モニン・オブコフの相似理論を用いた裸地畑の粗度長の決定 Determination of the aerodynamic roughness length of a bare soil field using Monin-Obukhov similarity theory

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Abstract

黄河上流域における秋灌漑の水収支を知る上で重要な冬季の畑地からの蒸発量を,日常観測要素である風速(1高度)と気温・湿度(2高度)を用い,単純な傾度法により測定することを目的として,作付け前の対象畑の粗度長(<i>z</i><sub>0</sub>)を決定するために集中観測を実施した。<br>2008年の4月末の2日間に,超音波風速・気温計により顕熱(<i>H</i>)と運動量(<i>τ</i>)の乱流フラックスを測定した。このとき,畑地は凍結してはおらず,また植生もなかった。これらの測定値をObukhov長の表式と,Monin・Obukhov理論に基づいた顕熱と運動量に関する輸送方程式に代入し,3種類の<i>z</i><sub>0</sub>の推定値を得た。<i>H</i>方程式を用いて得られた<i>z</i><sub>0</sub>の推定値は,中立・非中立成層に対する<i>τ</i>方程式に用いて得られた値に比べて小さく,かつ,風向に大きく依存した。<br><i>H</i>方程式を用いて得られた<i>z</i><sub>0</sub>とともに,Monin・Obukhov理論と繰返し法に基づいて推定された日蒸発量は,ボーエン比法による測定値の1.1~1.3倍になった。本結論に基づき,本畑地からの冬季蒸発量について解析した結果を次報に示す。

In order to measure the evaporation from a field in the upper Yellow River valley in winter, which is essential for a proper understanding of the water budget of the “autumn irrigation”, by a simple profile method using routine measurements of wind speed and direction made at one height and those of air temperature and humidity at two heights, an intensive observation was conducted to determine the aerodynamic roughness length (<i>z</i><sub>0</sub>) of the field bare of vegetation.<br>Measurements of turbulent fluxes of sensible heat (<i>H</i>) and momentum (<i>τ</i>) were made in the field with a sonic anemometer-thermometer for two days in late April of 2008, when the field was not frozen but without vegetation. These measurements were substituted into the expression of the Obukhov length, and equations for <i>H</i> and <i>τ</i> that were derived based on Monin-Obukhov similarity theory, and three kinds of estimates of <i>z</i><sub>0</sub> were obtained. The estimates of <i>z</i><sub>0</sub> made using the H equation were smaller than those made using the<i>τ</i>equations for neutral and non-neutral stratification and had a great dependence on the wind direction.<br>Daily evaporation calculated from the equation for the turbulent flux of water vapor, which was also derived based on Monin-Obukhov similarity theory, using the <i>z</i><sub>0</sub> estimated using the <i>H</i> equation and an iteration technique were 1.1∼1.3 times as large as the measurements made by the Bowen ratio method. Based on this conclusion, the evaporation from this field in winter was analyzed, which will be shown in the following paper.

Journal

  • Journal of Agricultural Meteorology

    Journal of Agricultural Meteorology 66(2), 117-124, 2010

    The Society of Agricultural Meteorology of Japan

Codes

  • NII Article ID (NAID)
    130004878264
  • NII NACSIS-CAT ID (NCID)
    AA11530034
  • Text Lang
    ENG
  • Article Type
    journal article
  • ISSN
    0021-8588
  • NDL Article ID
    10739806
  • NDL Call No.
    Z18-388
  • Data Source
    NDL  J-STAGE  JASI 
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