ALBERT

Description: The Weather Research and Forecasting (WRF) model was used to simulate the evolution of Tropical Storm Ivan (2004) in the southeast (SE) US using both the Yonsei University (YSU) and Mellor-Yamada-Janjić (MYJ) boundary layer parameterizations. In contrast with tropical cyclone (TC) simulations over the ocean, the effect of surface layer becomes secondary for a dissipating hurricane along its terrestrial track. Although these two schemes can reproduce Ivan reasonably well, our results suggest that the mixing properties for damped mechanical turbulent conditions (weakly stable) are strongly underestimated by both parameterizations. This underestimation impacts the thermodynamic properties of the storm, leading to significant differences in the storm areal extent and the simulated precipitation fields. Suggestions for further improvements are provided. An evaluation of the impact of using or not using a convective parameterization, specifically the Kain-Fritsch (KF) scheme, at 3 km grid spacing shows marginal impact on storm coverage, intensity and precipitation, except for the presence of widespread light rainfall in the Piedmont east of the mountains when the KF is employed. Analysis of the thermal structure of the simulated storm indicates that, in the inner-storm region, the KF is either not activated or primarily produces ( parameterized ) shallow convection. As a result, the net heating tendency associated with adiabatic and diabatic processes is almost unaltered inside the storm, together with a nearly equivalent surface momentum sink, leading to similar storm areal extent and intensity. Light rainfall to the east of the mountains can be due to the trigger mechanism of KF, which depends on boundary layer convergence, forcing parameterized deep convection near the coast, where surface roughness changes enhance convergence.

Description: The Weather Research and Forecasting (WRF) model was used to investigate the impact of Amazonian evapotranspiration (ET) on moisture transport and convection along the eastern flanks of the Andes (EADS). To isolate the role of surface ET, quasi-idealized simulations down to 1.2 km grid spacing were conducted, where over the Amazon lowlands (AMZL) and at every time step the surface sensible heat effects are exactly identical to the realistic reference runs while surface latent heat fluxes are prevented from entering the atmosphere. The results show that, without surface ET, daily precipitation within the AMZL decreases by as much as ~75%, but nearly doubles over the surrounding mountainous regions. This dramatic influence is attributed to a dipole structure of convergence-divergence anomalies over the AMZL, primarily due to the considerable cooling of the troposphere associated with suppressed convection. Further examination of moist static energy evolution indicates that the net decrease in convective available potential energy over the AMZL is due to the removal of surface ET that is only partially compensated by related regional circulation changes. Because of the concave shape of the Andean mountain range, enhanced low-level divergence promotes air mass accumulation to the east of the central EADS. This perturbation becomes sufficiently strong around nightfall and produces significant eastward low-level pressure gradient force, rendering stronger winds away from the Andes. Moisture convergence and convection over the EADS vary accordingly, strengthened in the day but attenuated at night. Nocturnal convective motion is, however, more widespread. Analytical solutions of simplified diagnostic equations of convective fraction suggest that reduction of lower troposphere evaporation is the driving mechanism. Additional exploratory experiments with varied surface ET magnitude demonstrate that the connection between the AMZL ET and EADS precipitation is robust.