Lidar Observations of the Typhoon Boundary Layer Within the Outer Rainbands
5.4 Averaged Wind-Speed Profiles
Investigation of the averaged wind-speed profiles in both stages II and III enable quantifica-tion of the general characteristics of the typhoon boundary layer. To examine the similarity between the dimensionless wind-speed profiles of Typhoons Dujuan and Soudelor, a range of wind directions was selected to ensure approximately equivalent mean wind directions.
Figure9compares the wind-speed profiles between the observations and theoretical models, clearly showing the increase of the wind speed with respect to the logarithmic profile in the mixed layer. The observations agree with the predictions of the Gryning, Blackadar, and Deaves and Harris profiles, indicating that features of the typhoon boundary layer formed
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(a) (b)
Fig. 9 Dimensionless form of the averaged wind-speed profiles in the typhoon boundary layer in comparison with the theoretical profiles in a stage II for the Typhoons Dujuan and Soudelor, and b stage III for Typhoon Dujuan
Table 1 Parameters of the wind-speed profile for neutral typhoon boundary layers
Stage of typhoon Dujuan (stage II) Dujuan (stage III) Soudelor (stage II) Averaging period 0400–2200 28
September
2300 28
September–0200 30 September
1700 7 August–0500 8 August
Direction of rotation (°) 346°–275° 260°–165° 352°–283°
UR(m s−1) 19.4 14 22.1
θR(°) 323 178 303.5
ZR(m) 119 125 115
θ (°) 4.12 2.94 3.18
u∗o(m s−1) 1.04 0.96 1.21
zo(m) 0.08 0.39 0.09
zh(m) 3045 2891 3542
Power exponent,α 0.18 0.23 0.17
Here,θ represents the direction difference within the boundary layer
within the outer rainbands can be explained using the finite mixing-length and Deaves and Harris models, and implying that the mesoscale variation caused by the typhoon–orography interaction discussed in Sect.3appears not to affect the boundary layer at relatively lower altitudes. Moreover, consistency of the wind-speed profiles between Typhoons Dujuan and Soudelor in stage II implies the presence of similarities for dimensionless heights z/zo< 3×
103when the typhoons have analogous features, such as track and storm intensity.
The roughness lengths in stage II were 0.08 m and 0.09 m for Typhoons Dujuan and Soudelor, respectively, corresponding to onshore flow over flat terrain. In contrast, the off-shore flow in stage III over both the flat terrain and suburban low-rise buildings, resulted in a larger roughness length of 0.39 m. These terrain types are in agreement with the pro-posed roughness category given by Choi (2009). Table1lists the parameters describing the averaged wind-speed profiles in the typhoon boundary layer.
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(a) (b)
Fig. 10 Dimensionless averaged wind-speed profiles in the typhoon boundary layer fitted using the power law in a stage II for Typhoons Dujuan and Soudelor, and b stage III for Typhoon Dujuan
Figure10demonstrates the power law fitted using the reference height ZRand wind speed UR, and describes the averaged wind-speed profiles remarkably well for z < 240 m, which is an altitude close to the observations of Tse et al. (2013) who fitted a power law to wind-speed profiles in the typhoon boundary layer up to 300 m. The power-law exponents areα 0.18 and 0.17 for Typhoons Dujuan and Soudelor in stage II, respectively, and 0.23 for Typhoon Dujuan in stage III. The larger value ofα in stage III is attributed to the increased terrain roughness because, in neutral stratification, the exponentα is dependent only on the surface roughness, and increases with the roughness length (Panofsky and Dutton1984; Song et al.
2016). Under typhoon conditions, Choi (1978) showed that the value ofα varies from 0.19 and 0.28 at a coastal site, which is consistent with the results here.
In contrast to the aforementioned values of the power exponents obtained in the coastal zone, Hurricane Kate in the Gulf of Mexico with wind speeds of 26.2 m s–1gaveα 0.12, which is associated with significantly lower roughness lengths between 10−4and 10−2m (Hsu2003). Using the formulations and, hence, the algorithm described in Hsu (2003) and Tsai et al. (2018), the values ofα calculated from the buoys C and S are 0.094 and 0.091 for Typhoon Dujuan, and 0.093 and 0.094 for Typhoon Soudelor in the period of stages II and III.
That both the results of the two buoys and those reported in Hsu (2003) indicate considerably smaller power exponents, suggesting that the formation of the typhoon wind-speed profiles in the coastal region is a ground-based boundary-layer process affected by the larger surface roughness on land. The development of the boundary layer over the sea was modelled by Peña and Gryning (2008) using the Charnock constant to quantify the variation in roughness length due to wind stress. Therefore, further observations are required to determine whether the theoretical model is also able to predict the typhoon boundary layer over undulating surfaces.
6 Conclusions
The typhoon boundary layer has been studied using a lidar wind profiler in different stages of the passage of two typhoons over Taiwan. For the relatively high wind speeds within the outer rainbands, the hourly mean wind-speed profiles in neutral stratification demonstrate
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that the surface scaling parameter u∗o/fcdetermines the depth of the surface layer and the structure of the mixed layer. With the dominance of frictional shear, corresponding to large values of u∗o/fc, the entire observation height may be described by a logarithmic profile up to a height of 240 m. In contrast, relatively small u∗o/fcvalues denote the importance of the Coriolis parameter, for which the finite mixing-length model produces wind speeds larger than those predicted by the logarithmic law in the mixed layer over the altitude of 80–160 m, representing the observed thickness of the surface layer here. In addition, during the typhoon passage, different types of boundary layers were observed, including uniformly-mixed layers in the early stage of the typhoon approach, and the jet-like profiles as the typhoon moves farther away.
Although the power law describes the averaged typhoon wind-speed profiles remarkably well, the power exponents obtained at the present coastal site are significantly larger than those over the sea, implying wind-speed profiles in the boundary layer are characterized by ground-based processes. The finite mixing-length theory used by the Gryning and Blackadar models and the empirical Deaves and Harris model explain the formation of the typhoon boundary layer over land, and reveal that the typhoon boundary layer formed in the outer rainbands is in agreement with the non-typhoon boundary layer described in Gryning et al.
(2007) and Peña et al. (2010). Finally, in filtering for analogous characteristics between Typhoons Dujuan and Soudelor, similar averaged non-dimensional wind-speed profiles are found for dimensionless heights z/zo< 3×103.
Acknowledgements The authors wish to express their acknowledgement to the Ministry of Science and Technology for project funding (MOST 105-3113-E-006-016-CC2 and MOST 107-3113-F006-002). We are also indebted to the Central Weather Bureau and Water Resources Agency in Taiwan for use of the buoy and meteorological data.
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Journal of Aeronautics, Astronautics and Aviation, Vol. 51, No. 2, pp. 141–158 (2019) 141 DOI: 10.6125/JoAAA.201906_51(2).01