Page 1© Crown copyright 2006 Precipitating Shallow Cumulus Case Intercomparison For the 9th GCSS Boundary Layer Cloud Workshop, September 2006, GISS New York, USA Ben Shipway, UK Met Office Met Office LEM Results

Page 2© Crown copyright 2006 LEM microphysics salient features  Single moment (prognostic mixing ratio) or double moment (also prognostic number concentration) bulk rain scheme.  Cloud scheme uses a fixed cloud number concentration.  Rain autoconversion is through Kessler scheme.  Raindrop fallspeed uses expression due to Uplinger.  Raindrop break-up terms are absent.  Although the LEM has a variable timestep, this was limitted to a maximum of 1s since binary collisions are assumed in the microphysics. (In actual fact timestep was generally <1s)

Page 3© Crown copyright 2006 Sensitivity tests Experiment NameDetails NOMICROMicrophysics turned off. 1-MSingle moment rain 1-M-RNaModified single moment rain MODRNc Change to cloud condensation nuclei MODVRChange to raindrop fall speed HIHi Resolution ABEL Simulations from Abel and Shipway

Page 4© Crown copyright 2006 Results from the control simulation

Page 5© Crown copyright 2006 Results from the control simulation The control simulations uses:  Double moment rain  Cloud number concentration (RNc) = 45cm -1  Upinger fall speed V R (D)=aD b exp(-fD); a=4854.1, b=1., f=195.  Horizontal resolution 100mx100m

Page 6© Crown copyright 2006 Results from the control simulation Control_pptControl_totql Control_totccControl_zi

Page 7© Crown copyright 2006 Results from the control simulation Control_qrbudgetControl_Q02Q03 Control_AControl_mflux

Page 8© Crown copyright 2006 Results from the control simulation

Page 9© Crown copyright 2006 Results from the control simulation  Inversion rises throughout 24 hours to ~2100m  Surface precipitation ~1-2 mm/day  In-cloud rain content up to ~.5g/kg  Rain values possibly too high – perhaps Kessler autoconversion is too efficient and/or cloud droplet concentration is too low.

Page 10© Crown copyright 2006 Control V NOMICRONOMICRO

Page 11© Crown copyright 2006 Control v NOMICRO Switched off the rain.

Page 12© Crown copyright 2006 Control v NOMICRO Control_Q02Q03NOMICRO_Q02Q03 ControlNOMICRO

Page 13© Crown copyright 2006 Control v NOMICRO Control_mfluxNOMICRO_mflux Control_ANOMICRO_A ControlNOMICRO

Page 14© Crown copyright 2006 Control v NOMICRO Control_ZINOMICRO_ZI Control_TKENOMICRO_TKE ControlNOMICRO

Page 15© Crown copyright 2006 Control v NOMICRO Control_THVBAVNOMICRO_THVBAV ControlNOMICRO

Page 16© Crown copyright 2006 Control v NOMICRO

Page 17© Crown copyright 2006 Control v NOMICRO  Detrainment/evaporation of cloud water at the top of the cloud layer serves to cool the environment and destabilize the layer.  Clouds then go deeper.

Page 18© Crown copyright 2006 Control V NOMICRO1-M

Page 19© Crown copyright 2006 Control v 1-M  Single moment rain is used.

Page 20© Crown copyright 2006 Control v 1-M Control_qrbudget1M_qrbudget Control_Q02Q031M_Q02Q03 Control1-M

Page 21© Crown copyright 2006 Control v 1-M Control_ppt1M_ppt Control_zi1M_zi Control1-M

Page 22© Crown copyright 2006 Control v 1-M Control_THVBAV1M_THVBAV Control_mflux1M_mflux Control1-M

Page 23© Crown copyright 2006 Control v 1-M

Page 24© Crown copyright 2006 Control v 1-M  Rain budget balance is very different to 2-M scheme; collection reduced and autoconversion more significant.  Underestimation of thein-cloud rain content.  Surface precip is reduced and is less variable.  As NOMICRO case more detrainment at cloud top (since less conversion to rain) resulting in elevation of the inversion.

Page 25© Crown copyright 2006 Control V NOMICRO1-M-RNa

Page 26© Crown copyright 2006 Control v 1-M-RNa  Single moment rain, diagnosis of rain number distribution through expression: With R Na =1.1x  In order to try to achieve larger in-cloud rain contents, the single moment scheme is modified such that R Na =3x10 19.

Page 27© Crown copyright 2006 Control v 1-M-RNa Control_qrbudget1MRNa_qrbudget Control_Q02Q031MRNa_Q02Q03 Control1-M-RNa

Page 28© Crown copyright 2006 Control v 1-M-RNa Control_ppt1MRNa_ppt Control_zi1MRNa_zi Control1-M-RNa

Page 29© Crown copyright 2006 Control v 1-M-RNa Control_zi1MRNa_zi Control_TKE1MRNa_TKE Control1-M-RNa

Page 30© Crown copyright 2006 Control v 1-M-RNa

Page 31© Crown copyright 2006 Control v 1-M-RNa  Although the change to R Na proved to be successful in increasing the in-cloud rain contents, the precipitation comes in isolated bursts and subsequently changes the cloud dynamics.  This sporadic precipitation is possibly due to the lack of size sorting with all particles effectively falling at the same rate in the 1-M scheme.(??)  There is further increased evaporation of rain in the boundary layer leading to increased cooling and generation of cold pools.(??)  Obvious issues with domain size in this case.

