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: 1) vertical mixing within PBL transports surface moisture to the PBL top, and 2) 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 (CAPE) 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.

Published edition:Huang, Y., M. Xue, X. Hu, E. Matin, H. Novoa, R. McPherson, A. Perez, I. Morales, 2023: Convection-Permitting Simulations of Precipitation over the Peruvian Central Andes: Strong Sensitivity to Planetary Boundary Layer Parameterization. Journal of Hydrometeorology, 24, 1969–1990. https://doi.org/10.1175/JHM-D-22-0173.1

To explore the potential impacts of climate change on precipitation and mesoscale convective systems (MCSs) in the Peruvian Central Andes, a region with complex terrain, two future and one historical convection-permitting regional climate simulations are conducted using the Weather Research and Forecasting (WRF) model. All simulations adopt consistent model configurations and two nested domains with grid spacings of 15 and 3 km covering the entire South America and the Peruvian Central Andes, respectively. The future simulations are run for 2070-2080 and driven by a bias-corrected global dataset derived from the Coupled Model Intercomparison Project Phase 6 (CMIP6) ensemble under the SSP2-4.5 and SSP5-8.5 emission scenarios. Results show geographically dependent changes in annual precipitation, with a consistent rise in the frequency of intense hourly precipitation across all examined regions. The western Amazon Basin shows a decrease in annual precipitation, while increases exist in parts of the Peruvian west coast and the east slope of the Andes under both future scenarios. In the warming scenarios, there is an overall increase in the frequency, precipitation intensity, and size of MCSs east of the Andes, with MCS precipitation volume increasing by up to ~22.2%. Despite consistently enhanced synoptic-scale low-level jets in future scenarios, changes in low-level dynamic convergence are inhomogeneous and predominantly influence annual precipitation changes. The increased convective available potential energy (CAPE), convective inhibition (CIN), and precipitable water (PW) in a warming climate suppress weak convection, while fostering a more unstable and moisture-rich atmosphere facilitating more intense convection and the formation and intensification of heavy precipitation-producing MCSs. The study highlights the value of convection-permitting climate simulations in projecting future severe weather hazards and informing climate adaptation strategies, especially in regions characterized by complex terrain.Keywords severe convective storms, future projections, convection-permitting, regional climate simulations, Peruvian Central AndesKey pointsConvection-permitting regional climate simulations are conducted to investigate the climate change impacts on precipitation and mesoscale convective systems in the Peruvian Central Andes. Intense hourly precipitation and organized convective storms become more frequent in the Peruvian Central Andes under a warming climate. Increased convective available potential energy (CAPE), convective inhibition (CIN), and precipitable water (PW) in a warming climate shift the convection population.

Using the Weather Research and Forecasting (WRF) model with two planetary boundary layer schemes, ACM2 and MYNN, convection-permitting model (CPM) regional climate simulations were conducted for a 6-year period at a 15-km grid spacing covering entire South America and a nested convection-permitting 3-km grid spacing covering the Peruvian central Andes region. These two CPM simulations along with a 4-km simulation covering South America produced by National Center for Atmospheric Research, three gridded global precipitation datasets, and rain gauge data in Peru and Brazil, are used to document the characteristics of precipitation and MCSs in the Peruvian central Andes region. Results show that all km-scale simulations generally capture the spatiotemporal patterns of precipitation and MCSs at both seasonal and diurnal scales, although biases exist in aspects such as precipitation intensity and MCS frequency, size, propagation speed, and associated precipitation intensity. The 3-km simulation using MYNN scheme generally outperforms the other simulations in capturing seasonal and diurnal precipitation over the mountain, while both it and the 4-km simulation demonstrate superior performance in the western Amazon Basin, based on the comparison to the gridded precipitation products and gauge data. Dynamic factors, primarily low-level jet and terrain-induced uplift, are the key drivers for precipitation and MCS genesis along the east slope of the Andes, while thermodynamic factors control the precipitation and MCS activity in the western Amazon Basin and over elevated mountainous regions. The study suggests aspects of the model needing improvement and the choice of better model configurations for future regional climate projections.