1. Ship observation and numerical simulation of the marine atmospheric boundary layer over the spring oceanic front in the northwestern South China Sea Rui Shi1, Ju Chen1, Xinyu Guo2, LiLi Zeng1, Jian Li1, Qiang Xie1,3, Dongxiao Wang1
State Key Laboratory of Tropical Oceanography, South China Sea Institute of Oceanology, Chinese Academy of Sciences, 164 West Xingang Road, Guangzhou , China
Center for Marine Environmental Studies, Ehime University, 2-5 Bunkyo-cho, Matsuyama, , Japan
Institute of Deep-sea Science and Engineering, Chinese Academy of Sciences, No.62, Fenghuang Road, Sanya, , China.
Abstract
The response of the marine atmospheric boundary layer (MABL) structure to the oceanic front was analyzed using GPS sounding data obtained during a survey in the northwestern South China Sea (NSCS) over a period of about one week in April Fifteen GPS soundings were deployed across the front with a sea surface temperature (SST) gradient of ~0.5×10−2–1.0×10−2 °C km−1. The Weather Research and Forecasting (WRF) model was used to further examine the influence of large-scale weather and the local air–sea coupling. The WRF model successfully simulated the change in the MABL structure across the front, which agreed well with the observation results. The simulations show that the synoptic pattern dominates the distribution of wind speed across the front. Meanwhile, the SST front works as a damping zone to reduce the enhancement of wind in the lower boundary layer flowing in a direction from the warm side to the cold side of the front. Momentum budget analysis shows that the most active and significant term affecting horizontal momentum over the frontal zone is the adjustment of the pressure gradient. It is found that the wide front in the NSCS provides enough distance for the slowly moving air parcel to be affected by the change in the underlying SST. The different thermal structure upwind and downwind of the front causes a baroclinic adjustment of the perturbation pressure from the surface to the mid-layer of the MABL, which dominates the change in the wind profile across the front.
Model results
(a, b) Virtual potential temperature (K), (c, d) mixing ratio (g kg−1) and (e, f) wind speed (m s−1) profiles observed by GPS soundings (black solid lines) and simulated by the WRF model (dashed lines) at sites (a, c, e) G2 and (b, d, f) G6.
Shi R, Chen J, Guo X, et al. Ship observations and numerical simulation of the marine atmospheric boundary layer over the spring oceanic front in the northwestern South China Sea. J. Geophys. Res. Atmospheres, 2017.
The definition of different regions used for the calculation of the momentum budget
Momentum Budget
Cruise Observation
Averaged sea surface temperature (SST, °C) during the observation period from (a) 16 to 17 April and from (b) 18 to 20 April 2013 in the northwestern SCS. (c) Averaged SST gradient for 16 and 17 April. The black triangles show GPS soundings launched during the cruise survey.
Horizontal- and time-averaged terms from the momentum budget from Eq. (3) from Zone 1 to Zone 2 (a, b) and Zone 2 to Zone 3 (c, d). Figures on the right side include the upstream momentum tendency (solid lines) and pressure gradient term along with the turbulent stress divergence and Coriolis force (dashed lines). The positive tendencies denote the increases in the total momentum of the air parcel.
Sketch map for a summary of this study
Key points
In situ observation of the change of atmospheric boundary layer structure over oceanic front in SCS 
Momentum budget from Lagrangian view to identify the primary term affecting horizontal momentum over the frontal zone
Maps of spatially high-pass-filtered 5-day (16–20 April 2016) average SST (°C, black contours) and 10-m equivalent neutral wind speed (m s–1, color shading) from (a) ASCAT data and (b) the WRF simulation. The blue line is the ship cruise transect. The ASCAT data used here are obtained from the Asia–Pacific Data Research Center.