New Tiedtke scheme

New Tiedtke is a cumulus parameterisation in WRF, descended from the Tiedtke scheme long used at ECMWF and revised substantially since. It is the third family available alongside Kain-Fritsch and the Grell schemes, and it differs from both in ways that matter for tropical work.

What separates it

Distinct convective modes. The scheme treats shallow, mid-level, and deep convection as separate types with their own closures rather than as one process at different depths. Shallow convection in particular is handled explicitly rather than as a weak version of deep convection — which matters over tropical oceans, where non-precipitating shallow cumulus does most of the boundary-layer moistening and almost none of the raining. A scheme that treats it as small deep convection tends to rain when it should only be moistening.

A moisture-convergence heritage. The original closure tied convective mass flux to large-scale moisture supply rather than to CAPE removal. Later versions incorporate CAPE-based elements, but the lineage shows: it responds to large-scale forcing rather than purely to local instability, which suits regimes where convection is organised by convergence rather than triggered locally.

Momentum transport. Unlike Kain-Fritsch, it includes convective momentum transport. For anything where convective organisation feeds back on the flow this is a real difference rather than a refinement.

Why it comes up in tropical work

The distinct shallow mode and the convergence-oriented closure both suit maritime tropical conditions better than schemes developed and tuned largely over mid-latitude continents. Its long operational use at ECMWF also means its behaviour in the tropics is unusually well documented compared with alternatives tested mainly over the continental United States.

For the Galapagos, where the model sits close to the boundary between a shallow stratocumulus regime and deep convection, having shallow convection treated in its own right rather than as a degenerate case is a meaningful advantage — see the experiment suite for how these choices were tested rather than assumed.

Its complexity is the trade: more internal machinery, more parameters, and correspondingly harder to reason about when it goes wrong.

See also: cumulus convection schemes, Kain-Fritsch, Grell-Freitas, and the tropical WRF setup.