Min

Impact of diurnal variability and meteorological factors on the PM2.5 - AOD relationship: Implications for PM2.5 remote sensing

PM2.5 retrieval from space is still challenging due to the elusive relationship between PM2.5 and aerosol optical depth (AOD), which is further complicated by meteorological factors. In this work, we investigated the diurnal cycle of PM2.5 in China, using ground-based PM measurements obtained at 226 sites of China Atmosphere Watch Network during the period of January 2013 to December 2015. Results showed that nearly half of the sites witnessed a PM2.5 maximum in the morning, in contrast to the least frequent occurrence (5%) in the afternoon when strong solar radiation received at the surface results in rapid vertical diffusion of aerosols and thus lower mass concentration. PM2.5 tends to peak equally in the morning and evening in North China Plain (NCP) with an amplitude of nearly twice or three times that in the Pearl River Delta (PRD), whereas the morning PM2.5 peak dominates in Yangtze River Delta (YRD) with a magnitude lying between those of NCP and PRD. The gridded correlation maps reveal varying correlations around each PM2.5 site, depending on the locations and seasons. Concerning the impact of aerosol diurnal variation on the correlation, the averaging schemes of PM2.5 using 3-h, 5-h, and 24-h time windows tend to have larger R biases, compared with the scheme of 1-h time window, indicating diurnal variation of aerosols plays a significant role in the establishment of explicit correlation between PM2.5 and AOD. In addition, high cloud fraction and relative humidity tend to weaken the correlation, regardless of geographical location. Therefore, the impact of meteorology could be one of the most plausible alternatives in explaining the varying R values observed, due to its non-negligible effect on MODIS AOD retrievals. Our findings have implications for PM2.5 remote sensing, as long as the aerosol diurnal cycle, along with meteorology, are explicitly considered in the future.

Developing the Science Product Algorithm Testbed for Chinese Next-Generation Geostationary Meteorological Satellites: Fengyun-4 Series

Fengyun-4A (FY-4A), the first of the Chinese next-generation geostationary meteorological satellites, launched in 2016, offers several advances over the FY-2: more spectral bands, faster imaging, and infrared hyperspectral measurements. To support the major objective of developing the prototypes of FY-4 science algorithms, two science product algorithm testbeds for imagers and sounders have been developed by the scientists in the FY-4 Algorithm Working Group (AWG). Both testbeds, written in FORTRAN and C programming languages for Linux or UNIX systems, have been tested successfully by using Intel/g compilers. Some important FY-4 science products, including cloud mask, cloud properties, and temperature profiles, have been retrieved successfully through using a proxy imager, Himawari-8/Advanced Himawari Imager (AHI), and sounder data, obtained from the Atmospheric InfraRed Sounder, thus demonstrating their robustness. In addition, in early 2016, the FY-4 AWG was developed based on the imager testbed—a near real-time processing system for Himawari-8/AHI data for use by Chinese weather forecasters. Consequently, robust and flexible science product algorithm testbeds have provided essential and productive tools for popularizing FY-4 data and developing substantial improvements in FY-4 products.

On-Orbit Spatial Quality Evaluation and Image Restoration of FengYun-3C/MERSI

The Medium Resolution Spectral Imager (MERSI) was installed as a key payload on the FengYun-3C (FY-3C) polar-orbit meteorological satellite, which was successfully launched on September 23, 2013. We used 14 months of continuous FY-3C/MERSI Level-1B data (from November 2013 to December 2014) to evaluate the on-orbit image spatial quality. Based on a polar ice block image, a sharp target modulation transfer function (MTF) estimation method is used to quantitatively estimate the MTF value at the Nyquist frequency, which is an index for the spatial quality. The results show very good stability in the first year and a relatively lower spatial quality (MTF approximately 0.15) along the FY-3C/MERSI scan direction. This lower spatial quality is primarily attributed to the known and fixed 27% overlapped scan mode of MERSI, which can significantly reduce the image contrast. By using this fixed overlapped proportion (27%), we develop a fast and robust image restoration algorithm based on the Gaussian elimination (GE) method with lower and upper triangular matrix decomposition (LU). The speed-up ratio of this GE with LU decomposition method can attain a value of 626.30 compared with the traditional GE method when it solves linear equations with 2048 MERSI scan pixels. After the image restoration process, significant enhancement in the image spatial quality along the scan direction for every band of FY-3C/MERSI can be found with an increased MTF value of approximately 0.30. However, we evaluate the possible effect of this restoration algorithm on the original digital number (DN) and reflectance values. We find a slight decrease in the total averaged DN (0.5) and reflectance (<; 0.5%, relative bias) values. The variation in DN or reflectance after the image restoration process exhibits a positive correlation with homogeneity of the original target. Moreover, a sensitivity study on the reflectance reveals that it has a more significant impact on the inhomogeneous pixel with a low DN value.

