption Spectroscopy (DOAS) technique to the synthetic reflectance LUT, the reflectance spectra can be associated with O2-O2 geometrical vertical column densities (VCDgeo) and continuum reflectance. This is the core of the retrieval method, since there is a one-to-one relationship between O2-O2 VCDgeo and continuum reflectance, on the one hand, and effective cloud fraction and effective cloud pressure, on the other hand, for a given illumination and observing geometry and given surface height and surface albedo. We first used the VLIDORT synthetic spectra to verify the HECORA algorithm and obtained good results in both the Lambertian cloud model and the scattering cloud model. Secondly, HECORA is applied to OMI and TROPOMI and compared with OMCLDO2, FRESCO+, and OCRA/ROCINN cloud products. Later, the cloud pressure results from TROPOMI observations obtained using HECORA and FRESCO+ are compared with the CALIOP Cloud Layer product. HECORA is closer to the CALIOP results under low cloud conditions, while FRESCO+ is closer to high clouds due to the higher sensitivity of the O2 A-band to cloud vertical information. Finally, HECORA is applied to the TROPOMI NO2 retrieval. Validation of the tropospheric NO2 VCD with ground-based MAX-DOAS measurements shows that choosing HECORA cloud products to correct for photon path variations on the TROPOMI tropospheric NO2 VCD retrievals has better performance than using FRESCO+ under low cloud conditions. In conclusion, this paper shows that the HECORA cloud products are in good agreement with the well-established cloud products and that they are suitable for correcting the effect of cloud in trace gas retrievals. Therefore, HECORA has the potential to be applied to EMI.

Highlights

About 67% of the Earth’s surface is covered by clouds [1]
The simulations are performed on Lambertian cloud and scattering cloud models
Based on the comparison in Section 3.3.2, we found that under low altitude cloud conditions, the cloud pressure retrieved by Hefei Environment Monitoring Instrument (EMI) Cloud Retrieval Algorithm (HECORA) is closer to that of the CALIOP cloud layer products than FRESCO+

Summary

Introduction

About 67% of the Earth’s surface is covered by clouds [1]. Clouds play a major role in the radiation budget of the Earth climate system and hydrological cycle through shielding the short-wave radiation from the sun and absorbing (and re-emitting) long-wave radiation from the Earth. When using satellite data to monitor atmospheric pollution, clouds are an important factor affecting the accuracy of trace gas retrieval for both stratospheric and tropospheric gases. The albedo and intra-cloud absorption effects increase the visibility of trace gas at and above the cloud top, while the shielding effect results in an underestimation of the trace gases vertical column densities. For the retrieval of tropospheric trace gases, the albedo effect and intra-cloud absorption effect may be beneficial. When the most portion of the trace gas is above the cloud, such as ozone, or the stratospheric column density is subtracted from the total column density to obtain the tropospheric column density, the albedo plays an important role [4,5,6]

Results

Discussion

Conclusion