Tuesday, 25 October 2005
Alvarado F and Atria (Hotel Albuquerque at Old Town)
Cloud radar observations of the eastern Pacific Intertropical Convergence Zone (ITCZ) cloud vertical structure are interpreted using soundings, large-scale divergences calculated from the precipitation radar Doppler velocities using the Mapes and Lin (2005) technique, and surface rainfall rates, acquired during the East Pacific Investigation of Climate (EPIC) experiment. Cloud boundaries are well-defined by the vertically-pointing 35 GHz cloud radar, despite some attenuation by rain. The EPIC experiment documented dry air intrusions centered at altitudes of 5.5 to 7.5 km; these appear to originate from the equatorial zone and the shallow Costa Rica oceanic thermocline dome. The suppression of surface-based convection by the dry layers is convincingly shown. A new observation is the sublimation of cirrus anvil ice overlying the dry air layers. In one comparison, ice sublimation-induced diabatic cooling inferred from the precipitation radar wind divergences of 60 +- 40 K/day over a ~100 km scale, is consistent with the sublimation of 0.2 +- 0.15 g/m^3 in ice water contents independently estimated from the cloud radar reflectivities. The association of the southerly dry air intrusions with overlying convective outflow (in the example shown, from Hurricane Juliette to the north) suggests the sublimation at their interface is also a climatological feature; the 6-7 km altitude preference of the southerly dry air intrusions is also evident in September-mean climatologies. The movement of dry free-tropospheric air into the EPIC domain is modulated on daily time scales by easterly wave activity.
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