Factors controlling the position of the Intertropical Convergence Zone on an aquaplanet
Factors controlling the position of the Intertropical Convergence Zone on an aquaplanet
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DOI：
10.1029/2012ms000199
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发表时间：
2012-04
影响因子：
6.8
通讯作者：
B. Möbis;B. Stevens
中科院分区：
文献类型：
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作者：
B. Möbis;B. Stevens
为了解影响热带辐合带(ITCZ)位置的性质，对使用ECHAM6 GCM在固定海表面温度(SST)下进行的水行星实验进行了研究。当使用Nordeng方案时，产生一个单一的ITCZ；当使用Tiedtke方案时，产生一个双ITCZ。发现ITCZ的位置取决于一个反馈环路过程，其中对流加热驱动气压梯度和风，这决定了地面蒸发的速率，从而影响边界层湿静态能量，最终耦合回对流加热的模式。这种反馈环路过程对海温廓线和对流方案的选择很敏感。然而，SST只有在控制边界层湿静态能量的情况下才是重要的。可以通过指定用于计算地表通量的风来打破反馈回路，这样就有可能控制边界层湿静力能量的大小，从而控制ITCZ的位置。云顶高度和对流加热主要取决于夹带率和对流层自由湿度。在双ITCZ的情况下，自由对流层下部的湿度在双ITCZ的赤道一侧比在极地一侧要高。因此，夹带率的增加有利于赤道一侧的对流。这就解释了为什么Nordeng方案只产生一个ITCZ，而Tiedtke方案产生两个ITCZ。
Aqua planet experiments performed with fixed sea surface temperatures (SST) using the ECHAM6 GCM are studied to understand properties that influence the position of the Intertropical Convergence Zone (ITCZ). A single ITCZ develops when using the Nordeng scheme and a double ITCZ when using the Tiedtke scheme. The position of the ITCZ is found to depend on a feedback loop process wherein convective heating drives pressure gradients and winds, which determine the rate of surface evaporation, which influences the boundary layer moist static energy, which finally couples back to the pattern of convective heating. This feedback loop process is sensitive to the SST profile and the choice of the convection scheme. However SSTs are only important in so far as they control the boundary layer moist static energy. The feed‐back loop can be broken by specifying the wind used to calculate surface fluxes, in so doing it is possible to control the magnitude of boundary layer moist static energy and hence the position of the ITCZ. The cloud top height and therefore the convective heating decisively depends on the entrainment rates and the free tropospheric humidity. In the double ITCZ case the humidity in the lower free troposphere is higher on the equatorward side of the double ITCZ compared to the poleward side. Therefore an increase of the entrainment rates favor convection on the equatorward side. This explains why the Nordeng scheme produces a single ITCZ, although the Tiedtke scheme produces a double ITCZ.