A New Generalised Mass-flux Convection Scheme for the Met Office Unified Model

1. A New Generalised Mass-flux Convection Scheme for the Met Office Unified Model Mike Whitall (Met Office, UK) Convection Parameterisation: Progress and Challenges. 15 July 2019 Met Office, Exeter, UK

2. CoMorph(Convection Morph) A new parameterisation scheme for moist convection in the Met Office Unified Model. Still a diagnostic mass-flux convection scheme, but with a total redesign, and flexible code structure to allow various assumptions to be relaxed… Aims: - Operational implementation in NWP and climate models. - Collaborative research tool; flexible code for testing new ideas in the mass-flux framework. Design goals: - Remove ad-hoc structural assumptions which have hampered progress in the past. - Allow representation of new physical processes which were previously neglected.

3. CoMorph – Back to Basics: - Convecting parcels launch from any height where there is local vertical instability. - Plumes from different unstable layers integrated independently. - Single parcel ascent / descent code for all plumes. - Parcel radius / entrainment rate depend on the turbulent mixing-length in the parcel’s source-layer. “Traditional” approach: - Complex empirical trigger functions. - A-priori diagnosis of a unique “cloud-base” height. - Plume can only start from surface or other prescribed height. - Separate schemes for “deep”, “shallow” and “mid-level” convection (must be pre-diagnosed which one to trigger). or (c.f. Gregory & Rowntree 1990!)

4. CoMorph – Back to Basics: - Mass-flux launched from any height just depends on the local vertical instability -N2: - Cloud-base mass-flux is an emergent property of the entraining-detraining plume-model, not a closure variable! “Traditional” approach: - Rescale the mass-flux afterwards to satisfy some vertically-integrated closure assumption (CAPE, moisture convergence, etc) for the cloud-base mass-flux. Separate dry / moist N2 computed for: - clear-sky (dry N2) - Liquid cloud - Mixed-phase cloud - Subsaturated ice / rain for both ascending and descending parcels (downdraft scheme is just the updraft scheme upside down!) Successive timesteps during onset of convection in CoMorph UM SCM simulation of AMMA case, Niger.

5. CoMorph – Back to Basics: - Dynamic initiation, entrainment and detrainment simulated by the parcel model removes any need for additional ad-hoc cloud-base trigger or closure rescaling. - Intermittency avoided by using implicit- in-time discretisation for initiating mass and detrainment at each height. “Traditional” approach: - Separate models / philosophies for “triggering”, “cloud-model” and “closure” allows inconsistencies and numerics problems. - e.g. post-hoc closure rescaling ignores triggering condition, causing intermittency.

6. Implicit Assumed-PDF-based detrainment - Buoyancy, T, q, u, v, etcassumed to have a power-law PDF within the bulk plume. - Separate parcel ascent calculations are done for the in-plume mean properties, and for a less dilute parcel “core”. - At each level-step, extrapolate the fraction of the PDF which becomes non-buoyant (due to entrainment, changes in environment Tv, etc) Tv’ = Tv parcel – Tvenv But how to define Tvenvwhen it is changing due to the convective increment? For smooth behaviour with Δt ~ 10 minutes, need implicit-in-time discretization, accounting for the convective heating:

7. Parcel Microphysical Processes The parcel updraft / downdraft model contains a simple mixed-phase microphysics scheme with 4 condensed water species: - Liquid cloud - Rain - Ice cloud - Graupel. All water phase-changes within the parcel are solved simultaneously / implicitly: - Condensation / evaporation. - Ice deposition / sublimation. - Melting. - Riming. Using interactive microphysics within the parcel has a significant impact on parcel buoyancy: - Faster ascents glaciate more gradually with height and retain higher water-loading. - Parcels that entrain falling ice from the environment glaciate lower-downdue to “seeding”.

8. SCM tests – ASTEX (Sc to Cu transition case). Cloud Fraction timeseries: UM SCM CoMorph CoMorph gives smoother transition from Sc to Cu, and more realistic increase in Sc-top height. Convection scheme much less noisy. LES from van der Dussen et al (2013) UM SCM GA7 (ctl)

9. Test-run with CoMorph in a UM climate simulation… Control run (GA7) uses standard CAPE closure mass-flux scheme. CAPE closure not sensitive enough to resolved forcings, e.g. Tropical waves. But CoMorph seems to be much too sensitive!

10. Huge spurious increase in rainfall along the ITCZ! Test-run with CoMorph in a UM climate simulation… CoMorph simulation has many “grid-point storms” (although in this case they seem to be driven by excessive parameterised convective mass-flux, not explicit convection). Convection collapsing towards more realistic scales of organisation? (which is grid-scale at N96?) Single grid-point updrafts known to suffer severe non-conservation of moisture under Semi-Lagrangianadvection…

11. The “Eternal Fountain of Semi-Lagrange”! Winds interpolated onto scalar points to get a vector for the back-trajectory. Equal and opposite convergent wind from both sides cancels out in the interpolation. Back trajectories in the ascending grid-column go straight down.

12. The “Eternal Fountain of Semi-Lagrange”! • Winds interpolated onto scalar points to get a vector for the back-trajectory. • Equal and opposite convergent wind from both sides cancels out in the interpolation. • Back trajectories in the ascending grid-column go straight down. • Any moist anomaly in the column just gets “copied upwards” and cannot be replaced by dryer air converging laterally. • Large spurious moisture source inside grid-point storms; runaway feedback…

13. The “Fountain Buster” scheme Diagnose grid-points with convergent winds from opposite directions (stagnation points). Add on Eulerian up-wind advection tendencies at these points to correct for the convergent in-flow missed by SL-advection. (capped between 0 and 1)

14. The “Fountain Buster” scheme UM + CoMorph “Fountain Buster” greatly reduces excessive rain-rates, by preventing a runaway feedback (spurious moisture production) that causes extreme grid-point storms. => Global UM can produce near-grid-scale convective organisation without producing nonsense! Instantaneous rain-rate histogram. UM + CoMorph + “Fountain Buster”

15. 5-day N320 NWP forecast, initialised from ECMWF analysis. Hovmoller plot of surface precip along the equator. CoMorph couples much more strongly to Tropical waves than the control UM simulation. But not very skilful at predicting where the large-scale waves are! e.g. large event over the Indian Ocean is not present at the start of the forecast?

16. Questions?