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Keith Williams

Met Office, UK

This presentation will discuss the sensitivity of weather forecasts to parametrizations, with a focus on moist processes. A range of examples from the Met Office Unified Model will be given including discussion of the role of convection, cloud and boundary layer processes. Diagnostic techniques to identify the likely source of sensitivity will also be discussed. Global examples of the Impacts on both the large scale circulation and local weather will be presented.

Victoria Sinclair¹, Helen Dacre²

¹University of Helsinki, Finland
²University of Reading, UK

Extra-tropical cyclones are known to be responsible for a large proportion of the poleward moisture transport in the mid-latitudes. However, not all cyclones are equal; some transport significantly more moisture polewards than others. The aim of the work presented here is to identify which cyclone characteristic (genesis latitude, maximum vorticity and poleward propagation speed), leads to the greatest variability in meridional moisture transport per cyclone. We use ERA-Interim reanalysis combined with an objective cyclone tracking algorithm (TRACK), a cyclone masking method and cyclone composites. Interestingly, the results show that cyclone intensity, quantified by maximum vorticity, has a weaker influence on how much moisture a cyclone can transport than other cyclone characteristics. This result suggests that studies considering how extra-tropical cyclones will change in the future should focus on cyclone track as well as cyclone intensity and number.

Heini Wernli

ETH, Switzerland

The prediction of the track, intensity and structure of extratropical cyclones is of key importance for accurately forecasting surface weather and its extremes. In addition to classical baroclinic instability typically induced by an upper-level cyclonic potential vorticity (PV) anomaly, moist diabatic processes that occur in saturated ascending airstreams are essential for the evolution of cyclones, the associated surface weather, and the downstream flow evolution. Results from studies that are mainly based on the analysis of IFS analyses and forecasts, will be shown to illustrate that (i) the diabatic PV production in the low troposphere is essential for the formation of intense extratropical cyclones, (ii) diabatic processes are particularly intense in so-called warm conveyor belts, i.e., in moist ascending airstreams associated with extratropical cyclones, (iii) different microphysical processes in and below clouds contribute to the diabatic formation of PV anomalies in cyclones, and (iv) exceptionally poor global model medium-range weather forecasts (so-called forecast busts) in Europe are often associated with errors in the representation of warm conveyor belts. The presentation will also provide a selection of first results from the recent field experiment NAWDEX, which was dedicated to observing diabatic processes in mid-latitude weather systems.

Christian M. Grams

Karlsruher Institut für Technologie, Germany

The large-scale extratropical circulation is dominated by Rossby wave activity along the upper-level midlatitude wave guide. Recent research revealed that moist processes can be as important in the evolution of Rossby wave packets as dry dynamics. This is primarily due to latent heat release in ascending air streams, i.e. warm conveyor belts (WCBs). Thereby moist warm air is transported poleward and ascends slantwise along the midlatitude baroclinic zone. Latent heat release due to condensation enhances this ascent and results in a net transport of air mass into the upper troposphere. This so-called diabatic outflow deflects the wave guide poleward, accelerates the jet stream, and an upper-level ridge builds or amplifies that might ultimately become a blocking anticyclone. Rossby wave activity in the Atlantic-European region often occurs in preferred quasi-stationary, persistent, and recurrent states called weather regimes. These explain most of the atmospheric variability on sub-seasonal time scales. However, current numerical weather prediction models struggle in correctly predicting weather regime life cycles.

This presentation will focus on the role of diabatic outflow in the life cycle of Atlantic-European weather regimes and how diabatic outflow affects predictability of the large-scale extratropical circulation. In the period prior to the onset of regimes characterized by blocking important changes of the location and frequency of WCBs occur. WCB activity increases significantly upstream of the incipient blocking anticyclone even before blocking is detectable. Diabatic outflow persists over the blocked region later. Thus diabatic outflow is key for the onset and maintenance of blocked regimes in the Atlantic-European region. The misrepresentation of WCB activity and associated diabatic outflow can result in severe forecast busts due to upscale error growth. Thereby the physical processes within a WCB might communicate and amplify small initial condition error to the large-scale extratropical circulation. This is exemplified for recent cases of regime onset.

