Converting Ieodo Ocean Research Station Wind Speed Observations to Reference Height Data for Real-Time Operational Use

이어도 해양과학기지 풍속 자료의 실시간 운용을 위한 기준 고도 변환 과정

Abstract

Most operational uses of wind speed data require measurements at, or estimates generated for, the reference height of 10 m above mean sea level (AMSL). On the Ieodo Ocean Research Station (IORS), wind speed is measured by instruments installed on the lighthouse tower of the roof deck at 42.3 m AMSL. This preliminary study indicates how these data can best be converted into synthetic 10 m wind speed data for operational uses via the Korea Hydrographic and Oceanographic Agency (KHOA) website. We tested three well-known conventional empirical neutral wind profile formulas (a power law (PL); a drag coefficient based logarithmic law (DCLL); and a roughness height based logarithmic law (RHLL)), and compared their results to those generated using a well-known, highly tested and validated logarithmic model (LMS) with a stability function (${\psi}_{\nu}$), to assess the potential use of each method for accurately synthesizing reference level wind speeds. From these experiments, we conclude that the reliable LMS technique and the RHLL technique are both useful for generating reference wind speed data from IORS observations, since these methods produced very similar results: comparisons between the RHLL and the LMS results showed relatively small bias values ($-0.001m\;s^{-1}$) and Root Mean Square Deviations (RMSD, $0.122m\;s^{-1}$). We also compared the synthetic wind speed data generated using each of the four neutral wind profile formulas under examination with Advanced SCATterometer (ASCAT) data. Comparisons revealed that the 'LMS without ${\psi}_{\nu}^{\prime}$ produced the best results, with only $0.191m\;s^{-1}$ of bias and $1.111m\;s^{-1}$ of RMSD. As well as comparing these four different approaches, we also explored potential refinements that could be applied within or through each approach. Firstly, we tested the effect of tidal variations in sea level height on wind speed calculations, through comparison of results generated with and without the adjustment of sea level heights for tidal effects. Tidal adjustment of the sea levels used in reference wind speed calculations resulted in remarkably small bias (<$0.0001m\;s^{-1}$) and RMSD (<$0.012m\;s^{-1}$) values when compared to calculations performed without adjustment, indicating that this tidal effect can be ignored for the purposes of IORS reference wind speed estimates. We also estimated surface roughness heights ($z_0$) based on RHLL and LMS calculations in order to explore the best parameterization of this factor, with results leading to our recommendation of a new $z_0$ parameterization derived from observed wind speed data. Lastly, we suggest the necessity of including a suitable, experimentally derived, surface drag coefficient and $z_0$ formulas within conventional wind profile formulas for situations characterized by strong wind (${\geq}33m\;s^{-1}$) conditions, since without this inclusion the wind adjustment approaches used in this study are only optimal for wind speeds ${\leq}25m\;s^{-1}$.

운용용으로 사용되는 대부분의 풍속자료는 10 m 기준 고도에서 측정 또는 생산된 자료이다. 이 연구는 이어도 해양과학기지 42.3 m 고도의 옥상 등대에서 측정 중인 풍속을 기준 고도의 풍속으로 변환시켜 국립해양조사원 누리집을 통해 실시간으로 제공하기 위한 사전 연구이다. 이를 위해 2015년에 이어도 기지에서 관측한 풍속을 대표적인 네 종류의 풍속 변환식 - 멱법칙식, 두 종류의 중립벽 로그법칙식(항력계수형, 거칠기 높이형), 대기 안정도 효과를 고려한 벽 로그법칙모델(안정도 고려 거칠기 높이형) -에 적용하였다. 관측 바람을 평가하는데 많이 사용되는 '안정도 고려 거칠기 높이형' 벽 로그법칙모델의 결과와 나머지 풍속 변환식 결과들을 서로 비교하였다. 그 결과 '거칠기 높이형' 벽 로그법칙식과 '안정도 고려 거칠기 높이형' 벽 로그법칙모델 간 편향과 평균 제곱근 편차는 각각 $-0.001m\;s^{-1}$와 $0.122m\;s^{-1}$로 가장 낮아 실시간 현업 운용 측면에서 상호 보완적으로 이 두 변환식을 함께 사용하는 것이 바람직하다는 결론을 도출하였다. 또한 이어도 해역에서 조석에 의한 풍속 관측 고도 변화가 풍속 변환에 미치는 영향을 분석하였다. 이들 변환식에 대한 조석 효과 고려 전후에 대한 비교 실험 결과, 편향과 평균 제곱근 편차는 각각 <$0.0001m\;s^{-1}$와 <$0.012m\;s^{-1}$로 그 영향은 미미하였다. 대기 표면 거칠기 높이를 사용하는 '거칠기 높이형' 벽 로그법칙식과 '안정도 고려 거칠기 높이형' 벽 로그 법칙모델을 이용하여 간편 풍속 변환식의 필수 입력값인 표면 거칠기 높이 값의 적절성에 관해 논의하였으며, 풍속 변환 정확도를 향상시킬 수 있는 표면 거칠기 높이 계산식을 제시하였다. 또한 인공위성 산란계(ASCAT) 풍속자료와 네 종류의 중립 연직 풍속 변환식들의 결과를 비교하여 이들 중 '안정도 고려 거칠기 높이형' 벽 로그법칙모델에서 안정도 항을 뺀 풍속 변환 모델의 정확도가 더 낫다는 결과를 제시하였다. 끝으로 이들 종래 $25m\;s^{-1}$ 이하 풍속에 최적화된 풍속 변환식들로부터 바람 항력계수를 산정 분석하여 강풍(${\geq}33m\;s^{-1}$) 환경에서도 적합한 풍속 변환식으로 개선 필요성에 관해 논의하였다.

