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.

Publications Andreas Schäfler et al. (2018), The North Atlantic Waveguide and Downstream Impact Experiment, BAMS 99, 1607-1637.
https://doi.org/10.1175/BAMS-D-17-0003.1

Euler, C. et al. (2019), Lagrangian Description of Air Masses Associated with Latent Heat Release in Tropical Storm Karl (2016) during Extratropical Transition, Mon. Wea. Rev. 147, 2657-2676. https://doi.org/10.1175/MWR-D-18-0422.1

Harvey, B., et al. (2020), Diabatic generation of negative potential vorticity and its impact on the North Atlantic jet stream, Q. J. Roy. Met. Soc. 146, 1477–1497.  https://doi.org/10.1002/qj.3747
Web sites NAWDEX flight campaign web portals of various project partners: DLR, KIT, ETHZ
Other material

Presentation by Ben Harvey (University of Reading) at the 5th OpenIFS User Meeting 2019 (PDF)

US National Hurricane Center Tropical Cyclone Report - Tropical Storm Karl (PDF)

3. Important dates

The dates shown below were obtained from the US NHC Tropical Cyclone Report and from ECMWF ERA5 reanalysis data. They represent important milestones in the development and lifetime of Storm Karl. The first four dates describe events during the tropical storm phase, the following events occurred after the extra-tropical conversion in the North Atlantic and later.

12 Sep 2016 Karl forms from tropical wave on the western coast of Africa)



tropical storm
15 Sep 2016 Karl strengthens into a tropical storm
21 Sep 2016 Karl weakens to a depression
25 Sep 2016 Karl strengthens again and reaches peak intensity (0600 UTC)
25 Sep 2016 Karl undergoes extra tropical conversion (12 UTC)



extra-tropical storm

26 Sep 2016 Karl merges with another ET low pressure system
? Karl causes maximum upper-tropospheric wind speed in the United Kingdom (Scotland)
? Karl causes maximum 6-hourly precipitation in Norway (Bergen)

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. 


Where can I download the data?

All required data packs for this case study can be downloaded from this site:

https://openifs.ecmwf.int/data/experiments/48r1/2016-09_Karl/ 

page Data Licence

(Note:  The data packs only include the experiment data, not the OpenIFS model source code.)

4.1. Available OpenIFS Experiments

Model
Cycle

 Exp ID Model
Grid		 Spectral
Truncation		 Vertical
Levels		 Data
Source		 Initial
Date		 Forecast
Length
48r1 ab7z N80 TL159 60 ERA5   2016-09-25   00:00 UTC   6 days (144 hours)
48r1 ab2a N128 TL255 91 ERA5   2016-09-25   00:00 UTC   6 days (144 hours)
48r1 ab89 O200 TCO199 91 ERA5 2016-09-25   00:00 UTC 6 days (144 hours)
48r1 ab8v N320 TL639 137 ERA5 2016-09-25   00:00 UTC 6 days (144 hours)
48r1 abdr N640 TL1279 137 ERA5 2016-09-25   00:00 UTC 6 days (144 hours)
48r1 ab8r O1280 TCO1279 137 ERA5 2016-09-25   00:00 UTC 6 days (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.


 
Experiment data download ...


Download this case study here:

https://openifs.ecmwf.int/data/experiments/48r1/2016-09_Karl 

page Data Licence

On this page ...

See also ...