Further reading:

<p>Ferranti, L., S. Corti, M. Janousek, 2015: Flow-dependent 
verification of the ECMWF ensemble over the Euro-Atlantic sector. Q.J.R.  Meteorol. Soc., 141, 916–924, <a href='https://doi.org/10.1002/qj.2411' target="blank">https://doi.org/10.1002/qj.2411</a>.</p>

<p>Grams, C.M., H.M. Archambault, 2016: The key role of diabatic outflow in 
amplifying the midlatitude flow: a representative case study of weather 
systems surrounding western North Pacific extratropical transition. Mon. Wea. Rev., 144, 3847–3869, <a href='https://doi.org/10.1175/MWR-D-15-0419.1' target="blank">https://doi.org/10.1175/MWR-D-15-0419.1</a>.</p>

<p>Grams, C.M., R. Beerli, S. Pfenninger, I. Staffell, H. Wernli, 2017: Balancing Europe’s wind-power output through spatial deployment informed by weather regimes. Nature Climate Change, 7, 557–562, <a href='https://doi.org/10.1038/nclimate3338' target="blank">https://doi.org/10.1038/nclimate3338</a>.</p>

<p>Joos, H., H. Wernli, 2012: Influence of microphysical processes on the potential vorticity development in a warm conveyor belt: a case-study with the limited-area model COSMO. Q.J.R. Meteorol. Soc., 138, 407–418, <a href='https://doi.org/10.1002/qj.934' target="blank">https://doi.org/10.1002/qj.934</a>.</p>

<p>Martínez-Alvarado, O., E. Madonna, S.L. Gray, H. Joos, 2016: A route to systematic error in forecasts of Rossby waves. Q.J.R. Meteorol. Soc., 142, 196–210, <a href='https://doi.org/10.1002/qj.2645' target="blank">https://doi.org/10.1002/qj.2645</a>.</p>

<p>Michel, C., G. Rivière, 2011: The link between Rossby wave breakings and 
weather regime transitions. J. Atmos. Sci., 68, 1730–1748, <a href='https://doi.org/10.1175/2011JAS3635.1' target="blank">https://doi.org/10.1175/2011JAS3635.1</a>.</p>

<p>Pfahl, S., C. Schwierz, M. Croci-Maspoli, C.M. Grams, H. Wernli, 2015: 
Importance of latent heat release in ascending air streams for atmospheric blocking. Nature Geosci, 8, 610–614, <a href='https://doi.org/10.1038/ngeo2487' target="blank">https://doi.org/10.1038/ngeo2487</a>.</p>

<p>Vautard, R., 1990: Multiple weather regimes over the North Atlantic: 
analysis of precursors and successors. Mon. Wea. Rev., 118, 2056–2081, 
<a href='https://doi.org/10.1175/1520-0493(1990)118<2056:MWROTN>2.0.CO;2' target="blank">https://doi.org/10.1175/1520-0493(1990)118<2056:MWROTN>2.0.CO;2</a>.</p>

1. Jake Bland (University of Reading, UK)

2. Yun Chen (National Meteorological Center of China)

3. Guokun Dai (Fudan University, China)

4. Mokhliss El-azaar (EHTP, Morocco)

5. Jian-Feng Gu (University of Reading, UK)

6. Ben Harvey (University of Reading)

Helen Dacre

University of Reading, UK

Extreme precipitation associated with extratropical cyclones can lead to flooding if cyclones track over land. However, the dynamical mechanisms by which moist air is transported into cyclones is poorly understood. We analyse airflows within a climatology of cyclones in order to understand how cyclones redistribute moisture stored in the atmosphere. This analysis shows that within a cyclones' warm sector the cyclone-relative airflow is rearwards relative to the cyclone propagation direction. This low-level airflow (termed the feeder airstream) slows down when it reaches the cold front resulting in moisture flux convergence and the formation of a band of high moisture content. One branch of the feeder airstream turns towards the cyclone centre supplying moisture to the base of the warm conveyor belt where it ascends and precipitation forms. The other branch turns away from the cyclone centre exporting moisture from the cyclone. As the cyclone travels, this export results in a filament of high moisture content marking the track of the cyclone (often used to identify atmospheric rivers). We find that both cyclone precipitation and water vapour transport increase when moisture in the feeder airstream increases, thus explaining the link between atmospheric rivers and the precipitation associated with warm conveyor belt ascent. Atmospheric moisture budgets calculated as cyclones pass over fixed domains relative to the cyclone tracks, show that continuous evaporation of moisture in the pre-cyclone environment moistens the feeder airstream. Evaporation behind the cold front acts to moisten the atmosphere in the wake of the cyclone passage, potentially preconditioning the environment for subsequent cyclone development.

