A Spectral Limited-area Model with Time-dependent Lateral Boundary Conditions and Its Application to a Multi-level Primitive Equation Model

Abstract. A major linear mesoscale convective system caused severe weather over northern France, Belgium, the Netherlands and northwestern Germany on 3 January 2014. The storm was classified as a cold-season derecho with widespread wind gusts exceeding 25 m s−1. While such derechos occasionally develop along cold fronts of extratropical cyclones, this system formed in a postfrontal air mass along a baroclinic surface pressure trough and was favoured by a strong large-scale air ascent induced by an intense mid-level jet. The lower-tropospheric environment was characterised by weak latent instability and strong vertical wind shear. Given the poor operational forecast of the storm, we analyse the role of initial and lateral boundary conditions to the storm's development by performing convection-resolving limited-area simulations with operational analysis and reanalysis datasets. The storm is best represented in simulations with high temporally and spatially resolved initial and lateral boundary conditions derived from ERA5, which provide the most realistic development of the essential surface pressure trough. Moreover, simulations at convection-resolving resolution enable a better representation of the observed derecho intensity. This case study is testimony to the usefulness of ensembles of convection-resolving simulations in overcoming the current shortcomings of forecasting cold-season convective storms, particularly for cases not associated with a cold front.

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A Time-Dependent Lateral Boundary Scheme for Limited-Area Primitive Equation Models

Monthly Weather Review ◽

10.1175/1520-0493(1976)104<0744:atdlbs>2.0.co;2 ◽

1976 ◽

Vol 104 (6) ◽

pp. 744-755 ◽

Cited By ~ 77

Author(s):

Donald J. Perkey ◽

Carl W. Kreitzberg

Keyword(s):

Time Dependent ◽

Limited Area ◽

Lateral Boundary ◽

Primitive Equation

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Soil Field Model Interoperability: Challenges and Impact on Screen Temperature Forecast Skill during the Nordic Winter

Journal of Hydrometeorology ◽

10.1175/jhm-d-11-095.1 ◽

2012 ◽

Vol 13 (4) ◽

pp. 1215-1232 ◽

Cited By ~ 3

Author(s):

Jørn Kristiansen ◽

Dag Bjørge ◽

John M. Edwards ◽

Gabriel G. Rooney

Keyword(s):

Soil Moisture ◽

High Resolution ◽

Boundary Conditions ◽

Limited Area ◽

Lateral Boundary ◽

Error Source ◽

Main Error ◽

Lateral Boundary Conditions ◽

The Impact ◽

Grid Length

Abstract The high-resolution (4-km grid length) Met Office (UKMO) Unified Model forecasts driven by the coarser-resolution (8-km grid length) High-Resolution Limited-Area Model (HIRLAM), UM4, often produce significantly colder screen-level (2 m) temperatures in winter over Norway than forecast with HIRLAM itself. To diagnose the main error source of this cold bias this study focuses on the forecast initial and lateral boundary conditions, particularly the initialization of soil moisture and temperature. The soil variables may be used differently by land surface schemes of varying complexity, representing a challenge to model interoperability. In a set of five experiments, daily UM4 forecasts are driven by alternating initial and lateral boundary conditions from two different parent models: HIRLAM and Met Office North Atlantic and Europe (NAE). The experiment period is November 2007. Points for scientific examination into the topics of model interoperability and sensitivity to soil initial conditions are identified. The soil moisture is the main error source and is therefore important also in winter, rather than being a challenge only in summer. The day-to-day variability in the forecast error is large with the larger errors on days with strong longwave heat loss at the surface (i.e., the forecast sensitivity to soil moisture content is significant but variable). The much drier soil in HIRLAM compared to NAE reduces the heat capacity of the soil layers and affects the heat flux from the surface soil layer to the surface. Normalizing the respective soil moisture fields reduces these differences. The impact of ground snow is quite limited.

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