Effects of Lower Troposphere Vertical Mixing on Simulated Clouds and Precipitation Over the Amazon During the Wet Season

Abstract

Planetary boundary layer (PBL) schemes parameterize unresolved turbulent mixing within the PBL and free troposphere (FT). Previous studies reported that precipitation simulation over the Amazon in South America is quite sensitive to PBL schemes and the exact relationship between the turbulent mixing and precipitation processes is, however, not disentangled. In this study, regional climate simulations over the Amazon in January–February 2019 are examined at process level to understand the precipitation sensitivity to PBL scheme. The focus is on two PBL schemes, the Yonsei University (YSU) scheme, and the asymmetric convective model v2 (ACM2) scheme, which show the largest difference in the simulated precipitation. During daytime, while the FT clouds simulated by YSU dissipate, clouds simulated by ACM2 maintain because of enhanced moisture supply due to the enhanced vertical moisture relay transport process: (a) vertical mixing within PBL transports surface moisture to the PBL top, and (b) FT mixing feeds the moisture into the FT cloud deck. Due to the thick cloud deck over Amazon simulated by ACM2, surface radiative heating is reduced and consequently the convective available potential energy is reduced. As a result, precipitation is weaker from ACM2. Two key parameters dictating the vertical mixing are identified, p, an exponent determining boundary layer mixing and λ, a scale dictating FT mixing. Sensitivity simulations with altered p, λ, and other treatments within YSU and ACM2 confirm the precipitation sensitivity. The FT mixing in the presence of clouds appears most critical to explain the sensitivity between YSU and ACM2.

Airborne Measurements of Scale-Dependent Latent Heat Flux Impacted by Water Vapor and Vertical Velocity over Heterogeneous Land Surfaces During the CHEESEHEAD19 Campaign

“…Clearly, the PBL will need to be parameterized in NWP models in the foreseeable future. Further studies should compare the different atmospheric fluxes between the airborne measurements and model simulation to improve the parameterization of PBL fluxes (Hu et al, 2023).…”

“…Water vapor and latent heat flux (LE) in the Planetary Boundary Layer (PBL) play critical roles in atmospheric dynamics, the hydrological cycle, radiation balance, and conversion of latent heat (Garratt, 1994;Pielke et al, 2003;Linné et al, 2006;Kiemle et al, 2007;Stevens & Bony, 2013;Stull, 2015;Hu et al, 2023). Relevant processes include surface evapotranspiration, transport and diffusion through the PBL, and cloud formation and dissipation (LeMone et al, 2019).…”

Airborne Measurements of Scale‐Dependent Latent Heat Flux Impacted by Water Vapor and Vertical Velocity Over Heterogeneous Land Surfaces During the CHEESEHEAD19 Campaign

“…By applying the wavelet technique to airborne in situ data collected at multiple flight levels, or, better, by combining full profiles of airborne Raman lidar r v data with Doppler lidar w measurements could effectively characterize the height-dependent scale-resolved LE. Further studies should also compare and synergize atmospheric fluxes between the airborne measurements and model simulations to improve the parameterization of the PBL fluxes (Hu et al, 2023). Aeronautics and Space Administration (NASA) 80NSSC20K0663, with additional support for scientific airborne data collection activities from the University of Wyoming King Air team.…”