Nedjeljka Žagar

University of Hamburg, Germany

Global numerical weather prediction (NWP) models are characterized by significant uncertainties in their initial state in the tropics. Furthermore, the largest simulated uncertainties of the global ensemble forecasts in the short range is also found in the tropics. This applies to all zonal wavenumbers and the larger the scale, the greater the uncertainty. My lecture will present evidence of these facts using the data from the ECMWF deterministic and ensemble prediction system to illustrate how these properties have evolved in recent years.

Moist processes in the tropics generate the spectrum of waves that propagate horizontally and vertically. For example, the short-term response to tropical heating anomalies resembling MJO projects predominantly on the inertio-gravity (IG) waves, especially the Kelvin wave. The equatorial IG waves make a large part of tropical variance on all scales and their role in the initial state for NWP is still not clear. This is related to complex moist dynamics, a lack of wind observations and assumptions made in data assimilation modelling. I will demonstrate that uncertainties in the forecasted IG spectra associated with moist convection are less successfully reduced on all scales than uncertainties associated with Rossby waves.

In order to explain some observed features of the NWP models and to investigate possible remedies, simplified models can be used provided they are still complex enough to capture the main dynamical and physical aspects of the phenomena of interest. I will introduce such a model for moist processes and data assimilation in the tropics and present its results that explain some observed features of full-scale NWP systems.

Klaus Wyser

Swedish Meteorological and Hydrological Institute

The 6th phase of the Coupled Model Intercomparison (CMIP6) has started and the EC-Earth community has started making pre-industrial, historical and future scenario experiments with the EC-Earth3 model. The data are uploaded to ESG data nodes and shared with the climate modeling community. In my talk I will briefly describe the development from the coupled IFS-NEMO model to the CMIP6 version of EC-Earth3, the tuning of the coupled model, and present an analysis of the first results from our CMIP6 simulations. The results will be put in context to other CMIP6 models in a common evaluation that will focus on the role of clouds for the climate.

Bethan Harris, Remi Tailleux, Christopher Holloway, Pier Luigi Vidale

University of Reading, UK

Investigating the evolution of Available Potential Energy (APE) in a tropical cyclone (TC) offers the opportunity to gain greater physical insight into the effects of diabatic processes on TC intensification and maintenance. For example, when TC processes are considered in terms of entropy, irreversible processes are by definition a source of entropy and therefore reduce the efficiency of the TC heat engine; local APE theory shows that in fact irreversible processes may be either a source or sink of APE and so they could in some situations promote intensification. We present a budget of APE for the idealised axisymmetric TC model of Rotunno and Emanuel [1987], with Craig’s modified microphysics scheme. The APE density is defined for each moist air parcel as the work released when the parcel moves from its initial position to its level of neutral buoyancy with respect to a reference state. A discretised APE density and APE production efficiencies are constructed to match the model equations. In the mature stage, the chief source of APE in the TC is the advection of APE along the near-surface radial inflow, with significant additional contributions from local APE production due to surface fluxes and the microphysics scheme. The intensification phase is characterised by increasing APE production efficiencies as well as changes in diabatic forcings. We outline the link between the APE budget and TC intensification in terms of the kinetic energy budget, and question whether the choice of APE reference state affects the interpretation of diabatic processes as sources or sinks of APE. We discuss how the components of the APE budget may be useful as process-oriented diagnostics for studying the role of moisture-convection coupling in TC intensification in NWP and climate models.

Pirkka Ollinaho

Finnish Meteorological Institute

Probabilistic forecasts provide information on how predictions of the atmospheric evolution may differ from the traditional "best guess" solution, i.e. the deterministic forecast. Ensemble prediction systems generate this probabilistic information through assessing uncertainties in both the model initial state and the model itself. In order to open up ensemble prediction research for a wider research community, we have recreated all 50+1 operational IFS ensemble initial states for OpenIFS. The data set covers one year (December 2016 to November 2017) twice a day. A range of model resolutions are provided to cover different research needs (TL159, TL399 and TL639).

An open-source workflow manager, called OpenEPS, is used to launch ensemble forecast experiments from the perturbed initial conditions. The experiments are run with OpenIFS CY40R1 and examine all the provided resolutions. The probabilistic skill of the experiments is assessed through ensemble mean RMSE and ensemble spread, and the Continuous Ranked Probability Score (CRPS) for several forecast variables. We will furthermore discuss how many ensemble members are required for conducting meaningful research experiments. This is inspired by a recent paper by Leutbecher (2018), who explored the ensemble size impacts with IFS. Here we redo the same experiments with the OpenIFS ensembles.

We will also illustrate the research potential of running your own ensemble of forecasts through a case study of typhoon Damrey, which severely affected Vietnam in 2017.

