Volume 56(4) — October 2007

Improvement of weather forecasts in polar regions

by Thor Erik Nordeng1, Gilbert Brunet2 and Jim Caughey3

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Introduction

  man & instrument

So far, there have been two so-called Polar Years separated in time by 50 years: the First International Polar Year took place in 1882-1883 and established a precedent for international science cooperation. The second took place 50 years later in 1932-1933, and investigated the global implications of the newly discovered “jet stream”. The third—the International Polar Year (IPY)—is taking place in 2007–2008 and is an international programme of coordinated, interdisciplinary scientific research and observations in the Earth’s polar regions.

From an enhanced observational network, the sophisticated use of new observations and a better understanding of physical processes in polar regions, it is hoped that IPY 2007–2008 will achieve a similar leap forward in skill in numerical weather and environmental prediction (NWEP) as was achieved by the First Global Atmospheric Research Programme Global Experiment (FGGE) year of 1979. NWEP constitutes one of the most important technological and societal successes of the last century. The positive impact of weather and environmental forecasting on health, safety and economic competitiveness is recognized worldwide. The benefit of NWEP applications in polar regions has been somewhat delayed, due to the higher priority of forecasting in the more densely populated mid-latitude and tropical regions. Concerns about an amplification of anthropogenic climate change at higher latitudes, combined with an increasing interest of many governments, require a better understanding of weather, environmental and climate processes in the polar regions so as to improve our ability to make reliable, quantitative predictions. The IPY provides the important international context to improve weather and environmental forecasting capabilities for the polar regions.

World Weather Research Programme (WWRP)-The Observing System Research and Predictability Experiment (THORPEX)

IPY and WWRP-THORPEX

WMO’s WWRP-THORPEX has the basic objective of accelerating improvements in global and regional numerical weather prediction skill, especially in relation to high- impact weather events. Within the IPY, WWRP-THORPEX will play an important role with the following general objectives:

  • Assess, and seek to improve the quality of, operational analyses and research re-analysis products in the polar regions;
  • Address improving data-assimilation techniques for the polar regions, including sensitivity studies;
  • Assess the skill in predicting polar- to-global high-impact weather events for different observing strategies at higher latitudes;
  • Demonstrate the utility of improved utilization of ensemble weather forecast products for high-impact weather events and for IPY operations, when applicable;
  • Develop recommendations for the design of the Global Observing System (including the middle atmosphere) in polar regions for weather prediction;
  • Conduct field campaigns during the IPY intensive observing period to assist achievement of the goals;
  • Improve representation of high-latitude clouds, cloud/radiation interaction and other key energy exchanges;
  • Develop and evaluate snow (including blowing snow) models;
  • Address two-way interactions of polar and sub-polar weather regimes;
  • Improve/implement/evaluate detailed dynamic-thermodynamic sea-ice models coupled with ocean currents in the Arctic basin;
  • Implement and evaluate assimilation of sea-surface temperature (SST) and ice-motion observations with variational data-assimilation systems.

WWRP-THORPEX will also play a major role as a partner with the climate forecast community in bridging the gap between weather and climate forecasting, leading to better understanding, improved techniques and more skilful forecasts for the often neglected 10-60 day range between the weather and climate time-scales.

Education, outreach and com­munication with northern com­munities will be important throughout IPY. A variety of activities will be organized, including community meetings, on-site visits, joint research projects and Web­sites. As an example, researchers in the Canadian Storm Studies of the Arctic (STAR) project have established linkages with Nunavut territory agencies and Inuit organizations. These include the Nunavut Government (Environment Department, Nunavut Department of Community and Government Services, Nunavut Department of Economic Development and Transportation), the Nunavut Research Institute and the Nunavut Arctic College.

STAR researchers are already collaborating to maintain portable weather stations in which data are being used to investigate site plans for new housing developments in Iqaluit. STAR will also collaborate with the Clyde River Hunters and Trappers Association to establish a research centre in the community that will document and reconcile Inuit observations of change with those made using conventional scientific methods with the objective of developing adaptation strategies. STAR will thus provide an employment opportunity for local people/students as upper-air technicians with Environment Canada.

