A Study of the Role of Daytime Land–Atmosphere Interactions on Nocturnal Convective Activity in the Southern Great Plains during CLASIC

Abstract

his study examines whether and how land?atmosphere interactions can have an impact on nocturnal convection over the southern Great Plains (SGP) through numerical simulations of an intense nocturnal mesoscale convective system (MCS) on 19?20 June 2007 with the Weather Research and Forecasting (WRF) Model. High-resolution nested simulations were conducted using realistic and idealized land surfaces and two planetary boundary layer (PBL) parameterizations (PBLp): Yonsei University (YSU) and Mellor?Yamada?Janji? (MYJ). Differences in timing and amount of MCS precipitation among observations and model results were examined in the light of daytime land?atmosphere interactions, nocturnal prestorm environment, and cold pool strength. At the meso-? scale, land cover and soil type have as much of an effect on the simulated prestorm environment as the choice of PBLp: MYJ simulations exhibit strong sensitivity to changes in the land surface in contrast to negligible impact in the case of YSU. At the end of the afternoon, as the boundary layer collapses, a more homogeneous and deeper PBL (and stronger low-level shear) is evident for YSU as compared to MYJ when initial conditions and land surface properties are the same. At the meso-? scale, propagation speed is faster and organization (bow echo morphology) and cold pool strength are enhanced when nocturnal PBL heights are higher, and there is stronger low-level shear in the prestorm environment independent of the boundary layer parameterization for different land surface conditions. A comparison of one- and two-way nested MYJ results demonstrates how daytime land?atmosphere interactions modify the prestorm environment remotely through advection of low-level thermodynamic features. This remote feedback strongly impacts the MCS development phase as well as its spatial organization and propagation velocity and, consequently, nocturnal rainfall. These results indicate that synoptic- and meso-α-scale dynamics can play an important role in determining the spatial and temporal scales over which precipitation feedbacks of land?atmosphere interactions emerge regionally. Finally, this study demonstrates the high degree of uncertainty in defining the spatial and temporal scales of land?atmosphere interactions where and when organized convection is dominant.