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Dynamics, Structure and Importance of Deep Atmospheric Convection on Earth, Mars, and Titan

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Dynamics, Structure and Importance of Deep Atmospheric Convection on Earth, Mars, and Titan Comparative Climatology III 2018 (LPI Contrib. No. 2065) 2032.pdf DYNAMICS, STRUCTURE AND IMPORTANCE OF DEEP ATMOSPHERIC CONVECTION ON EARTH, MARS, AND TITAN. S. C. R. Rafkin, Southwest Research Institute, 1050 Walnut Street, Suite 300, Boulder, CO, [email protected]. Introduction: Earth, Mars, and Titan (EMT) all ex- and secondary indirect impacts on diabatic heating, if at hibit deep convection driven by diabatic heating. On all. Earth and Titan the heating is a result of latent heat re- Convective Organization: The storm environment lease associated with the phase change of water and me- can result in convective organization. For Earth, the thane, respectively. On Mars, radiative heating of dust necessary environment for the development of squall provides buoyancy. From a dynamical standpoint, the lines, mesoscale convective complexes, and tropical cy- response of an atmosphere to diabatic heating is inde- clones are well known, e.g., [4]. Model simulations pendent of the source of that heating—condensation, show that Titan could develop squall lines similar to dust, aliens, or magic—it doesn’t matter. Consequently, Earth under similar environmental wind shear onditions the dynamics in the atmospheres of EMT respond in [5]. On Mars, dust storms may behave more closely to similar ways and, likewise, the dynamics feedback and tropical systems on Earth, requiring weak wind shear modulate deep convection in similar ways. Deep con- and deep adiabatic layers in order to grow and mature vection is also known to play a critical role in the water [6]. vapor and momentum budget of Earth. Within the last Deep Convective Transport: The importance of decade, deep convection has been shown to be im- deep convection on Earth has been known for well over portant for the Mars dust cycle. The methane cycle of half a century, e.g., [7][8]. Rafkin [9] hypothesized that Titan is also likely to be sensitive to the infrequent but deep transport on Mars driven by topographic circula- intense deep convection, but this is largely hypothetical tions might play a similar role. Later modeling studies, based on limited modeling results and even more sparse e.g., [6][10], indicated that radiative heating of dust observational evidence. could also produce deep convective dust plumes and Deep Atmospheric Convection: Deep convection larger-scale convective structures. Vertical dust profiles results when the combination of diabatic heating and ad- from the Mars Climate Sounder (MCS) revealed ele- iabatic cooling results in air that is warmer than its en- vated maxima of dust mixing consistent with what vironment, e.g., [1]. Dynamics are agnostic about the would be expected by rapid, deep transport e.g., [11]. source of the diabatic heating; the governing equations This observational evidence confirmed that the dust cy- for this process contains a generic source term (often cle, and therefore the thermal structure and dynamics of represented by the symbol 푄̇ . All that matters is the Mars, was sensitive to deep convection. structure and temporal evolution of that heating. All Methane convection on Titan is strongly analogous other things being equal, an atmosphere will respond to Earth, but there are key differences. First, the fre- identically to identical heating regardless of the origin quency and coverage of Titan’s convection is far less of that heating. Consequently, to the extent that atmos- compared to Earth. This might be expected to limit the pheres of EMT are similar in structure, they respond relative importance of Titan’s convection on global similarly to heating and the behavior of Mars dust scale cycles. Second, Titan’s convection is far deeper storms, Titan methane storms, and Earth thunderstorms and contains relatively greater abundances of vapor and should be similar. condensate compared to Earth. Thus, even though more The evolution of a convective storm on EMT can be sparse and less frequent than on Earth, the vigor and different, because of the way the storms themselves transport of Titan’s convection may compensate in the feedback or affect their environment. For example, the Titan climate system. Titan general circulation model cold outflow from an Earth or Titan storm can separate simulations [12] found that latent heating from deep the convective cell from the unstable air that it relies convection had important effects on the Hadley Cell. upon for maintenance or growth, e.g., [2][3]. Dust does Observations of Titan, largely from Cassini, are gener- not evaporate, so dust storms cannot produce cold out- ally too infrequent to unambiguously or definitively flow through microphysical processes. However, shad- identify the role or importance of deep convection, but owing by the storm could produce cooler air due the based on experience with Earth and Mars, it should be strong interaction of radiation and dust. Further, winds expected that the deep convection will produce noticea- generated from a Mars storm can directly enhance the ble impacts on the abundance and structure of methane dust lifting which leads to a direct diabatic effect, and its photochemical byproducts in the upper tropo- whereas increased winds on Earth or Titan have muted sphere and lower stratosphere. Rapid and deep injection of high methane abundance air into the stratosphere is Comparative Climatology III 2018 (LPI Contrib. No. 2065) 2032.pdf generally not considered in photochemical models of Titan; diffusion is the primary transport term. Conclusions: The similarities and differences be- tween deep convection on Earth, are compared and con- trasted. While the basic physics of deep convection is driven by diabatic heating, the storms can alter their evolution and feedback to the environment in different ways. The organization of convective systems on Titan and Mars appear to have close analogs to system on Earth. The importance of convection in any atmos- phere, not just EMT, is likely to have an important ef- fect. Neglect of deep convective processes and transport is, therefore, likely to leave a gap in the under- standing of atmospheric climate systems. References: [1] Rennó, N. O., and Ingersoll, A. P. (1996). JAS, 53(4), 572-585. [2] Weisman, M. L., and Klemp, J. B. (1986), In Mesoscale meteorology and forecasting, 331-358. [3] Hueso, R., and Sánchez- Lavega, A. (2006), Nature, 442(7101), 428. [4] Weis- man, M. L., and Klemp, J. B. (1982). MWR, 110(6), 504-520. [5] Rafkin, S. C., and Barth, E. L. (2015), JGR, 120(4), 739-759. [6] Rafkin, S. C. (2009). JGR, 114(E1). [7] Riehl, H., and Malkus, J. (1961). Tellus, 13(2), 181-213. [8] Zipser, E. J. (2003). In Cloud Sys- tems, Hurricanes, and the Tropical Rainfall Measuring Mission (TRMM), 49-58. [9] Rafkin, S. C. (2003), Mars Atmosphere Observation and Modeling Workshop. [10] Spiga, A., et al., (2013). JGR Planets, 118(4), 746-767. [11] Heavens, N. G., et al., (2011). JGR Planets 116(E1). [12] Mitchell et al. (2009), Icarus, 203, 250- 264. Additional Information: If you have any questions or need additional information regarding the preparation of your abstract, call the LPI at 281-486-2142 or -2188 (or send an e-mail message to [email protected]). Please DO NOT Submit Duplicates of Your Ab- stract; should you find it necessary to replace or repair your abstract PRIOR TO the submission deadline, re- turn to the abstract submission portion of the meeting portal and click on the “Update” link that appears next to the title of the abstract you submitted. .
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