Page 1© Crown copyright 2006 Precipitating Shallow Cumulus Case Intercomparison For the 9th GCSS Boundary Layer Cloud Workshop, 18-22 September 2006, GISS.

© Crown copyright 2006 Precipitating Shallow Cumulus Case Intercomparison For the 9th GCSS Boundary Layer Cloud Workshop, 18-22 September 2006, GISS New York, USA Ben Shipway, UK Met Office Met Office LEM Results

© 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)

© 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

© 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

© 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.

© 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.

© Crown copyright 2006 Control v 1-M-RNa  Single moment rain, diagnosis of rain number distribution through expression: With R Na =1.1x10 15.  In order to try to achieve larger in-cloud rain contents, the single moment scheme is modified such that R Na =3x10 19.

© 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.

© 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.

© 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]

© 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.

© 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.

© 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.

© 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.

Page 1© Crown copyright 2006 Precipitating Shallow Cumulus Case Intercomparison For the 9th GCSS Boundary Layer Cloud Workshop, 18-22 September 2006, GISS.

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Page 1© Crown copyright 2006 Precipitating Shallow Cumulus Case Intercomparison For the 9th GCSS Boundary Layer Cloud Workshop, 18-22 September 2006, GISS.

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Presentation on theme: "Page 1© Crown copyright 2006 Precipitating Shallow Cumulus Case Intercomparison For the 9th GCSS Boundary Layer Cloud Workshop, 18-22 September 2006, GISS."— Presentation transcript:

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