The Impact of the Winter Monsoon on Marine Surface-Layer Turbulence

Abstract

During winter the East-Asia monsoon is the dominant climatological phenomenon in the north-west Pacific, especially around the Japan Islands. Previous studies based on satellite and radiosonde observations revealed that cold and dry strong flow from Siberia over the warm Kuroshio Extension enhances instability of the marine boundary layer and reduces vertical wind shear. Since active convection manifests as intensification of near-surface turbulence, in situ meteorological buoy data were analyzed, confirming that turbulence intensity or equivalently the gust factor increases during the winter north-westerly monsoon. Concurrently, a hindcast atmospheric simulation was conducted to quantify the thermal and vapour fluxes in the marine boundary layer. When large gust factors or turbulence intensity are observed, the buoyancy production plays an important role in the turbulent kinetic energy budget. To further clarify the respective roles of the sensible and latent heat fluxes at the sea surface, atmospheric model experiments showed that the sensible heat flux predominantly contributes to the increase in turbulence intensity. On the other hand, the latent heat flux mainly affects the generation of clouds at the top of the marine boundary layer.

Appendix: Relationship Between Gust Factor and Turbulence Intensity

Assuming that the wind fluctuation is Gaussian, the gust factor is equivalent to the turbulence intensity as shown in Eq. 4. This relationship has two important implications: the first is that the turbulence intensity calculated in the atmospheric model can be treated in the same way as the gust factor. Secondly the gust factor can be used as an indicator of the intensity of turbulence when only mean and maximum wind speeds in an averaging period are measured. Here, we validate this relationship by available in-situ data.

Fig. 12

Scatter diagram of turbulence intensity and gust factor at NOWPHAS Central Iwate. Blue dots observations, red dots and error bar the mean and standard deviation of the observations, and black line Eq. 9

Ports and Harbours Bureau, Ministry of Land, Infrastructure, Transport, and Tourism has deployed the NOWPHAS (Nationwide Ocean Wave information network for Ports and HArbourS) GPS wave buoy about 15 km off the Pacific coast of Japan island. The wave buoy at the NOWPHAS Central Iwate site $(39.627^{\circ }\hbox {N}, 142.187^{\circ }\hbox {E})$ is equipped with an anemometer that provides wind fluctuations continuously at a 1-Hz sampling rate. Therefore, the time series from the NOWPHAS Central Iwate site can be used to derive the correlation between the gust factor and turbulence intensity.

From the NOWPHAS observation in 2012, the gust factor and turbulence intensity were estimated for wind speeds $>10 \hbox { m s}^{-1}$. The relationship of the observed gust factor with respect to observed turbulence intensity is compared against the Gaussian model (Fig. 12). Here, the Gaussian model (Eq. 4) was reduced to,

$$\begin{aligned} G=1+2.935 \, TI, \end{aligned}$$

(9)

for the gust duration time (t) of 1 s and averaging period (T) of 10 min. The Gaussian model (black line) shows quite good agreement with the NOWPHAS observation (red dots and error bar). An analysis conducted using ship-borne wind measurements (not shown here) showed a similar relationship as well. Therefore, the complementary relationship between gust factor and turbulence intensity (Eq. 4) is validated. Consequently, the analysis based on the in situ data (Fig. 9) and that based on atmospheric model (Fig. 10) are equivalent and indeed the results are consistent.