WG on role of PBL in deep convection

Thanks to all WG participants for a fine discussion and Steve Klein for note-taking.

Progress in past decade

• PBL and transition-layer dynamics solve the mass flux closure/triggering problem for parameterization of shallow (and likely deep) Cu. CAPE seen to be a necessary condition but not the control parameter.

• CRM/LES can now model Cu ensemble/PBL interaction with enough realism to help us to improve parameterization.

• We have lots of repeatable observations of the diurnal cycle of shallow and deep Cu clouds and regime transitions using radars and single-column observations (e.g. tight connection between surface RH and LCL, evolution of cloud and PBL statistics.)

Challenges and Future Directions

1. Parameterization of PBL heterogeneity

- PDF of T/q and horizontal power spectrum of w (roles of precipitation-driven downdrafts, surface properties, upscale cascade).

- Do mesoscale wakes from stratiform precipitation require special treatment or is it better to consider them a resolved-scale process?

- Better downdraft parameterizations required? Stochasticity?

2. Is the coupling of lateral Cu entrainment spectrum to PBL eddy scale and the size spectrum of clouds at cloud base as tight as some recent idealized CRM simulations suggest (or is subgrid free tropospheric heterogeneity in q,T also important)?

3. Improved and more widely used parameterizations of convective momentum transport to improve predictions of surface wind stress/fluxes.

Resolution/technical issues

1. What to do re PBL/shallow cumulus parameterization when model resolution falls to 2-5 km in NWP applications?

2. What is adequate vertical resolution of PBL/transition layer for fluxes, Cu mass-flux and initial parcel properties for deep Cu params that use boundary/transition layer closure? (100-200 m?) How do we balance the refinement of horizontal vs. vertical resolution for deep convective PBL?

3. Separation of and handoff between shallow and deep Cu params –best solved by unification of Cu scheme.

Speculative topics

1. Do we need extra subgrid ‘memory’ in PBL scheme (e.g. prognostic TKE or eddy scale info) to maintain long-lived and redeveloping convective systems? Could interaction between horizontal grid boxes help represent this problem? Cellular automata as a tool

2. Need for temporal memory in Cu convection to help with Cu lifecycle (e.g. diurnal cycle of land convection)?