Upscale energy transfer from mesoscale convective systems to African easterly waves

The 30-second take

• Two 40-day, roughly 5-kilometer Met Office simulations show a vertical dipole of interscale kinetic-energy transfer around the storms, with strong upscale transfer in the anvils below 1000 kilometers.
• At 200 hectopascals the upscale transfer leads African easterly-wave troughs, and growing continental waves in the explicit run show stronger transfer from the storms into the wave.
• Tropical dynamicists get a kilometer-scale energy budget that is weak when convection is parameterized and visible when the storms are actually simulated.

What the paper actually did

African easterly waves organize the day-to-day rainfall of the West African monsoon through a two-way coupling with mesoscale convective systems. The composite of the two is a standard example of tropical multiscale interaction: deep convection feeds back onto a larger circulation. The authors support that picture with a turbulence diagnostic. They compute, for the first time in this setting, the pattern of interscale kinetic-energy transfer across spatial scales around the convective systems and around phases of the wave lifecycle. The data are two pan-tropical 40-day simulations with the Met Office Unified Model at about 5 kilometer grid spacing. The transfer is calculated with the LoSSETT package. Explicitly simulated storms show a distinct vertical dipole at scales below 1000 kilometers, with strong upscale transfer in the upper troposphere inside the anvils. Regionally, the upscale transfer is strongest where organized convection is active. At 200 hectopascals, upscale transfer consistently leads the wave troughs. Growing continental waves in the explicit simulation are marked by stronger upscale transfer from the convective-system scale to the wave scale. A case study shows that this transfer can contribute to the eddy-kinetic-energy growth of a strengthening trough. In a simulation that parameterizes convection instead, the signal is weak. There, zonal-mean energetics show enhanced barotropic conversion, and a stronger African easterly jet supports a stronger wave population.

What makes this disruptive

The usual story can be told entirely with potential vorticity. This paper adds the kinetic-energy handoff across scales, computed in a model that actually contains the storms. The handoff has a shape: a dipole below 1000 kilometers, anvil-level upscale transfer, and a lead ahead of the trough at 200 hectopascals. The parameterized run is the control that matters. When the storms are not explicit, the upscale signal fades and the waves are fed more by the jet. Resolving convection changes the energy pathway, not only the rainfall map.

Why it matters (outside the lab)

Forecasts of the West African monsoon, and of the tropical cyclones that some of these waves become, depend on whether the storms are passengers or drivers. An upscale transfer that leads the trough, and that strengthens while continental waves grow, is evidence that the storms help build the wave in the explicit model. Groups running kilometer-scale tropical simulations get a diagnostic, LoSSETT, already applied to a 40-day pair of runs they can compare against their own parameterized setups.

Limitations & open questions

The evidence is two 40-day simulations, one with explicit convection and one with parameterized convection, not a multi-year observational energy budget. The case study says the transfer can contribute to eddy-kinetic-energy growth. The abstract does not give the fraction of that growth it explains. Signals are weak when convection is parameterized, so the result is tied to resolving the storms. Grid spacing is about 5 kilometers, which is convection-permitting rather than a claim that every cloud process is resolved.