Fig. 1. (a) Location (●) of the Ieodo Ocean Research Station (IORS) and (b) its structure and each deck height from the elevation level (EL).

Fig. 2. Weather monitoring equipment, measuring winds (①), humidity and air temperatures (② and ③), and barometric pressure (④), and the heights at which they were installed above the roof deck (33.5 m from mean sea level) of the Ieodo Ocean Research Station before (a) and after (b) the passage of Typhoon Muifa on August 7, 2011.

Fig. 3. Variation in kinematic viscosity (v) with variation in air temperature.

Fig. 4. Time series plots of differences in (a) adjusted 10-m wind speeds between $U_{10}^{LKBns}$ and $U_{10}^{LKBs}$ and in (b) temperatures between sea surface (T_s) and air (T) measured at the Ieodo Research Station in 2015 and (c) time series plot of values of atmospheric stability function (Ψ_u) calculated from Eq. (12).

Fig. 5. Comparisons between converted 10-m wind speed values from $U_{10}^{LKBs}$ and converted values from each different formula (i.e., $U_{10}^{PW}$ , $U_{10}^{LP83}$ , $U_{10}^{S88}$ , and $U_{10}^{S88}$(v(T)), using 1-hr interval wind records from the roof deck at 42.3 m above the mean sea level of the Ieodo Ocean Research Station in 2015.

Fig. 6. Variations in the surface roughness length (z₀) consisting of the aerodynamic roughness length due mainly to the shorter surface waves (z_c) and the aerodynamic roughness length for a smooth surface (z_s), which are calculated from Eq. (8) with α = 0.001, v = 1.5 × 10^-5 m²s^-1 and g = 9.8 m²s^-1. .

Fig. 7. Surface roughness lengths (z₀) estimated from (a) $U_{10}^{S88}$ (Eq. (7)) and (b) $U_{10}^{LKBs}$ (Eq. (12)) and their fits (thick black lines) of wind speed measurements (U_z) to the z₀. Blue (A) and red (B) dots in panel (b) denote values of z₀ under conditions of Ψ_u and Ψ_u > 0, respectively.

Fig. 8. Wind speed measurements (U_z) vs difference between U_z and 10-m wind speeds (U₁₀) adjusted from $U_{10}^{LKBs}$ (Eq. (12)) for the cases of Ψ_u > 0 (red dots) and Ψ_u < 0 (blue dots).

Fig. 9. Comparisons between $U_{10}^{ASCAT}$ and converted 10-m wind speed values from each different formula ( $U_{10}^{PW}$ , $U_{10}^{LP83}$ , $U_{10}^{S88}$ and $U_{10}^{LKBns}$ ), using 1-hr interval wind records measured from the roof deck at 42.3 m above the mean sea level of the Ieodo Ocean Research Station in 2015.

Fig. 10. (a) Drag coefficients calculated from $C_{d10}^{V88}$ (Eq. 4), $C_{d10}^{Z12}$ (Eq. 22) and $C_{d10}^{S88}$ (Eq. 10) under the wind speeds (U_z) of ≤ 60ms-1 and (b) difference of 10-m wind speeds adjusted from $U_{10}^{LP83NW}$ (using $C_{d10}^{Z12}$) and $U_{10}^{LP83}$ (using $C_{d10}^{V83}$) using wind speeds (U_z) of ≤ 60ms^-1 with an interval of 0.1ms^-1.

Table 1. Meteorological sensors (wind, humidity, temperature and barometric pressure) installed above the roof deck of the Ieodo Ocean Research Station

Table 2. Wind profile formulas used for adjusting sea winds (U_z) measured above the roof deck (z = 42.3 m) of the Ieodo Ocean Research Station to a 10-m reference height (z₁₀ = 10 m)

Table 3. Sensitivity analysis of the LKB with respect to the input parameters using meteorological and oceanographic records measured at the Ieodo Ocean Research Station in 2015

Table 4. Comparisons of adjusted 10-m wind speeds between $U_{10}^{LKBs}$ and $U_{10}^{PW}$ ($U_{10}^{LP83}$ , $U_{10}^{S88}$ , $U_{10}^{S88}$ (v(T))) according to observed wind speed ranges

Table 5. The five main tidal harmonic constants estimated from harmonic analysis of 2015 yearlong sea level observation records at the Ieodo Ocean Research Station

Table 6. Sensitivity experiments of four vertical wind conversion equations ($U_{10}^{PW}$ , $U_{10}^{LP83}$, $U_{10}^{S88}$ and $U_{10}^{LKBs}$ ) on the effect of variation in sea-level heights due to tides, using meteorological and oceanographic records measured at the Ieodo Ocean Research Station in 2015

Table 7. Four sensitivity experiments on use of different surface roughness lengths (z₀) in Eq. (7), Eq. (12) and Eq. (18) using meteorological and oceanographic records measured at the Ieodo Ocean Research Station in 2015

Table 8. Maximum wind speeds observed at the Ieodo Ocean Research Station from 2004 and 2017 and their count number of occurrence