1. Introduction

Tropical Storm Karl developed over the tropical North Atlantic in September 2016 and reached its maximum tropical intensity on 25 September. As Karl moved rapidly north-eastwards, it encountered the mid-latitude baroclinic zone and underwent extratropical transition. The resulting extratropical cyclone subsequently interacted with the North Atlantic jet and a pre-existing low-pressure system.

The evolution of ex-Karl was associated with substantial changes to the large-scale circulation over the North Atlantic. During 26–27 September, a pronounced upper-level ridge developed downstream and an unusually intense upper-tropospheric jet streak formed north of Scotland. The subsequent circulation was characterised by strong poleward moisture transport towards Scandinavia, and a downstream cyclone produced heavy precipitation over western Norway on 28–29 September.

Karl was one of the weather systems investigated during the North Atlantic Waveguide and Downstream Impact Experiment (NAWDEX). The event provides an interesting example of the interaction between a transitioning tropical cyclone and the mid-latitude circulation, and has subsequently been studied in relation to extratropical transition, diabatic processes, jet-stream dynamics and atmospheric predictability.

In this case study, a six-day deterministic OpenIFS forecast initialised at 00 UTC on 25 September 2016 is used to follow the extratropical transition of Karl and the subsequent evolution of the North Atlantic circulation. The forecast provides an opportunity to examine the development of the surface cyclone, its interaction with the upper-level flow, the formation of the intense North Atlantic jet streak, and the downstream precipitation event over Norway.

2. Literature

This table lists published references which directly describe relevant atmospheric processes for this case study. They can be used for background reading and provide further inspriration for the future expansion of this case study.

3. Experiment Design

3.1. Scientific Motivation

The extratropical transition of Tropical Storm Karl provides a useful case for examining how a tropical weather system interacts with the mid-latitude circulation. After transition, ex-Karl interacted with the North Atlantic flow during the development of a downstream ridge and an intense upper-tropospheric jet streak north of Scotland. The subsequent circulation also transported moisture towards Scandinavia and was associated with heavy precipitation over western Norway.
The objective of this worked example is to investigate the following question:

• How well does a deterministic OpenIFS forecast reproduce Karl’s extratropical transition and the subsequent evolution of the North Atlantic circulation?

The focus is on following the main synoptic developments rather than establishing a detailed causal attribution of the downstream weather.

3.2. Methodology

The worked example uses a single six-day OpenIFS 48r1 forecast at T255L91 resolution (N128 grid), initialised at 00 UTC on 25 September 2016, with model output available every six hours. The analysis follows the event chronologically using a small set of standard diagnostics:

• mean sea-level pressure to follow Karl and the subsequent surface cyclones;
• 850 hPa temperature and wind to identify the baroclinic zone, frontal development and low-level flow;
• 500 hPa geopotential height to examine the evolving trough and downstream ridge;
• 300 hPa wind speed to follow the North Atlantic jet and identify the intense upper-tropospheric jet streak;
• atmospheric moisture, where available, to examine transport towards Scandinavia;
• accumulated precipitation to identify the downstream precipitation event over western Norway.

You are encouraged to compare selected forecast times with the corresponding ECMWF analysis to assess how the simulated large-scale circulation evolves with forecast lead time.

4. OpenIFS initial experiment data

• We provide experiment data packs that allow to simulate the time period of Storm Karl in the North Atlantic with the global OpenIFS forecasting model. The initial experiment data and boundary conditions were created from ECMWF reanalysis data.
• The experiment data packs include a model namelist file (fort.4) which contain suggested 6-hourly outputs for instantaneous and derived model fields and diagnostics.
• All OpenIFS experiments begin on 25th September 2016 00:00 UTC, shortly before the extra tropical conversion of Storm Karl. 
• The above recommendation for the N128 / T255 grid is intended to satisfy most computational hardware requirements. However, we offer the experiment data packs at arange of grid resolutions and different types of Gaussian model grids.

4.1. Available OpenIFS Experiments

All experiments are initialised from ERA5 reanalysis data on 2016-09-25 00:00 UTC and include data for a 6 day forecast (144 hours).

The Storm Karl case study is used as the primary case study in the ECMWF training course "A hands-on introduction to Numerical Weather Prediction Modelling" which is held at ECMWF in Reading, UK, approximately every 2 years. 

5. Licensing and Attribution

The OpenIFS experiment data packages were produced with the OpenIFS Data Hub and their use is subject to the following Data Licensing conditions.