The WWRP-THORPEX IPY cluster

WMO and the International Council for Science (ICSU) established the IPY Joint Committee (JC) and issued a call for Expressions of Intent (EOI) to be received by November 2004. The IPY-THORPEX cluster was formed from those EOIs that related closely to the objectives outlined above. More than 30 projects were noted as potential members of the cluster and of these (at the time of writing) 10 were actively participating.

In this article, we will first present some of the meteorological conditions that are particular to the polar regions (with a focus on the Arctic), consider some challenges with regard to numerical weather prediction, note the importance of WMO Global Observing Systems (and the EUMETNET Composite Observing System, EUCOS) and, finally, briefly describe some of the projects within the IPY-THORPEX cluster.

Meteorological conditions and challenges for weather prediction

The area is data-sparse, at least for conventional observations. Figure 1 shows the availability of surface and radiosonde observations between 1 and 15 October 2005. Blue dots are stations reporting more than 90 per cent of the time, while all other colours show stations with less complete reports. In the centre of the polar basin and on the Antarctic Plateau, there are virtually no conventional observations. There is no help from commercial aircrafts either (AMDAR data) as few flights cross the area. The use of satellites will therefore play an important role. This is not straightforward, however, as satellite retrievals in polar regions are difficult, owing to the snow- and ice-covered land surface, as well as cold, low- level clouds consisting mainly of ice crystals.

SYNOPS & Temps Figure 1 — Synops (left panels) and TEMPs (right panels) received at Main Telecommunication Network centres during the period 1-15  October 2005 in the Arctic (upper panels) and the Antarctic (lower panels)

Because of the low troposphere and large horizontal variability in stability and temperature, small-scale systems with rapid developments are not uncommon. The most well known examples of this are polar lows but heavy precipitation (snow) from convective systems, low-level fronts and corresponding jets, and mountain lee waves trapped under an inversion, all cause problems for meteorological forecasting.

Routine verification at the Norwegian Meteorological Institute reveals that the overall quality of numerical weather prediction performance is not as good in these high latitudes as further south (see Figure 2). This is probably because of the poorer data coverage, as well as a higher percentage of small-scale systems.

graphic Figure 2 — RMS error of mean sea-level pressure forecasts with the Norwegian limited area model system (HIRLAM) over a two-year period; the Barents Sea in red and the North Sea in blue

Polar processes may have a significant impact on weather phenomena at high and middle latitudes in Europe. The combination of Greenland’s high topography and the data-sparse area of northern Canada make these areas sources of surprise developments which may affect Europe with relatively short lead times. Klinker and Ferrant (2000) investigated the relatively poor performance of the European Centre for Medium-Range Weather Forecasts (ECMWF) model for the summer of 1999 as compared with the summer of 1998 and showed that analysis errors in the polar area can have a detrimental impact on forecast skill over Europe. The result is likely to be dependent on the flow conditions. For the period they considered, a strong baroclinic flow extended from Greenland over the North Atlantic into Europe.

This influence between high- and mid-latitude weather phenomena is not only flow-dependent but also reciprocal. Adjoint sensitivity studies, as exemplified by Figure 3, indicate that disturbances (or initial condition errors) originating in lower latitudes can rapidly amplify while propagating into polar regions. These studies will be indispensable for the strategic planning of next-generation upper- air and space-based observation networks.

map Figure 3 — Example of adjoint sensitivity map, based on the 10 leading singular vectors (SVs) for the case of 5 January 2005. SVs were calculated using the Canadian Global Environmental Multiscale (GEM) model and an optimization time of 48 h. At initial time, the global total-energy norm was used and, at the final time, the total energy norm was restricted to latitudes larger than 70°N. The 48-h forecast error was projected on the leading 10 final-time SVs, to define the linear combination of SVs that best explains the forecast error at that time. This figure shows the total energy (in J/kg) of this linear combination of SVs at initial time and indicates the spatial distribution of the most rapidly growing component of initial condition errors that will impact the 48-h forecast north of 70°N (Figure prepared by Ayrton Zedra).

