Brian J. Soden

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

[1]xa0Global high-resolution (3-hourly, 0.1° × 0.1° longitude-latitude) water vapor (6.7 μm) and window (11 μm) 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 models 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.

The diurnal cycle of convection, clouds, and water vapor in the tropical upper troposphere

Hourly observations of the 6.7 µm water vapor radiances from geostationary satellites are used to document the diurnal cycle in upper tropospheric water vapor and its relationship to cloud cover and convection. A coherent diurnal cycle in tropical water vapor is observed which lags the variations in cloud cover by approximately 2 hours. The variations in upper tropospheric cloud and water vapor occur (roughly) in phase with changes in deep convection over land, but nearly 12 hours out of phase with those over ocean. This feature is shown to be associated with differences in the vertical structure of land and ocean convection and offers a useful test of convective parameterizations in atmospheric models.

Diurnal cycle of summertime deep convection over North America: A satellite perspective

[1]xa0High-resolution (0.1° × 0.1°) geostationary satellite infrared radiances at 11 μm in combination with gridded (2.5° × 2.0°) hourly surface precipitation observations are employed to document the spatial structure of the diurnal cycle of summertime deep convection and associated precipitation over North America. Comparison of the diurnal cycle pattern between the satellite retrieval and surface observations demonstrates the reliability of satellite radiances for inferring the diurnal cycle of precipitation, especially the diurnal phase. On the basis of the satellite radiances, we find that over most land regions, deep convection peaks in the late afternoon and early evening, a few hours later than the peak of land surface temperature. However, strong regional variations exist in both the diurnal phase and amplitude, implying that topography, land-sea contrast, and coastline curvature play an important role in modulating the diurnal cycle. Examples of such effects are highlighted over Florida, the Great Plains, and the North American monsoon region.

The impact of tropical convection and cirrus on upper tropospheric humidity: A Lagrangian analysis of satellite measurements

[1]xa0Geostationary satellite observations are used in conjunction with an objective pattern-tracking algorithm to describe the Lagrangian evolution of convection, clouds and water vapor in the tropical upper troposphere. This analysis reveals that larger convective events within a Lagrangian air mass are associated with larger and longer-lived cirrus anvil shields. Convective systems which generate larger cirrus shields are, in turn, associated with higher downstream humidity levels following the anvils dissipation. In the absence of cirrus, the clear-sky upper troposphere is shown to dry at a rate consistent with radiatively-driven subsidence. The presence of cirrus anvils following a convective event is shown to reduce the rate of drying and for large anvils can even change its sign. Analysis of the Lagrangian tendencies suggests that this moistening effect is not attributable to the evaporation of cirrus condensate, but instead results from the same dynamical mechanisms responsible for the formation and maintenance of the cirrus anvil.

Variations in atmosphere‐ocean solar absorption under clear skies: A comparison of observations and models

Satellite observations of the clear-sky, atmosphere-ocean solar absorption are compared to simulations from a global climate model. Variations in solar absorption are analyzed under conditions of constant solar geometry to highlight spatially coherent features. From this analysis, the observed zonal and interannual variability in clear-sky solar absorption is shown to be substantially larger than that predicted by the model. Explanations for the greater observed variability are considered in terms of aerosols, surface wind effects, and water vapor.