Diurnal cycles of cloud cover and its vertical distribution over the Tibetan Plateau revealed by satellite observations, reanalysis datasets, and CMIP6 outputs

[摘要] Diurnal variations in cloud cover and cloud vertical distributionare of great importance to Earth–atmosphere system radiative budgets andclimate change. However, thus far these topics have received insufficientattention, especially on the Tibetan Plateau (TP). This study focuses on thediurnal variations in total cloud cover, cloud vertical distribution, andcirrus clouds and their relationship to meteorological factors over the TPbased on active and passive satellite observations, reanalysis data, andCMIP6 outputs. Our results are consistent with previous studies but providenew insights. The results show that total cloud cover peaks at 06:00–09:00 UTC, especially over the eastern TP, but the spatial and temporaldistributions of clouds from different datasets are inconsistent. This could to someextent be attributed to subvisible clouds missed by passivesatellites and models. Compared with satellite observations, the amplitudesof the diurnal variations in total cloud cover obtained by the reanalysisand CMIP6 models are obviously smaller. CATS can capture the varying pattern ofthe vertical distribution of clouds and corresponding height of peak cloudcover at middle and high atmosphere levels, although it underestimates thecloud cover of low-level clouds, especially over the southern TP. Comparedwith CATS, ERA5 cannot capture the complete diurnal variations in verticaldistribution of clouds and MERRA-2 has a poorer performance. We furtherfind that cirrus clouds, which are widespread over the TP, show significantdiurnal variations with averaged peak cloud cover over 0.35 at 15:00 UTC.Unlike in the tropics, where thin cirrus ( 0.03  opticaldepth  0.3 ) dominate, opaque cirrus clouds ( 0.3  opticaldepth  3 ) are the dominant cirrus clouds over the TP. The seasonaland regional averaged cloud cover of opaque cirrus reaches a daily maximumof 0.18 at 11:00 UTC, and its diurnal cycle is strong positive correlationwith that of 250 hPa relative humidity and 250 hPa vertical velocity.Although subvisible clouds (optical depth  0.03 ), which have apotential impact on the radiation budget, are the fewest among cirrus cloudsover the TP, the seasonal and regional averaged peak cloud cover can reach0.09 at 22:00 UTC, and their diurnal cycle correlates with that of the250 hPa relative humidity, 2 m temperature, and 250 hPa vertical velocity.Our results will be helpful to improve the simulation and retrieval of totalcloud cover and cloud vertical distribution and further provide anobservational constraint for simulations of the diurnal cycle of surfaceradiation budget and precipitation over the TP region.

[学科分类] 医学（综合）