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JAMSTEC
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National Institute of Polar Research
Forecasting Extreme Summertime Arctic Cyclone with Observations over Arctic Ocean
-Toward More Accurate Prediction for Safer Arctic Sea Cruise-
Overview
Dr. Akira Yamazaki, Application Laboratory at the Japan Agency for Marine-Earth Science and Technology (JAMSTEC: Asahiko Taira, President) and Dr. Jun Inoue, Associate Professor at the Inter-university Research Institute Corporation Research Organization of Information and Systems, National Institute of Polar Research conducted forecasting experiments for the Great Arctic cyclone of 2012 (AC12) occurred in early August of 2012, jointly with the Alfred Wegener Institute in Germany. The experiments used atmospheric general circulation models (AGCM)*1 and a data assimilation system*2 developed on the JAMSTEC Earth Simulator supercomputer. As a result, it was found that radiosonde observation*3 data obtained during the period of AC12 activity over the Arctic Ocean by the German icebreaker research vessel Polarstern gives significant impacts on improving the forecast accuracy.
By using the data assimilation system, the research team assimilated the radiosonde observation data with an atmospheric general circulation model (AGCM) during mid July to early August 2012, and then investigated whether it could reproduce and forecast the event of AC12. To estimate the impact of the radiosonde observation data on the forecast, two forecasting experiments were carried out by using two initial fields; one by including radiosonde data, and the other excluding it.
As a result, the formation and development of AC12 was successfully predicted in the experiment using the initial field with the Polarstern radiosonde observations, while it was not well without the observation data. This result implies that upper-air observation by radiosonde over the Arctic Ocean would be a big key to improving accuracy of weather forecasts in the Arctic regions.
Prediction of the Arctic low-pressure systems is likely to considerably affect vessel operations in the Arctic Ocean. In terms of both observations and AGCM simulations, this study showed how it is effective to observe the polar region for prediction of the low pressure systems. It is necessary to continue the research because not only the low pressure disturbance during summer but also other disturbances such as strong wind continuing for several days along the edge of high pressure systems over the Arctic Ocean could be a threat for cruising vessels in the region.
In addition, it has been pointed out that an unusual cold spell in mid-latitude regions seems to be related to sea ice extent and volume in the Arctic Ocean. It is, therefore, expected to acquire additional observational data of different seasons in the Arctic region and evaluate impacts of the data on various weather and climate systems under continued international cooperation.
These study results were posted on an academic journal, Journal of Geophysical Research: Atmospheres published by the American Geophysical Union, on April 27th (JST). This project was supported by funds from Grant-in-Aid for Scientific Research (24241009).
Title: Impact of radiosonde observations on forecasting summertime Arctic cyclone formation
Authors: Akira Yamazaki1, Jun Inoue2,3, Klaus Dethloff4, Marion Maturilli4, Gert König-Langlo4
Affiliation: 1. Application Laboratory, JAMSTEC 2. Arctic Environment Research Center, National Institute of Polar Research 3. Institute of Arctic Climate and Environment Research (IACE), JAMSTEC 4. Helmholtz Centre for Polar and Marine Research, Alfred Wegener Institute
*1 Atmospheric general circulation model (AGCM)
It is a program to simulate atmospheric flows in the Earth, including temporal and spatial changes of pressure, temperature and winds based on hydrodynamic and thermodynamic equations. AGCMs can reproduce various atmospheric phenomena with a range of several days to years, so they are used to investigate mechanisms and predictabilities of them.
*2 Data assimilation
Data assimilation is a method to synthesize observation data with numerical simulations, and thus enables to create initial values for forecasting weather or climate by an AGCM, etc. More precise initial values generated by data assimilation systems lead to plausible predictions with higher accuracy.