Figure 4 is a typical result from a state-of-the-art limited-area model. Superimposed on the mean sea-level pressure contour lines is a satellite picture showing that the central area of the synoptic scale low actually consists of a number of smaller-scale vortices. The forecasting challenge in situations such as these is to decide which of the local vorticity maxima will intensify, in which direction and at what speed they will move and how strong the corresponding winds will be.

forecast Figure 4 — Forecast mean sea-level pressure from the operational HIRLAM model at the Norwegian Meteorological Institute valid at 03 UTC 23 January 2003

Simple linear baroclinic theory (small perturbations on basic flow) shows that maximum growth in polar regions may occur for much smaller horizontal scales than at middle latitudes. This is caused by a low tropopause, low static stability and a relative large Coriolis parameter. Most polar lows will, however, develop as a finite disturbance mechanism (Montgomery and Farrel, 1992), where a disturbance (e.g. a potential vorticity maximum) at upper levels interacts with a low-level baroclinic zone. Due to large contrasts in low-level characteristics (open sea, ice-covered sea, snow-covered land, “warm” and cold ocean currents, etc.) there are ample possibilities for interactions with transient upper- level polar vortex PV maxima. Release of latent heat from convection comes as an additional energy source to baroclinic instability.

It is therefore necessary to have Latest issue, as well as good parameterization of physical processes in order to simulate these phenomena properly. Unfortunately, numerical weather prediction models seem to have problems at high latitudes. The ECMWF model, for instance, has been compared to measurements taken during the Sheba campaign and there are large differences between modelled and observed quantities (Figure 5). This is of some concern, since the transition of low-level air due to strong heat and moisture fluxes in cold air outbreaks is an important factor for preconditioning the atmosphere to be favourable for polar low developments (Nordeng and Rasmussen, 1992).

model Figure 5 — Modelled and observed heat flux (a) and stability (b) November-December 1997 from the SHEBA programme: stability is defined as the difference in potential temperature between 300 m and 2 m (from Beesley et al., 2000) .

For a correct description of radiative property schemes and for the parameterization of microphysics, the fraction of cloud water content in its various phases needs to be described. Present schemes seem to have problems in describing the (surprisingly) high liquid-water content found even at low temperatures (Figure 6).

graphic Figure 6 — The relationship between cloud base temperature and condensate phase as inferred from the depolarization ratio (del). Reading vertically from a given temperature on the horizontal axis, the distance to the solid line is the fraction of clouds that are liquid, the distance between the solid line and the dashed line is the fraction of clouds whose phase is ambiguous, and the remainder are ice clouds. The dotted line represents how cloud condensate is partitioned between liquid and ice as a function of temperature in the ECMWF model (scale on right).

A correct description of cloud properties is equally important in order to assimilate satellite radiances by variational methods into numerical weather prediction models. Polar regions are data-sparse in terms of conventional observations but data- rich for polar-orbiting satellites and one will have to rely on satellite data. It is typical, however, that the areas are cloud-covered (in particular the Arctic). Another challenging task is the difficulty (for the radiation schemes) in distinguishing between cold surfaces (ice and snow) and clouds.

WMO global observing systems

At present, WMO operates or co-sponsors the following observing systems:

  • Global Observing System of the World Weather Watch (GOS/WWW)—physical parameters of the atmosphere;
  • Global Atmosphere Watch (GAW)—chemical parameters of the atmosphere, including ozone;
  • Global Ocean Observing System (GOOS)—physical, chemical and biological parameters of the ocean;
  • World Hydrological Cycle Observing System (WHYCOS), part of the Global Terrestrial Observing System (GTOS)—hydrological cycle parameters;
  • GCOS Terrestrial Network for Permafrost (GTN-P) and GCOS Terrestrial Network for Glaciers (GTN-G)—cryosphere parameters.

They will all contribute in a significant way during the IPY. The WWW Global Observing System during the IPY will, in particular: re-activate existing and establish new surface and upper-air stations; increase the number of drifting buoys; Voluntary Observing Ships and Aircraft Meteorological Data Relay flights; and use existing and new operational polar-orbiting satellite series, especially satellites with capabilities for polar regions.

