Marine atmospheric boundary layer and low-level cloud responses to the Kuroshio Extension front in the early summer of 2012: three-vessel simultaneous observations and numerical simulations


Intensive atmospheric observations were carried out with five research vessels in total across the sea surface temperature (SST) front along the Kuroshio Extension in the early summer of 2012, to identify the effects of the front on the thermal structure and cloud formation in the marine atmospheric boundary layer (MABL). Three of the vessels were aligned together along the 143°E meridian with latitudinal separation of as small as 30′ or 45′, going back and forth across the SST front for in situ observations during 2–6 July. The SST front was quite sharp and moved northward by about 50 km in 3 days, which was not well represented in objectively analyzed SST data sets. The observations captured rapid changes of the mesoscale MABL structure across the SST front, which were particularly evident in cloud base height and downward longwave radiation (DLR) at the surface. The higher base of low-level clouds observed over the warmer water resulted from stronger turbulent mixing in the MABL, which became prominent under the northerlies. The most frequently measured DLR value was greater by 20 W m−2 to the south of the SST front than to the north. High-resolution atmospheric model experiments conducted with and without the frontal SST gradient have confirmed its critical importance for the MABL structure and low-level clouds. These imprints of the SST front simulated in the models are sensitive to SST data assigned at the lower boundary of the model.

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    The Japanese standard time (JST) is 9 h ahead of UTC.


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This work was supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, Grants-in-Aid for Scientific Research on Innovative Areas (22106007, 22106003, 22106004, 22106005, 22106006 and 22106009). The authors would like to sincerely thank the captains, crews, and cruise leaders of R/V Seisui-maru, R/V Wakataka-maru, and R/V Tansei-maru. Weather charts, MGDSST and MSM data were provided by the Japan Meteorological Agency. Dr. Miyazawa of JAMSTEC and his colleagues kindly provided their JCOPE2 product. The authors thank Dr. Y. Wang of IPRC, University of Hawaii, for providing the IPRC-RAM. The authors are also extremely grateful to Prof. T. Murayama of Tokyo University of Marine Science and Technology, Dr. H. Tomita of Nagoya University, and all the colleagues who got involved in the intensive observation campaign. The editor and anonymous reviewers provided us with valuable comments and helped us to improve the paper.

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Correspondence to Yoshimi Kawai.

Appendix: Simulations by the AR-WRF

Appendix: Simulations by the AR-WRF

The simulation results of the AR-WRF are qualitatively similar to those of the IPRC-RAM, but the AR-WRF underestimates cloud height to the south of the SST front for the event on 3 July (Fig. 17). And the amount of cloud liquid water in the AR-WRF is much larger than in the IPRC-RAM. These tendencies are also seen on the other days. Although our conclusion that the SST front significantly affected cloud height is consistent, there have still been problems in evaluating the effect quantitatively.

Fig. 17

Same as in Fig. 14, but for data simulated by the AR-WRF

We point out that the simulation results can also be affected by the initial and lateral boundary condition entered into the regional models. Figure 18 shows time-mean cloud liquid water simulated in the AR-WRF with lateral boundary and initial conditions taken from three global atmospheric reanalysis data sets: the NCEP-FNL, the European Center for Medium-Range Weather Forecast (ECMWF) Re-Analysis Interim (ERA-Interim) (Dee et al. 2011), and the JMA Climate Data Assimilation System (JCDAS) analysis (Onogi et al. 2007 ). The elevated cloud deck to the south of the SST front relative to its north is reproduced as an impact of the lower boundary condition regardless of the lateral boundary and initial conditions, but the amount of cloud liquid water itself does depend on those conditions. The use of the JCDAS yields the greatest amount of cloud liquid water below 400 m height and least amount above 500 m height to the south of the front. These differences in model-simulated cloud liquid water can be caused by differences in humidity, temperature, and trajectories of air mass that comes into the model domain through the lateral boundaries. We need to pay attention to the effect of the lateral boundary in the investigation of air–sea interaction around the SST front area.

Fig. 18

Cloud liquid water averaged throughout the experiment period by the AR-WRF (g kg−1) with the use of a NCEP-FNL, b ERA-Interim, and c JCDAS as the lateral boundary and initial conditions

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Kawai, Y., Miyama, T., Iizuka, S. et al. Marine atmospheric boundary layer and low-level cloud responses to the Kuroshio Extension front in the early summer of 2012: three-vessel simultaneous observations and numerical simulations. J Oceanogr 71, 511–526 (2015).

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  • Kuroshio extension
  • Intensive observation
  • Early summer
  • SST front
  • Mid-latitude air–sea interaction
  • Longwave radiation
  • Low-level cloud
  • Water vapor
  • Ceilometer
  • Model experiment