Abstract
Variable solar heating drives the seasonal variability of Titan’s lower atmospheric dynamics, and therefore its hydrological cycle. Circulation models that have been developed to examine this methane cycle tend to produce a globally oscillating Hadley circulation, the upwelling arm of which follows a diurnal-mean insolation maximum that reaches the pole in summertime (e.g. Mitchell et al., 2006; Schneider et al., 2012). These models use highly simplified parameterizations of radiative transfer, designed to fit Huygens measurements from the equatorial regions; they do not account for the increased attenuation of sunlight at higher latitudes due to Titan’s curvature. Haze scattering in Titan’s atmosphere complicates the calculation of the radiation field that reaches the troposphere. However, based on Huygens DISR measurements, Tomasko et al. (2008) computed solar heating rates as a function of altitude for different latitudes, and at different seasons, including a scattering model. In their results, the maximum heating, during solstice, below ~50 km (i.e., in the troposphere) occurred at mid-latitudes, not the poles as might be assumed from the insolation distribution at the top of the atmosphere. Based on these results, we calculated an insolation distribution near the surface that differs significantly from that used in previous models (Lora et al., 2011). This has implications for the circulation, which we explored with a very simple box model that accounts only for thermally driven advection: Forced with the calculated insolation distribution, the model produces surface temperatures in agreement with observations (Jennings et al. 2009), and a circulation pattern significantly different than the one produced with the simplified distribution from the top of the atmosphere.
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