Thread by @pppapin

1) Let's reason out why the #GFS tracks #Florence NE of the #ECWMF even now 3 days till verification? Look at the upper-level cirrus canopy b/c cloud-radiative feedback is important.

The #GFS has a thicker & more expansive cirrus plume NW of #Florence. What is its significance?

2) The cirrus in a #TCs outer core enables cloud-radiative feedback, w/ lw warming under & lw cooling above clouds.

Warming under clouds enhances convective activity in the outer core, broadening the tangential wind field.

Fig. from Fovell et al. (2016) :journals.ametsoc.org/doi/10.1175/AM…

3) What happens when you broaden the tangential wind field? You induce stronger "beta drift" which draws the TC further north & west due to advection of planetary vorticity around the TC field.

See Fiorino & Elseberry (1989):journals.ametsoc.org/doi/abs/10.117…

4) How consistent is this NW expansion of dense cirrus in the #GFS. It is a consistent feature & hence the GFS remains right of the #ECMWF.

5) But how do we get the clouds in the first place? This is where microphysics come in. The GFS uses a fairly simple microphysics scheme that is two decades old (Zhao & Carr 1997): journals.ametsoc.org/doi/abs/10.117…

Also: dtcenter.org/GMTB/gfs_phys_…

6) It is possible this old scheme has a bias in producing large cirrus plumes that result in boosting the #TC size & its beta drift footprint.

The good news is that the new #FV3 uses a different microphysics scheme in its model from #GFDL, with 6 species including graupel.

7) Indeed, when you compare the new #FV3 vs. the #GFS, it depicts a track further to the south & west. It has been consistently further south & west vs. the operational GFS. I suggest this is related to the new microphysics & dynamical core.

8) Anyway, my point is that model differences are not just due to changes in initial conditions, but rather the model physics themselves that result in different solutions.

The ridge strength is key, but so is how each model depicts #Florence's outer core structure.

Fin.

The enhanced cirrus is NE of Florence, not NW.