A Bimodal Diagnostic Cloud Fraction Parameterization. Part I: Motivating Analysis and Scheme Description
- 1 March 2021
- journal article
- research article
- Published by American Meteorological Society in Monthly Weather Review
- Vol. 149 (3), 841-857
- https://doi.org/10.1175/mwr-d-20-0224.1
Abstract
Cloud fraction parameterizations are beneficial to regional, convection-permitting numerical weather prediction. For its operational regional mid-latitude forecasts, the UK Met Office uses a diagnostic cloud fraction scheme which relies on a unimodal, symmetric subgrid saturation-departure distribution. This scheme has been shown before to underestimate cloud cover and hence an empirically-based bias correction is used operationally to improve performance. This first of a series of two papers proposes a new diagnostic cloud scheme as a more physically-based alternative to the operational bias correction. The new cloud scheme identifies entrainment zones associated with strong temperature inversions. For model grid boxes located in this entrainment zone, co-located moist and dry Gaussian modes are used to represent the subgrid conditions. The mean and width of the Gaussian modes, inferred from the turbulent characteristics, are then used to diagnose cloud water content and cloud fraction. It is shown that the new scheme diagnoses enhanced cloud cover for a given grid-box mean humidity, similar to the current operational approach. It does so, however, in a physically meaningful way. Using observed aircraft data and ground-based retrievals over the Southern Great Plains in the US, it is shown that the new scheme improves the relation between cloud fraction, relative humidity and liquid water content. An emergent property of the scheme is its ability to infer skewed and bimodal distributions from the large-scale state that qualitatively compare well against observations. A detailed evaluation and resolution sensitivity study will follow in part II. Cloud fraction parameterizations are beneficial to regional, convection-permitting numerical weather prediction. For its operational regional mid-latitude forecasts, the UK Met Office uses a diagnostic cloud fraction scheme which relies on a unimodal, symmetric subgrid saturation-departure distribution. This scheme has been shown before to underestimate cloud cover and hence an empirically-based bias correction is used operationally to improve performance. This first of a series of two papers proposes a new diagnostic cloud scheme as a more physically-based alternative to the operational bias correction. The new cloud scheme identifies entrainment zones associated with strong temperature inversions. For model grid boxes located in this entrainment zone, co-located moist and dry Gaussian modes are used to represent the subgrid conditions. The mean and width of the Gaussian modes, inferred from the turbulent characteristics, are then used to diagnose cloud water content and cloud fraction. It is shown that the new scheme diagnoses enhanced cloud cover for a given grid-box mean humidity, similar to the current operational approach. It does so, however, in a physically meaningful way. Using observed aircraft data and ground-based retrievals over the Southern Great Plains in the US, it is shown that the new scheme improves the relation between cloud fraction, relative humidity and liquid water content. An emergent property of the scheme is its ability to infer skewed and bimodal distributions from the large-scale state that qualitatively compare well against observations. A detailed evaluation and resolution sensitivity study will follow in part II.Keywords
This publication has 35 references indexed in Scilit:
- Unified Modeling and Prediction of Weather and Climate: A 25-Year JourneyBulletin of the American Meteorological Society, 2012
- Evaluation of two cloud parametrization schemes using ARM and Cloud‐Net observationsQuarterly Journal of the Royal Meteorological Society, 2011
- Parametrization of area cloud fractionAtmospheric Science Letters, 2010
- Can Water Vapour Raman Lidar Resolve Profiles of Turbulent Variables in the Convective Boundary Layer?Boundary-Layer Meteorology, 2010
- On the use of PDF schemes to parameterize sub‐grid cloudsGeophysical Research Letters, 2009
- Development of an Improved Turbulence Closure Model for the Atmospheric Boundary LayerJournal of the Meteorological Society of Japan. Ser. II, 2009
- PC2: A prognostic cloud fraction and condensation scheme. I: Scheme descriptionQuarterly Journal of the Royal Meteorological Society, 2008
- Characteristics of High-Resolution Versions of the Met Office Unified Model for Forecasting Convection over the United KingdomMonthly Weather Review, 2008
- Snow Size Distribution Parameterization for Midlatitude and Tropical Ice CloudsJournal of the Atmospheric Sciences, 2007
- CloudnetBulletin of the American Meteorological Society, 2007