Verification of a seeder–feeder orographic precipitation enhancement scheme accounting for low‐level blocking

• Published in: Quarterly journal of the Royal Meteorological Society Vol. 145; no. 724; pp. 2909 - 2932
• Main Authors: Smith, Samantha A., Field, Paul R., Vosper, Simon B., Derbyshire, Steve H.
• Format: Journal Article
• Language: English
• Published: Chichester, UK John Wiley & Sons, Ltd 01.10.2019; Wiley Subscription Services, Inc
• Subjects: Albedo; climate simulation; Cloud albedo; Clouds; Convection; Convective precipitation; Drying; Duration; Froude number; Hydrologic cycle; Hydrological cycle; Hydrology; low‐level flow blocking; Microphysics; Mixing ratio; Mountains; Orographic precipitation; orographic precipitation enhancement; Orography; parametrization; Precipitation; precipitation forecasting; Satellite observation; Satellites; seeder–feeder mechanism; Simulation; Water mixing
• ISSN: 0035-9009; 1477-870X
• DOI: 10.1002/qj.3584

A subgrid parametrization scheme representing the enhancement of precipitation due to subgrid orography via the seeder–feeder (SF) effect has been modified to account for flow blocking in small Froude number situations. The scheme was validated in a set of limited‐area model simulations with a 1.5 km grid spacing, in which the orography was degraded by varying amounts. For simulations in which the largest orographic scales are still fairly well represented, the SF scheme was able to reduce the precipitation deficit by 30 to 70%. For simulations where the hills were completely subgrid, the SF scheme was still able to reduce the precipitation deficit by 10 to 30%. As well as increasing the integrated precipitation in global simulations with various grid spacings, some of the precipitation production was shifted from the convection scheme, with its very simple microphysical representation, to the microphysics scheme. In a long‐duration climate simulation, the SF scheme enhancements of orographic precipitation perturb the large‐scale hydrological cycle, as evidenced by the far‐field changes in both microphysical and convective precipitation. Changes over major mountain ranges were similar to those described for the case‐studies, so long as the upstream precipitation impinging on the mountains was not reduced by these changes in the global hydrological cycle. Increased atmospheric drying reduces column‐integrated resolved cloud water mixing ratios over mountains, reducing a positive bias in the amount of optically thick cloud with low‐ to mid‐level tops compared to ISCCP satellite observations. The associated reductions in cloud albedo slightly reduced the RMS error in the top‐of‐atmosphere outgoing short‐wave radiative fluxes over mountains compared to CERES satellite observations. A new seeder–feeder scheme, now also accounting for low‐level blocking by sub‐grid orography, is shown to increase orographic precipitation in low‐resolution simulations towards that produced by a 1.5 km resolution simulation, with some of the precipitation formation being shifted from the convection scheme to the microphysics scheme. Increased drying means that a positive cloud water bias over mountains producing strong precipitation enhancement is reduced.