Life cycle of deep convective systems in the midlatitude and tropics: from observations to simulations

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Description

Deep convective systems (DCSs) produce heavy rainfall and large cirrus anvil cloud shields, and they play an important role in the climate system through their impact on the general circulation and cloud radiative feedback. To improve understanding of the life cycle of DCSs, a Lagrangian framework is used to investigate DCSs in the midlatitudes and the tropics. An automated cloud tracking method is used in conjunction with a multi-sensor hybrid classification to analyze the evolution of DCS structure over the central U.S. (Feng et al. 2012). Composite analysis from 4221 tracked DCSs during two warm seasons shows that for short to medium systems (lifetimes <6 hours), the lifetime is mainly attributed to the intensity of the initial convection. Systems that last longer than 6 hours are associated with up to 50% higher midtropospheric relative humidity and up to 40% stronger middle to upper tropospheric wind shear. Such environment allows continuous growth of the stratiform rain region, thus prolonging the system lifetime. Areal coverage of thick anvil clouds is strongly correlated with the size and intensity of convection. Ambient upper tropospheric wind speed and shear also contribute to convective anvil production.

This Lagrangian framework is then applied to evaluate long-term high-resolution simulations by the Weather Research and Forecasting (WRF) model in the tropical western Pacific. In general, the simulated DCSs reproduce many satellite-observed cloud statistics, but the cloud size and convective intensity are very sensitive to the choice of different microphysics schemes. Two-moment Morrison schemes produce much larger DCSs than the WRF Single-Moment Six-Class Microphysics Scheme (WSM6) scheme due to more anvil clouds and hence agree better with observations. However, Morrison schemes overestimated precipitation by about 50%, while WSM6 overestimated by 35%. Diurnal cycles over land agree well with observations; but over the ocean, different microphysics schemes show considerable variances. As a result of the sensitivity to microphysics, differences in the simulated cloud radiative forcing at the top-of-atmosphere, surface, and atmospheric column are quantified. Results from this work suggest that while general features of the cloud life cycle of DCSs simulated by WRF agree reasonably well with observations, various cautions must be given when using these simulations for different purposes, given their wide range of results from the choice of microphysics schemes alone.