Having an ensemble data assimilation system and an AGCM, JAMSTEC has carried out various studies to evaluate impacts of specific observations. Moreover, JAMSTEC created a global atmospheric ensemble reanalysis dataset called ALERA2, which is based on the system comprised of the AGCM AFES (AGCM for Earth Simulator) and the assimilation code LEKTF (local ensemble transform Kalman filter) developed on the Earth Simulator.
http://www.jamstec.go.jp/esc/research/oreda/products/alera2.html
*3 Radiosonde observation (figure 5)
Measurement of vertical distribution of weather elements such as air temperature and wind by using balloon-borne radiosonde instruments. It can measure atmospheric conditions from the surface to the stratosphere beyond the tropopause extending up to about 10-15 km above the Earth's surface. The radiosonde observations are usually carried out twice a day (or once depending on location) in almost all countries. Observational data are reported real-time through GTS (Global Telecommunication System) to be used by operational weather forecast centers in each country.
Figure 1: Locations of radiosonde observations performed by the Polarstern (red dots) and sea level pressure (hectopascal, contour) on August 6, 2012 at the mature stage of the Great Arctic Cyclone (AC12). A large distance between them is clear here. The colored areas indicate sea ice distribution. AC12 was formed in mid-Eurasia and then developed moving northeastward.
Figure 2:
(Left) Pressure difference (colored) at the upper troposphere (about 10km altitude) during the AC12 formation period in the initial fields (experiments) in which Polarstern radiosonde observations were assimilated and in which excluded. Cold (warm) colors indicate the stronger (weaker) tropopause “polar vortex” in the initial fields with the assimilation. It is found that the experiment with the assimilation reproduced the stronger tropopause polar vortex over mid-Eurasia where AC12 was formed. The radiosonde obsevarion data affected even the eastern side from the observation points due to strong westerlies in the upper troposphere, and thus helped improve reproducibility of the polar vortex about 1,000km away from the observation points.
(Right) Conceptual diagram showing the AC12 formation. A tropopause polar vortex corresponding to a low pressure system at the upper troposphere circulates counterclockwise. This circulation influences near the earth’s surface through the mid troposphere, producing southerlies near the surface at the east side of the polar vortex. The warm air transported by the southerlies forms the warm sector of AC12, which results in A12 formation.
Figure 3: Forecast results of AC 12 formation and development in the experiments with (red) and without (blue) the Polarstern radiosonde observations. The values indicate time sequences of AC12 central pressure (hectopascals). These forecasts are based on ensemble-forecast experiments (multiple numerical predictions by using multiple initial values slightly different from each other, having their mean and spread values), which enables forecasting with probability information. The thin lines indicate the results of all ensemble predictions (probability values) while the thick lines mean of all ensembles (deterministic forecast values). The black line is analyzed values obtained by the data assimilation system, corresponding to the real-time change of the AC12 central pressure. With the Polarstern observations (shown in red lines), the forecast predicts decreasing of central pressure (AC12 formation and development) starting from around August 4. It indicated improvement in accuracy of predicting AC12 formation and development, comparing to the experiment without radiosonde observations.
Figure 4: Special radiosonde observation networks in the Arctic region at the initiative of Japan
(Left) Special observations between September 11 to 24, 2013
(Right) Special observations between September 6 to 25, 2013.
On land, radiosonde observations were performed in frequency increased by two to six times. Fixed-point observations were carried out on the research vessel, Mirai for several weeks.
Figure 5: Radiosonde observations from Mirai
Contacts:
- (For this research)
JAMSTEC - Akira Yamazaki, Scientist, Climate Variability Prediction and Application Research Group (CVPARG), Application Laboratory (APL)
- Inter-university Research Institute Corporation Research Organization of Information and Systems
- Jun Inoue, Arctic Environment Research Center, National Institute of Polar Research (also concurrently serving as Visiting Senior Scientist concurrently, Institute of Arctic Climate and Environment Research at JAMSTEC)
- (For press release)
JAMSTEC - Hiroyasu Matsui, Manager, Press Division, Public Relations Department
- Inter-university Research Institute Corporation, Research Organization of Information and Systems, National Institute of Polar Research
- Hiromi Obama, Public Relations Section