Counter-Gradient Momentum Transport Through Subtropical Shallow Convection in ICON-LEM Simulations

Area of Science:

• Atmospheric Science
• Meteorology
• Climate Dynamics

Background:

• Subtropical shallow convection is known to transport heat and water vapor upward.
• Its role in transporting horizontal momentum and affecting trade winds is less understood.

Purpose of the Study:

• To investigate the characteristics of shallow convective momentum transport (CMT).
• To determine the impact of CMT on trade wind deceleration in the subtropics.

Main Methods:

• Utilized unique multiday large-eddy simulations (LES) over the tropical Atlantic using the ICON-LES model.
• Analyzed trade-wind profiles during boreal winter conditions.
• Examined momentum flux budgets, including conditionally sampled updrafts and downdrafts.

Main Results:

• CMT acts as an apparent friction, decelerating the north-easterly trade winds.
• This decelerating effect is strongest below the cloud base.
• A counter-gradient zonal momentum flux was observed in the cloud layer, penetrating deeper than in traditional shallow cumulus studies.
• Convectively triggered nonhydrostatic pressure gradients and surrounding horizontal circulations drive the counter-gradient flux in the cloud layer.

Conclusions:

• Shallow convection significantly impacts trade wind dynamics through momentum transport.
• Nonhydrostatic pressure gradients and horizontal circulations are key mechanisms driving counter-gradient momentum transport in shallow convection.
• Accurate simulation of these effects requires large domain sizes and realistic boundary conditions in atmospheric models.