Further reading:

<p>Leutbecher M., 2018: Ensemble size: How suboptimal is less than infinity? Q. J. R. Meteorol. Soc. 2018, 1–22, <a href='https://doi.org/10.1002/qj.3387' target="blank">https://doi.org/10.1002/qj.3387</a>.</p>

Xavier Yepes Arbós, Etienne Tourigny, Mario Acosta Cobos, Francisco Doblas-Reyes

Barcelona Supercomputing Center, Spain

Models used in weather and climate forecasts must initialize the state of the Earth system (atmosphere, ocean, land and cryosphere) from observations. Retrospective forecasts (also called hindcasts) in weather or climate prediction mode are used to evaluate the model’s performance and are typically initialized from reanalysis products (such as ERA-Interim). However, the horizontal and vertical resolution of the reanalyses can be different and additional operations are needed to fit the grid model of the hindcast.

The EC-Earth climate model used, among others, by BSC relies on atmospheric initial conditions derived from ERA-Interim. These data must be obtained for several start dates (4 each year) for the entire period covered by the reanalysis product. Until recently, these data were generated at ECMWF HPC facilities using an IFS experiment with the prepIFS infrastructure.

The procedure used is:

1. Download the reanalysis data at its native resolution using MARS requests.
2. Running IFS in mode 927 to make use of FullPos, a powerful post-processing package that performs horizontal and vertical interpolations.
3. Post-processing the result and archival on MARS.

While this procedure is robust and relatively fast, it relies on ECMWF infrastructure and support. Moreover, it is vulnerable to changes in the HPC platform used.

To overcome these shortcomings, the scripts have been simplified and adapted to run on other HPC platforms using the OpenIFS model cycle 40r1. The advantage of the new scripts is that they can be easily deployed on any machine with minimal effort by the user. Currently, the retrieval script is run on the ecgate cluster at ECMWF for efficiency and ease of implementation, relying on the MARS client, but it could be adapted to use the CDS API instead of MARS client. In addition, they are automated using the open-source Autosubmit workflow manager to minimize the user intervention as well as errors, and to optimize some processes.

The results show that the differences of the interpolated GRIB files produced by IFS on ECMWF and by OpenIFS on MareNostrum are negligible. Moreover, several configurations have been successfully interpolated, including configuration T1279L137, which takes very little time, proving that the implementation is fast.

Hannah Christensen

University of Oxford, UK

Stochastic parametrisations are used in weather and climate models to represent model error. Designing new stochastic schemes has been the target of much innovative research over the last decade, with a focus on developing physically motivated schemes. We present a technique for systematically deriving new stochastic parametrisations or for constraining existing stochastic approaches. We take a high-resolution model simulation and coarse-grain it to the desired forecast model resolution. This provides the initial conditions and forcing data needed to drive the OpenIFS Single Column Model (SCM). By comparing the SCM parametrised tendencies with the evolution of the high-resolution model, we can measure the ‘error’ in the SCM tendencies. As a case study, we use this approach to assess the physical basis of the ‘Stochastically Perturbed Parametrisation Tendencies’ (SPPT) scheme, used operationally in the ECMWF Ensemble Prediction System to represent model uncertainty. We provide justification for the multiplicative nature of SPPT, and for the large temporal and spatial scales used in the stochastic perturbations. However, we also identify issues with the SPPT scheme and motivate improvements. In particular, our results indicate that an alternative approach is needed to represent uncertainty in the convection scheme. It is hoped this new coarse-graining technique will improve both holistic and process-based approaches to stochastic parametrisation.

Further reading:

<p>Christensen, H.M.: Constraining stochastic parametrisation schemes 
using high-resolution simulations. For submission to Q. J. Roy. Meteor. 
Soc.</p>

<p>Christensen, H.M., Dawson, A., Holloway, C.E., 2018: Forcing Single-Column Models Using High-Resolution Model Simulations. J. Adv. Model. Earth Syst., 10(8), 1833-1857, <a href='https://doi.org/10.1029/2017MS001189' target="blank">https://doi.org/10.1029/2017MS001189</a>.</p>

• Build the model
• Study and set the namelist
• Running on Cray (basic commands to submit a job, check the job status etc.)
• Run the T21 test
• Diagnostics
• gribapi/eccodes tools for examine GRIB files
• GRIB 2

• Case study: storm Desmond
• Short study the initial conditions
• Set the namelist
• Run the model (T255 experiment for 3 days)
• Post-processing
• Collect ERA5 data for validation
• Introduction to Metview
• Visualization of the forecast outputs and ERA5 with Metview