Diurnal cycle of convection, clouds, and water vapor in the tropical upper troposphere: Satellites versus a general circulation model

Abstract

[1] Global high-resolution (3-hourly, 0.1°Â 0.1°longitude-latitude) water vapor (6.7 mm) and window (11 mm) radiances from multiple geostationary satellites are used to document the diurnal cycle of upper tropospheric relative humidity (UTH) and its relationship to deep convection and high clouds in the whole tropics and to evaluate the ability of the new Geophysical Fluid Dynamics Laboratory (GFDL) global atmosphere and land model (AM2/LM2) to simulate these diurnal variations. Similar to the diurnal cycle of deep convection and high clouds, coherent diurnal variations in UTH are also observed over the deep convective regions, where the daily mean UTH is high. In addition, the diurnal cycle in UTH also features a land-sea contrast: stronger over land but weaker over ocean. UTH tends to peak around midnight over ocean in contrast to 0300 LST over land. Furthermore, UTH is observed to lag high cloud cover by $6 hours, and the latter further lags deep convection, implying that deep convection serves to moisten the upper troposphere through the evaporation of the cirrus anvil clouds generated by deep convection. Compared to the satellite observations, AM2/LM2 can roughly capture the diurnal phases of deep convection, high cloud cover, and UTH over land; however, the magnitudes are noticeably weaker in the model. Over the oceans the AM2/LM2 has difficulty in simulating both the diurnal phase and amplitude of these quantities. These results reveal some important deficiencies in the model's convection and cloud parameterization schemes and suggest the lack of a diurnal cycle in SST may be a shortcoming in the boundary forcing for atmospheric models.

Insights into the diurnal cycle of global Earth outgoing radiation using a numerical weather prediction model

“…Diurnal variability in the Earth system that defines such signatures has been studied extensively (e.g., Nitta and Sekine, 1994;Webster at al., 1996;Soden, 2000;Yang and Slingo, 2001;Wood et al, 2002;Nesbitt and Zipser, 45 2003;Taylor, 2012). However, discrepancies persist when comparing the diurnal cycles in observations and models (e.g., Betts and Jakob, 2002;Dai and Trenberth, 2004;Slingo et al, 2004;Tian et al, 2004;Itterly and Taylor, 2014). These discrepancies highlight a lack of understanding, yet it is essential we can correctly represent diurnal variability since it constitutes a fundamental forcing cycle for our weather and climate.…”

“…CC BY 4.0 License. and window (11 μm) channel radiances from multiple geostationary satellites spanning global longitude, Tian et al (2004) showed that deep convection in the tropics acts to moisten the upper troposphere via the evaporation of anvil clouds generated by deep convection. This increase in UTH can be prevalent over large spatial extents and will delay the increase in OLR after the convective cloud has dissipated due to continued absorption of the more intense radiation originating from warmer, lower 385 altitudes.…”

“…The study by Tian et al (2004) suggested a lag of approximately 6 hours between high cloud cover and UTH. In a similar but more spatially-and temporally-limited analysis, Soden (2000) suggested a lag time of approximately 2 hours.…”

Comparison of satellite limb-sounding humidity climatologies of the uppermost tropical troposphere

“…The diurnal cycle in UTH is reported to have an amplitude of 2-6% (Soden, 2000;Tian et al, 2004), with different peak-times for regions above ocean or land. However, as the amplitude of the diurnal cycle is relatively low compared to the accuracy of the individual measurements, this effect is also considered to be of minor importance here.…”

View-angle dependent AIRS cloud radiances: Implications for tropical anvil structures

“…Global Circulation Models (GCMs) simulations are sensitive to cloud microphysics when we calculate heating profiles due to clouds [Clement and Soden, 2005]. These clouds comprise a significant portion of stratiform precipitation [Cheng and Houze, 1979] and water vapor transport from the lower atmosphere to upper atmosphere [Tian et al, 2004], which are also closely associated with properties of cloud particles inside anvils (e.g., microphysical composition, organization, etc.). Tropical anvil clouds have been widely studied from the radiative and hydrological perspectives during the last two decades with satellite observations and high-resolution GCM simulations [e.g., Houze, 1982;Johnson and Young, 1983;Yuan et al, 2011], but cloud dynamics, a more difficult problem, has received relatively less attention.…”