Performance evaluation for on-orbit modulation transfer function of FengYun-3C medium resolution spectral imager (MERSI) using polar ice and snow

Modulation transfer function (MTF) can be used to evaluate spatial quality of an satellite imaging sensor using a sharp edge, a pulse target, or bar pattern target. This investigation evaluates on-orbit MTF performance of FengYun (FY)-3C MERSI with 20 Bands with 1 km and 250 m spatial resolutions using polar ice and snow as a sharp edge, which was launched on September 23 of 2013. The MTF is calculated by using a Fourier transformation on the line spread function (LSF) though a simple differentiation of the edge spread function (ESF). The final MTF Nyqusit frequencies of the most of MERSI Bands along FY-3C flight direction are higher than 0.30, which are satisfy the original design requirements of 0.25 (250 m) and 0.27 (1km). But the Nyquist frequencies of all Bands along FY-3C scanning direction are around 0.13 that are clearly lower than 0.25/0.27. This relatively worse spatial quality of image along FY-3C scanning direction is primarily attributed to the 27% overlapped scan mode of MERSI sensor for every pixel. The objective of design defect in FY-3C MERSI instrument system is to enhance the signal-to-noise ratio (SNR) of scanning image. To overcome the drawback, in future, we will develop some deblurring methods to restore FY-3C/MERSI image along scanning direction.

Radiance‐Based Evaluation of WRF Cloud Properties Over East Asia: Direct Comparison With FY‐2E Observations

Radiance-based comparison between model-simulated and satellite-observed atmosphere shows its unique advantage for model evaluation. This study couples a fast radiative transfer model, namely, the community radiative transfer model, and the Weather Research and Forecasting (WRF) to generate synthetic satellite observed infrared (IR) brightness temperatures (BTs) over East Asia. Simulated IR BTs are used to evaluate WRF cloud properties by direct comparison with observations from the Visible and Infrared Spin-Scan Radiometer (onboard the FengYun-2E geostationary satellite). A deep cyclone over North China is investigated as our case study. The simulated and observed BTs in the 6.8-μm channel demonstrate agreement with an averaged bias of <3 K, whereas the performances in two IR window channels are relatively poor with averaged BT differences almost 10 K. For typical precipitation regions, BT differences between the 6.8and 10.8-μm channels indicate that the WRF simulation underestimates lowto midlevel clouds but reproduces high-level clouds better. Overall, the WRF has solid capabilities to capture the cloud characteristics broadly, whereas some limitations are shown in the cloud temporal and variation cloud cover, especially 3 days after the WRF initialization.

Performance assessment of FY-3C/MERSI on early orbit

FY-3C/MERSI has some remarkable improvements compared to the previous MERSIs including better spectral response function (SRF) consistency of different detectors within one band, increasing the capability of lunar observation by space view (SV) and the improvement of radiometric response stability of solar bands. During the In-orbit verification (IOV) commissioning phase, early results that indicate the MERSI representative performance were derived, including the signal noise ratio (SNR), dynamic range, MTF, B2B registration, calibration bias and instrument stability. The SNRs at the solar bands (Bands 1–4 and 6-20) was largely beyond the specifications except for two NIR bands. The in-flight calibration and verification for these bands are also heavily relied on the vicarious techniques such as China radiometric calibration sites(CRCS), cross-calibration, lunar calibration, DCC calibration, stability monitoring using Pseudo Invariant Calibration Sites (PICS) and multi-site radiance simulation. This paper will give the results of the above several calibration methods and monitoring the instrument degradation in early on-orbit time.