THORPEX Pacific Asian Regional Campaign (T-PARC)

The THORPEX Pacific Asian Regional Campaign (T-PARC) was a multi-national field campaign that addressed the shorter-range dynamics and forecast skill of high-impact weather events in one region (Eastern Asian and the western North Pacific) and the downstream impact on the

medium-range dynamics and forecast skill of another region (in particular, the eastern North Pacific and North America). Although many significant weather events occur over eastern Asia and the western North Pacific, the focus of T-PARC was on various aspects of typhoon activity, which included formation, intensification, structure change, motion, and extratropical transition. Because of the significant impact of typhoon activity on the region of eastern Asia and the western North Pacific, T-PARC was comprised of several affiliated programmes. The experimental design (Figure 5) for T-PARC addressed three primary components: (1) A tropical measurement strategy to examine circulations of the tropical western North Pacific monsoon environment as they related to tropical cyclone formation, tropical cyclone intensification, and tropical cyclone structure change.

(2) Extratropical transition (ET) and downstream impacts was based on the poleward movement of a decaying tropical cyclone and the resulting intense cyclogenesis that results from its interaction with the midlatitude circulation. (3) Identification of regions in which extra observations may reduce numerical forecast error growth associated with forecasts of tropical cyclone track over the western North Pacific. Results addressed multi-scale factors in tropical cyclone formation, impacts of tropical cyclones on midlatitude flow characteristics, and the role of in situ observations in improving tropical cyclone track forecasts.

Figure 5. Aircraft missions conducted in four typhoons during the TPARC field programme DLR  Falcon

DOTSTAR

NRL  P-­‐3

USAF  WC-­‐130J

Key issues related to prediction of tropical cyclone structure were defined by the first ever multiple plane observing missions into one typhoon over the western North Pacific. While Typhoon Sinlaku was at peak intensity of 130 kt, aircraft missions concentrated on examination of the outer rainband structures in terms of wind distribution and formation of secondary eyewall formation (Wu et al.

2012). A following two aircraft mission was conducted to observe a region of explosive

re-development of deep convection (Figure 6). Sanabia (2010) related the deep towers of vertical vorticity to stretching due to updrafts that had peak intensity of near 30 m s^-1 between 10 -12 km, with convergence into the column below 10 km and divergence aloft. Therefore, multi-aircraft missions addressed significant unknown factors related to tropical cyclone formation, structure, and wind distribution.

Figure 6. Vertical cross section of radar reflectivity (shaded according to color bar, dBz), wind horizontal and vertical winds in the plane of the cross section (vectors, reference vector in

the lower left, m s^-1) and vertical vorticity (contours 10^-3 s^-1).

Source: After Sanabia 2010

A similar measurement strategy was followed to make observations of a typhoon during the transformation stage of extratropical transition. A three-plane mission was again conducted to measure the changes in tropical cyclone structure as the storm interacted with the midlatitude westerlies. Foerster et al. (2014) examined the airborne Doppler radar data (Figure 7) collected as the NRL P-3 flew through the centre of TY Sinlaku during the extratropical transition. The end of the extratropical transition process was observed during a two-plane mission with dual Doppler radar data (Quinting et al. 2014) that documented a strong convective outbreak and the interaction of the remnants of Sinlaku with the midlatitude jet.

Decreased predictability, which is defined as an increase in standard deviation in 500 hPa height among individual and a collection of ensemble prediction systems, is often observed during the interaction between a decaying tropical cyclone and a midlatitude jet. Keller et al (2011) used the THORPEX Interactive Grand Global Ensemble (TIGGE) database to show that increased variability was consistently observed over the central North Pacific between 160^oE and 160^oW with

subsequent relative maxima occurring downstream at intervals of approximately 40 deg. of longitude (Figure 8).

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Figure 7. Reflectivity (shaded, dBz) of the core of TY Sinlaku derived from airborne Doppler radar on board the NRL P-3 near 0000 UTC 21 September 2008.

Source: After Foerster et al. 2014

Figure 8. The 500 hPa height standard deviation (m) among ensemble forecasts initiated at 1200 UTC 15 September 2008 from the (a) ECMWF; (b) NCEP; (c) JMA operational ensemble prediction systems; and (d) all systems contained in the TIGGE database. The standard deviation is averaged

between 40^oN-60^oN. The black dots define the forecast positions of TY Sinlaku in all ensemble members. The red dot defines the location of TY Sinlaku at the time of ET.

Source: After Keller et al. 2011

The T-PARC observations (Figure 5) were assimilated in a number of global and regional models to draw conclusions on the benefit of targeted dropsonde observations for typhoon and mid-latitude forecasts (Weissmann et al. 2011, Harnisch and Weissmann 2010, Chou et al.

2011, Wu et al. 2012, Wu et al. 2013), to evaluate different targeting strategies and the potential of lidar instruments for the initialization of weather prediction models. Major findings from these studies include (Weissmann et al. 2011): (a) Targeted dropsondes overall improve typhoon track predictions, but their impact significantly depends on the assimilation system; (b) targeted dropsondes only have a small impact on mid-latitude forecasts and the impact is mainly due to improved typhoon tracks that indirectly lead to mid-latitude improvements; (c) dropsondes in the vicinity of typhoons have the largest impact, whereas the impact of

dropsondes in distant sensitive regions and the core and eyewall region is small; (d) wind lidar observations have a comparably high impact, which underlines high expectations for planned space-based lidar and suggests considering the deployment of wind lidars on commercial airplanes in the future; (e) the average impact of water vapour lidar observations is small, but forecasts can be affected considerably under certain conditions; (f) lidar cloud top observations can be used to adjust the height assignment of satellite-derived atmospheric motion vectors and by this significantly reduce their wind errors.

4.2.3.6 Winter THORPEX - Pacific Asian Regional Campaign (T-PARC)