The EUMETNET4 Composite Observing System (EUCOS) will be an important element during the IPY and is involved in a number of ways. The involvement will support the goals of EUCOS in terms of improving regional numerical weather prediction in the European domain and, by implication, information on how to improve observing networks so that regional numerical weather prediction can be improved. Additional funding has been allocated to support observing related activities over and above the continued operational delivery of observations from the EUCOS programme in the northern IPY region.

These include additional AMDAR data from E-AMDAR aircraft crossing the IPY region; additional radiosonde ascents from the E-ASAP fleet in the area, predominately from Danish and Icelandic ships; additional radiosondes from the land-based radiosonde networks of EUMETNET Members; data-quality monitoring services; and development of a data targeting system within the framework of the EURORISK-PREVIEW Programme sponsored by the European Commission (to deliver additional meteorological observations over key sensitive regions, to better understand issues surrounding data targeting and facilitate future services that might aid the more accurate prediction and early warning of high-impact weather events over Europe and reduce forecast uncertainties).

The IPY-THORPEX cluster

The IPY-THORPEX cluster currently comprises 10 individual IPY projects from nine countries with the following main objectives:

  • Explore the use of satellite data and optimized observations to improve high-impact weather forecasts (for Polar THORPEX Regional Campaigns (TReCs) and/or provide additional observations in real-time over the WMO Global Telecommunication System;
  • Better understand physical/dynamical processes in polar regions;
  • Achieve a better understanding of small-scale weather phenomena;
  • Utilize improved forecasts to the benefit of society, the economy and the environment;
  • Utilize the THORPEX Interactive Grand Global Ensemble (TIGGE) of weather forecasts for polar prediction.

A Research and Implementation Plan has been written to help coordinate overall activities. The projects span a number of scientific issues from climate research to weather prediction. In brief they comprise:

The Greenland flow Distortion Experiment

The focus is upon Greenland tip jets, air-sea interactions, barrier winds and mesoscale cyclones—the field campaign took place in February 2007 ( Ian Renfrew, University of East Anglia, United Kingdom).

Storm Studies of the Arctic

Includes enhanced observations in the eastern Canadian Arctic, gap flow, air-sea interactions, orographic precipitation, interaction of cyclones with topography etc. (John Hanesiak, University of Manitoba, and Ron Stewart, McGill University, Canada).

Concordiasi

Infrared Atmospheric Sounding Interferometer (IASI) assimilation in the Antarctic, assimilation of dropsondes launched from driftsondes, polar processes, the circumpolar vortex, using IASI data for climate monitoring, stable boundary layers, polar clouds and ozone, etc. (Florence Rabier, Météo-France).

Norwegian IPY-THORPEX

Optimization of new satellite data, improved modelling of the latent heat cycle, extreme weather, improved operational NWP, ensemble simulations. (Jon Egill Kristjansson, Univerity of Oslo, Norway).

Thorpex Arctic Weather and Environmental Prediction Initiative (TAWEPI)

Study of various aspects of Arctic weather and the Arctic climate system (snow processes, polar clouds, sea-ice and ozone layer); develop and validate a regional weather prediction model and the use of satellite observations over the Arctic. The research will be done in various Canadian provinces, through collaboration between government, universities and northern communities. The research will also improve science’s understanding of the Arctic and its influence on world weather (Ayrton Zadra, Environment Canada).

Greenland jets

Will consider mesocale flows, including orographic disturbances, mesocylones and surface fluxes. (Andreas Dornbrack, German Aerospace Centre).

GREENEX

Considers forecasting of small-scale weather phenomena, including extremes. Meso- and fine-scale flows in the vicinity of orography and sea ice and downstream weather development as well as scale interactions (Haraldur Olafsson, Iceland, in cooperation with the German Aeospace Centre).

Arctic Regional Climate Model Intercomparison Project

Targeted observations from the North Pole station over the Arctic Ocean; feedback between the planetary boundary layer and meso-cyclones; climate processes and feedbacks within the coupled Artic climate system (Klaus Dethloff, Alfred-Wegener Institute, Germany).

