The aim of this article is to describe the reference configuration of the convection-permitting numerical weather prediction (NWP) model HARMONIE-AROME, which is used for operational short-range weather forecasts in Denmark, Estonia, Finland, Iceland, Ireland, Lithuania, the Netherlands, Norway, Spain, and Sweden. It is developed, maintained, and validated as part of the shared ALADIN-HIRLAM system by a collaboration of 26 countries in Europe and northern Africa on short-range mesoscale NWP. HARMONIE-AROME is based on the model AROME developed within the ALADIN consortium. Along with the joint modeling framework, AROME was implemented and utilized in both northern and southern European conditions by the above listed countries, and this activity has led to extensive updates to the model's physical parameterizations. In this paper the authors present the differences in model dynamics and physical parameterizations compared with AROME, as well as important configuration choices of the reference, such as lateral boundary conditions, model levels, horizontal resolution, model time step, as well as topography, physiography, and aerosol databases used. Separate documentation will be provided for the atmospheric and surface data-assimilation algorithms and observation types used, as well as a separate description of the ensemble prediction system based on HARMONIE-AROME, which is called HarmonEPS.
S~I M MA R Y A three-time-level semi-lagrangian global spectral model was introduced operationally at the European Centre for Medium-Range Weather Forecasts in I99 I . This paper first documents some later refinements to the three-time-level scheme, and then describes its conversion to a two-time-level scheme. Experimental results are presented to show that the two-time-level scheme maintains the accuracy of its three-time-level predecessor, while being considerably more computationally efficient. In principle, a two-time-level semi-Lagrangian scheme provides a further doubling of efficiency, through a procedure which is usually referred to as 'doubling the time step'. This is slightly misleading, as in a three-time-level leapfrog scheme the length of each time step in the usual notation is 2 A t , but successive time steps overlap by A?. More precisely, the two-time-level scheme doubles the efficiency by eliminating this overlapping, so that only half the number of time steps is needed to complete the forecast. Viewed in this light, it is clear that the time truncation error can be the same for a three-time-level scheme and for the corresponding two-time-level scheme with a 'doubled' time step. For a two-time-level scheme to be accurate as well as efficient, it is important that second-order accuracy in time be maintained in the trajectory calculations. A simple way to achieve this was independently suggested by McDonald and Bates (1 987) and Temperton and Staniforth (1987), and formed the basis for later developments. Haugen (1992, 1993) described a two-time-level semi-Lagrangian limitedarea model; subsequently, Gustafsson and McDonald (1996) presented a comparison between spectral and finite-difference versions of this model. Recent applications of two-time-level semi-Lagrangian schemes to global finite-difference medium-range forecast models have been described by Chen and Bates ( 1996) and Moorthi ( 1997). CBtC et al. ( 1998a, 1998b) describe a multi-purpose variable-resolution global finite-element model based on a two-time-level semi-Lagrangian scheme, while Qian et al. (1998) incorporate a two-time-level scheme in a global non-hydrostatic model.In this paper, a two-time-level reformulation of the semi-Lagrangian global spectral model documented in R95 is presented. This version of the two-time-level scheme was used in the operational ECMWF forecast model from December 1996 until April 1998. First, section 2 describes some modifications to the three-time-level scheme which were implemented during its operational lifetime. The conversion to a two-time-level scheme is then described in section 3. Experimental results are presented in section 4, followed
SUMMARYA new treatment of the two-time-level semi-Lagrangian scheme is presented which avoids extrapolation in time of the velocities used for the computation of the trajectories and for the nonlinear terms of the evolution equations.Extrapolation in time is used in the original two-time-level scheme in order to make it centred in time and therefore second-order accurate. This time-extrapolation can lead to instabilities. In the new scheme the secondorder accuracy is achieved by means of a Taylor series expansion to second order around the departure point of the semi-Lagrangian trajectory thus avoiding extrapolation of the first-order term. The estimate of the secondorder term in this expansion is obtained by means of a stable time extrapolation from the previous time step and therefore the scheme is named the 'Stable Extrapolation Two-Time-Level Scheme' or SETTLS.Some cases of noise in forecast fields, arising from an instability of the time extrapolation in the version of the two-time-level semi-Lagrangian scheme used operationally at the European Centre for Medium-Range Weather Forecasts (ECMWF) between December 1996 and April 1998, are diagnosed; it is shown that the noise can be removed by the use of the SETTLS.
Breakup of the polar stratospheric vortex in the Northern Hemisphere is an event that is known to be predictable for up to a week or so ahead. This is illustrated using data from the 45-yr ECMWF Re-Analysis (ERA-40) for the sudden warmings of January 1958 and February 1979 and operational ECMWF data for February 2003. It is then shown that a similar level of skill was achieved in operational forecasts for the split of the southern stratospheric vortex in late September 2002. The highly unusual flow conditions nevertheless exposed a computational instability of the forecast model. Analyses and forecasts from reruns using improved versions of the forecasting system are presented. Isentropic maps of potential vorticity and specific humidity provide striking pictures of the advective processes at work. Forecasts as well as analyses are shown to be in good agreement with radiosonde measurements of the temperature changes associated with vortex movement, distortion, and breakup during August and September. Forecasts from 17 September onward capture the remarkable temperature rise of about 60°C recorded at 20 hPa by the Halley radiosonde station as the vortex split. Objective forecast verification and data denial experiments are used to characterize the performance of the observing and data assimilation systems and to infer overall forecast, analysis, and observation accuracy. The observations and analyses from 1957 onward in the ERA-40 archive confirm the extreme nature of the 2002 event. Secondary vortex development by barotropic instability is also discussed; in analyses for early October 2002, the process is active in the breakup of the weaker of the two vortices formed by the late-September split.
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