Abstract
Abstract. To better understand the accuracy of cloud top heights (CTHs) derived from passive satellite data, ground-based Ka-band radar measurements from 2016 and 2017 in Beijing are compared with CTH data inferred from the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Advanced Himawari Imager (AHI). Relative to the radar CTHs, the MODIS CTHs are found to be underestimated by−1.10 ± 2.53 km on average and 49 % of CTH differences are within 1.0 km. The AHI CTHs are underestimated by −1.10 ± 2.27 km and 42 % are within 1.0 km. Both the MODIS and AHI CTH retrieval accuracy depends strongly on the cloud depth (CD). Large differences are mainly due to the retrieval of thin clouds of CD <1 km, especially when the cloud base height is higher than 4 km. For clouds with CD >1 km, the mean CTH difference decreases to -0.48±1.70 km for MODIS and to -0.76±1.63 km for AHI. It is found that MODIS CTHs with higher values (i.e. >6 km) show smaller discrepancy with radar CTH than those MODIS CTHs with lower values (i.e. <4 km). Statistical analysis illustrate that the CTH difference between the two satellite instruments is lower than the difference between the satellite instrument and the ground-based Ka-band radar. The monthly accuracy of both CTH retrieval algorithms is investigated and it is found that summer has the smallest retrieval difference.
Highlights
Clouds play important role in the water and energy budgets in the Earth–atmosphere system (Ramanathan et al, 1989; Liou, 1992; Cess et al, 1996; Boucher et al, 2013)
Comparisons of stratocumulus cloud top heights (CTHs) simulated from general circulation model (GCM) with those retrieved from satellites suggested that either satellite retrievals placed stratocumulus clouds too high in the atmosphere or GCM cloud tops were biased too low (Rossow and Schiffer, 1999)
In order to compare with passive satellite data, for clouds detected in a period, the radar CTH is calculated as the mean CTH of all cloudy profiles but not upper-level cloud if there are multilayer clouds
Summary
Clouds play important role in the water and energy budgets in the Earth–atmosphere system (Ramanathan et al, 1989; Liou, 1992; Cess et al, 1996; Boucher et al, 2013). Satellites measure large-scale cloud systems, but the CTHs retrieved from passive IR instruments are still subject to large uncertainties. This study assesses the accuracy of the CTHs derived from passive satellites through comparison with surface active radar data. Xi et al (2014) found that daytime CTH of marine boundary layer cloud retrieved from MODIS was 0.063 km higher on average than what surface lidar and radar had measured. This study compares the retrieved MODIS CTHs with radar measurements in Beijing over a long period and provides further knowledge about the uncertainty of MODIS CTH products. We compare and evaluate the CTHs retrieved from the passive satellite instruments on board a polar-orbiting satellite and a geostationary satellite with those measured by a surface active radar in Beijing over a long period. This work quantifies the satellite CTH retrieval accuracy and provides a reference and usage guidance for the application of the CTH datasets in meteorological research, such as climate model simulations for Beijing and northern China
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