Impacts of surface fluxes on severe Arctic storms, climate change and Arctic coastal orographic processes

Includes studies of storm activity in the western Arctic in the context of surface fluxes from changing ice, ocean and land-surface conditions. Studies of coastal ocean processes and assessment of severe weather and climate factors that can impact human communities (Will Perrie, Bedford Institute of Oceanography, Canada).

THORPEX Pacific Asian Regional Campaign (T-PARC)

Includes studies of extra-tropical transition) and links between tropical/mid-latitiude and polar weather (David Parsons, National Center for Atmospheric Research, USA).

IPY-THORPEX is supported by EUCOS and ECMWF, which will provide targeted runs and assimilate observations from field campaigns.

As can be seen, these activities are mainly focused on the Arctic region. One of them, however, has its focus on the Antarctic and one of its aims is to validate and improve the assimilation of Atmospheric Infrared Sounder/IASI satellite data in numerical models with emphasis on polar latitudes. Other important issues that will be investigated in IPY-THORPEX are the role of Greenland in terms of flow distortion and its effect on local and middle-latitude weather prediction, as well as the thermohaline circulation in the ocean; comparison of Arctic regional climate models; exploration of the use of satellite data and optimized observations to improve high-impact weather forecasts and improved understanding of physical/dynamical processes in polar regions with emphasis on small-scale weather phenomena.

Summary

The WWRP-THORPEX IPY projects are expected to bring new knowledge and understanding of meteorological conditions and processes at high latitudes. This includes understanding the physics of small-scale systems (e.g. polar lows) and the role of Arctic mountain ranges such as in Greenland and the role of high latitudes in the climate system.

Dedicated in situ observations may be mainly limited to the IPY period, although it is hoped that some “legacy” observations will continue; it is clear, however, that most of the future observations to be used by numerical weather and environment prediction will come from satellites that have a unique vantage point of the polar regions atmosphere-ocean-ice systems. By the combined use of available in situ observations taken during the IPY with remotely sensed observations and better parameterization schemes, it is expected that weather and environmental forecasts will be significantly improved.

The development of new weather and environmental prediction systems trough the WWRP-THORPEX IPY projects will be a step forward in the generation of meteorological, hydrological and ice information needed to continuously monitor and forecast the polar regions’ present environmental state with unprecedented accuracy that will permit quantification of their spatial and temporal variability on a wide range of scales, from a few hours to weeks.

These improvements in turn will generate socio-economic benefits for polar communities. This will increase sustainability of Arctic communities, since the merging of traditional Inuit and meteorological knowledge of the evolving high-latitude climate and weather systems will be facilitated. Another example of an socio-economic advance is that our understanding of the climatology and physics of the low-level wind field in the Arctic will assist in the assessment of the potential use of wind power instead of burning fossil fuels in remote northern communities. This will contribute towards improving the health of northern communities as pollution will be reduced as well as to a reduction in greenhouse gas emissions.

References

Beesley, A., C.S Bretherton, C. Jakob, E.L. Andreas, J.M. Intrieri and T.A. Uttal, 2000: A comparison of cloud and boundary layer variables in the ECMWF forecast model with observations at surface heat budget of the Arctic Ocean (SHEBA) ice camp. J. Geo. Res., Vol. 105, D10, 12337-12349.

Klinker, E., and L. Ferranti, 2000: Forecasting system performance in summer 1999. Part 1—Diagnostics related to the forecast performance during spring and summer 1999. Technical Memorandum No. 321. European Centre for Medium-Range Weather Forecasts.

Montgomery M. T. and B. F. Farrell, 1992: Polar low dynamics. J. Atmos. Sci., 49, 2484–2505.

Nordeng, T.E. and E. Rasmussen, 1992: A most beautiful polar low. A case study of a polar low development in the Bear Island region. Tellus, 44A, 81-99.

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1 Norwegian Meteorological Institute, Oslo, Norway
2 Environnement Canada, Québec, Canada
3 WWRP-THORPEX, IPO, WMO
4 EUMETNET is a network grouping 22 European National Meteorological Services (as of 31 August 2007).

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