Rectification of the Diurnal Cycle and the Impact of Islands on the Tropical Climate Timothy W. Cronin*, Kerry A. Emanuel Program in Atmospheres, Oceans, and Climate, Massachusetts Institute of Technology 77 Massachusetts Avenue, Cambridge 02139; Motivation Tropical islands and continents are rainier than nearby ocean areas (figure below; Sobel et al., 2011) Convection is more vigorous over islands and continents (e.g., Williams et al., 2004) Why? Is the enhancement of island rainfall and greater convective intensity over land related to the diurnal cycle? Acknowledgments Thanks to Marat Khairoutdinov for supplying me with the model code, and to Allison Wing for teaching me how to use it. This work has been supported by NSF Grant # , “The Effect of Near-Equatorial Islands on Climate,” which is a collaboration with Peter Molnar. References Dayem, K.E., and David C. Noone, and Peter Molnar, Tropical western Pacific warm pool and maritime continent precipitation rates and their contrasting relationships with the Walker Circulation. J. Geophys. Res., 112, D Khairoutdinov, M.F., and D.A. Randall, Cloud Resolving Modeling of the ARM Summer 1997 IOP: Model Formulation, Results, Uncertainties, and Sensitivities. J. Atmos. Sci., 60, pp Mulligan, M., Global Gridded 1km TRMM Rainfall Climatology and Derivatives. Version 1.0. Database: Sobel, A.H., C.D. Burleyson, and S.E. Yuter, Rain on Small Tropical Islands. J. Geophys. Res., 116, D Williams, E., T. Chan, and D. Boccipio, Islands as Miniature Continents: Another Look at the Land-Ocean Lightning Contrast. J. Geophys. Res., 109, D Future Work Further sensitivity experiments on island size, wetness, and treatment of the surface heat capacity Comparison of radiative and dynamical mechanisms: role of advection of moist static energy in the subcloud layer Simulations of an equatorial beta channel with embedded island or islands Exploration of effects of diurnal cycle and land/sea breeze dynamics on soil moisture-precipitation feedback Experimental Design The islands of the Maritime Continent (MC) are substantially rainier than nearby ocean areas Rainfall data: Mulligan (2006) database: land mask: / Poster # A51C-0070 Walker Circulation strength correlates better with MC precipitation than warm pool precipitation (Dayem et al., 2007) 3-5 million years ago: permanent El Nino, and substantially less island area in the West Pacific? Could the interaction of islands and the diurnal cycle strongly affect the Walker Circulation? 384 x 384 km, 3 km resolution; 64 vertical levels Non-rotating; no background wind Top-of-atmosphere insolation: March 20, 45 N Diurnal average insolation: W m -2 Varied Island Size and Surface Wetness Results are averaged over simulation days Island = 5 cm slab Ocean = 1 m slab Simulate Radiative-Convective Equilibrium with a mixed lower boundary using the SAM Cloud-Permitting Model See Khairoutdinov and Randall (2003) for details on SAM Results: Precipitation r = 48 kmr = 96 km r = 24 kmr = 48 km; dry mm/day Simulated islands are rainier than the surrounding ocean Island size and surface wetness affect the rainfall enhancement Dry-island results suggest a strongly negative soil moisture- precipitation feedback Results: Temperature Islands warm the troposphere relative to an all-ocean simulation Vertical structure of warming is roughly moist –adiabatic with increased stability near the surface (nocturnal BL) Dry island has larger impact than wet island Discussion Cloud radiative effects cause most of the island-induced warming: in these simulations, clouds have a warming effect due to their timing, height, and fractional cover Diurnal timing of clouds (figure below) is extremely important for net cloud radiative effect Cloud radiative forcing affects the magnitude of island rainfall enhancement, but dynamics (e.g., land-sea breezes) are more important