Page 32© Crown copyright 2006 Control V NOMICROMODRNc

Page 33© Crown copyright 2006 Control v MODRNc  The Kessler autoconversion scheme is parametrized as where the threshold for autoconversion is calculated as with. Taking n l =45cm -1 results in a threshold mixing ratio of  Here we use n l =240cm -1 (Khairoutdinov & Kogan marine stratocumulus), resulting in a threshold mixing ratio of  i.e. autoconversion is less efficient.

Page 34© Crown copyright 2006 Control v MODRNc Control_qrbudgetMODRNc_qrbudget Control_Q02Q03MODRNc_Q02Q03 ControlMODRNc

Page 35© Crown copyright 2006 Control v MODRNc Control_pptMODRNc_ppt Control_ziMODRNc_zi ControlMODRNc

Page 36© Crown copyright 2006 Control v MODRNc Control_THVBAVMODRNc_THVBAV Control_mfluxMODRNc_mflux ControlMODRNc

Page 37© Crown copyright 2006 Control V NOMICROMODVR

Page 38© Crown copyright 2006 Control v MODVR  Control fall speed is that due to uplinger (cyan)  Here we use the LEM default which is shown by the red curve. This overestimates fall speed for drops 3mm.  [The drizzle formulation (green) was tried, but this unsurprisingly failed as soon as any rain was created]

Page 39© Crown copyright 2006 Control v MODVR Control_qrbudgetMODVR _qrbudget Control_Q02Q03MODVR _Q02Q03 ControlMODVR

Page 40© Crown copyright 2006 Control v MODVR Control_pptMODVR _ppt Control_ziMODVR _zi ControlMODVR

Page 41© Crown copyright 2006 Control v MODVR Control_THVBAVMODVR _THVBAV Control_mfluxMODVR _mflux ControlMODVR

Page 42© Crown copyright 2006 Control V NOMICROHI

Page 43© Crown copyright 2006 Control v HI  Final sensitivity test was to increase the horizontal resolution to 50mx50m (domain size remains the same)

Page 44© Crown copyright 2006 Control v HI Control_qrbudgetHI _qrbudget Control_Q02Q03HI _Q02Q03 ControlHI

Page 45© Crown copyright 2006 Control v HI Control_pptHI _ppt Control_ziHI _zi ControlHI

Page 46© Crown copyright 2006 Control v HI Control_THVBAVHI _THVBAV Control_mfluxHI _mflux ControlHI

Page 47© Crown copyright 2006 Control v HI

Page 48© Crown copyright 2006 Control v HI  Higher resolution allows smaller clouds to form and better resolves the turbulent structure of the clouds.  This enables a more gradual evolution of the cloud field, and possibly prevents the aliasing of cloud sizes onto the grid.

Page 49© Crown copyright 2006 Control V NOMICROABEL

Page 50© Crown copyright 2006 Control v ABEL  Previous work of Abel & Shipway produced quasi-equilibrium simulations of slightly deeper warm rain convection (~3km cloud top) based on RICO observations from 19 th January 2006.

Page 51© Crown copyright 2006 Control v ABEL Control_qrbudgetABEL _qrbudget Control_Q02Q03ABEL _Q02Q03 ControlABEL

Page 52© Crown copyright 2006 Control v ABEL Control_pptABEL _ppt Control_ziABEL _zi ControlABEL

Page 53© Crown copyright 2006 Control v ABEL Control_THVBAVABEL _THVBAV Control_mfluxABEL _mflux ControlABEL

Page 54© Crown copyright 2006 Control v ABEL

Page 55© Crown copyright 2006 Control v ABEL  The precipitation rates of the ABEL simulations are of the same order as those produced here (despite the different forcings)  Partitioning of cloud water and rain water in the clouds is also similar.

Page 56© Crown copyright 2006 Conclusions?

Page 57© Crown copyright 2006 What does this all mean?  Double moment rain is preferable to single moment rain. (Single moment rain seems to be able to get the precip or the in-cloud rain correct, but not both.)  The current double moment autoconversion may be too enthusiastic.  While the effects on organization in the 1-M-RNa experiment are perhaps extreme, they do indicate that the microphysics formulations can have important and significant effects on the boundary layer/cloud dynamics.  Hi resolution allows for smaller clouds to be created and better resolves the turbulent structure.  Overall properties of the control simulation look similar to those produced by Abel & Shipway for slightly deeper convection.  More results from changes to rain drop fallspeed and cloud number concentration will be shown by Steve Abel on Thursday.