Turbulence, Shallow Convection, Clouds Meeting October 9th 2023

Attendees: Bjørg Jenny Engdahl, Emily Gleeson, Karl-Ivar Ivarsson, Marvin Kahnert, Gudrun Nina Peterson, Wim de Rooij, Metodija Shapkalijevski, Xiaohui Zhao.

Some Notes:

Convective Time Scale (Tau): Wim presented (Link) on the convective time scale (Tau), work that has been accelerated due to the cloud issues present in CY46. The convective time scale is 600 seconds by default but Wim has created a formulation where Tau depends on the height in the cloud layer (related to the ensemble of clouds that the convection scheme should represent). There are convective and turbulent components to the variance (related to cloud cover) but missing terms due to heterogeneity, and advection of cirrus clouds e.g., are pragmatically represented by an extra variance term. The convective time scale depends on cloud size and is important for the variance. Tau should not be a constant but should increase inside the cloud. STATNEW (the new cloud scheme, without the bugs in the previous scheme) is a bit worse for variance (less low cloud) compared to LES data but the new formulation improves this. For the ARMCU case cloud cover looks better with Wim's new Tau formulation. More importantly, estimates of the variance look good for the hours where convective transport compares well to the LES. Emily showed results for clear sky index where the new Tau was implemented and the impact was clear - more analysis will be done.

T2m/XRIMAX: Marvin showed analysis of T2m over the AROME Arctic domain. The drag coefficients are functions of XRIMAX and are used in the T2m calculation. Therefore since XRIMAX=0.0, T2m never sees the effects of a stable BL. T2m follows the temperature of the lowest model level closely. Marvin implemented a fix for this so that T2m is closer to the surface temperature. On closer analysis, he saw a strong cooling, esp by the coast where they already have a negative bias (surprising impact by the coast where the BL should be more well-mixed, advection over warmer sea etc). He looked at valley and coast points and see the strange effect of the surface cooling rapidly (also seen in MEPS and CARRA so not a new issue) while the atmosphere stays warmer (coastal points). Link to Marvin's Slides: https://docs.google.com/presentation/d/13gAAtHefyDFQVIXDkUaFOvBy6JgAMfELrIJLPlBerfg/edit#slide=id.p6

W Threshold: Wim also showed results of a convection case where the MF scheme is switched off, a scale aware version is used or a scale aware based including a vertical velocity threshold option (taking resolution into account and based on Khain et al. 2020) . Also looked at impact of dynamics - small effect over his domain due to the lack of significant orography. If you shut down the MF completely the results become unreasonable (v. unstable). The w-threshold version looks promising so far. Also showed an open cell case. In the default set-up there are no cells and a lack of precipitation. With the w threshold, cells start to form (but look closed rather than open) and there's more precipitation (confirm obs). Emily showed CSI plots compared a REF with the w threshold option - an increase in cloud is clear - more analysis to follow.

Visit of Marvin and Meto to Wim to work on scale aware convection. Details to be finalised. MetNo can support Marvin. Emily to check with Jeanette re DAP funding.

CY46 paper: We discussed it and agreed it should go ahead. All happy to help out. Will start with a MWR template and hope that that's the right journal for it.