Shallow Convection and Boundary-layer Fluxes: the Parameterization of Post-cold-front Clouds

Booth, J., City College of New York

Cloud Processes

Warm Boundary Layer Processes

Lamraoui F, J Booth, C Naud, M Jensen, and K Johnson. 2019. "The interaction between boundary layer and convection schemes in a WRF simulation of post‐cold‐frontal clouds over the ARM East North Atlantic site." Journal of Geophysical Research: Atmospheres, 124(8), doi:10.1029/2018JD029370.


Figure 1: WRF-Simulated Cloud distribution using different PBL-CU configurations. White indicates clouds; blue: the ocean and cloud free regions; cold front is marked in red. The post-cold-frontal region is within the black box. The boundary-layer schemes are the Yonsei University (YSU), Mellor-Yamada-Janjić (MYJ), and Asymmetric Convective Model (ACM2) schemes, and the convection schemes are the Kain-Fritsch (K-F), Tiedtke, and New Simplified Arakawa-Schubert (NSAS) schemes.


Figure 2: Frequency of occurrences of cloud fraction derived from Aqua and Terra MODIS data (solid black), ENA Radar (dotted black), WRF 12-hours (shaded red) and WRF 2 time steps (shaded blue) obtained in PCF region on 2015-12-25 for nine configurations 3PBLx4CU. Boundary-layer schemes: YSU, MYJ, ACM2. Convection schemes: K-F, Tiedtke, NSAS.


Figure 1: WRF-Simulated Cloud distribution using different PBL-CU configurations. White indicates clouds; blue: the ocean and cloud free regions; cold front is marked in red. The post-cold-frontal region is within the black box. The boundary-layer schemes are the Yonsei University (YSU), Mellor-Yamada-Janjić (MYJ), and Asymmetric Convective Model (ACM2) schemes, and the convection schemes are the Kain-Fritsch (K-F), Tiedtke, and New Simplified Arakawa-Schubert (NSAS) schemes.

Figure 2: Frequency of occurrences of cloud fraction derived from Aqua and Terra MODIS data (solid black), ENA Radar (dotted black), WRF 12-hours (shaded red) and WRF 2 time steps (shaded blue) obtained in PCF region on 2015-12-25 for nine configurations 3PBLx4CU. Boundary-layer schemes: YSU, MYJ, ACM2. Convection schemes: K-F, Tiedtke, NSAS.

Science

Using a regional numerical model, we investigate the physical processes necessary to produce clouds in the wake of cold fronts. We find that the modeled clouds are most sensitive to the strength of the modeled vertical mass fluxes, regardless of which physics in the model generates the fluxes.

Impact

This work constrains the focus of future work aimed at improving the simulation of low-level oceanic clouds. It points to a small subset of mechanisms that must be properly modeled, both in terms of their individual behavior and their interactions.

Summary

The correct representation of low-level mid-latitude clouds found in the wake of cold fronts strongly relies on the representation of planetary boundary layer (PBL) and convection processes, which are typically parameterized separately in numerical models. This study investigates how distinct pairs of PBL and convection parameterization schemes represent cloud fraction in the post-cold-frontal region. The simulations focus on the region of the DOE-ARM Eastern North Atlantic observation site in the Azores Islands in the wake of a cold front. Different PBL and convection schemes are combined to create 12 distinct configurations. The simulations produce a wide range of cloud fractions (Figures 1, 2). A skill score is used to quantitatively assess the performance of each configuration with respect to ground-based radar data. The key processes that are found to significantly impact the cloud fraction distribution are the strength of the PBL decoupling, the vertical wind shear, the strength of detrainment for shallow convection, and the occurrence of drizzle. This indicates that to successfully simulate post-cold-frontal clouds, modeled physics must balance strong internal vertical mixing and weak exchange with the free troposphere.