The semiannual oscillation (SAO) in the tropical middle atmosphere and its gravity wave driving in reanalyses and satellite observations
The semiannual oscillation (SAO) in the tropical middle atmosphere and its gravity wave driving in reanalyses and satellite observations
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DOI：
10.5194/acp-21-13763-2021
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发表时间：
2021-09-16
影响因子：
6.3
通讯作者：
Riese, Martin
中科院分区：
文献类型：
--
作者：
Ern, Manfred;Diallo, Mohamadou;Riese, Martin
重力波在驱动热带地区纬向风半年振荡（SAO）方面发挥着重要作用。然而，我们缺乏对这种强迫的详细了解，而且对重力波的全球观测的直接估计也很少。在 2002 年至 2018 年期间，我们通过四种不同的重新分析来研究 SAO：ERA-Interim、JRA-55、ERA-5 和 MERRA-2。与 SPARC 纬向风气候学和微波临边探测器 (MLS) 和使用宽带发射辐射测量 (SABRE) 卫星观测的大气探测所得出的准地转风进行比较表明，重新分析再现了 SAO 的一些基本特征。然而，根据模型设置的不同，也存在很大的差异。特别是，MERRA-2 似乎受益于重力波阻力参数化的专门调整和 MLS 观测的同化。为了研究重力波与背景风的相互作用，将 SABRE 卫星观测得出的重力波动量通量的绝对值和绝对重力波阻力的代理与不同的风数据集进行了比较：SPARC 风气候学；数据集结合了低海拔的 ERA-Interim 和高海拔的 MLS 或 SABRE 准地转风；以及结合了 ERA-Interim、SABRE 准地转风和定时多普勒干涉仪 (TIDI) 直接风观测的数据集。在中层和中层下部，SABRE 绝对重力波阻力代理与背景风的正垂直梯度密切相关，表明重力波主要有助于驱动 SAO 东风相位及其随时间的向下传播。在 75-85 公里高度，SABRE 绝对重力波阻力代理与背景风的绝对值相关性更好，表明重力波振幅饱和对 SAO 风有更直接的强迫。约 80 公里以上 SABRE 重力波阻力主要受潮汐而非 SAO 控制。再分析再现了SAO重力波驱动的一些基本特征：所有再分析都表明平流层顶区域SAO东向阶段有更强的重力波驱动。对于较高顶型号 ERA-5 和 MERRA-2，中层下部也是如此。然而，所有重新分析都受到模型上层固有阻尼的限制，导致模型顶部附近出现不切实际的特征。我们对 SABER 的分析和重力波阻力的再分析表明，在自由运行的大气环流模型中，SAO 重力波强迫的大小通常太弱；因此，需要更现实的表示。
Gravity waves play a significant role in driving the semiannual oscillation (SAO) of the zonal wind in the tropics. However, detailed knowledge of this forcing is missing, and direct estimates from global observations of gravity waves are sparse. For the period 2002-2018, we investigate the SAO in four different reanalyses: ERA-Interim, JRA-55, ERA-5, and MERRA-2. Comparison with the SPARC zonal wind climatology and quasi-geostrophic winds derived from Microwave Limb Sounder (MLS) and Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) satellite observations show that the reanalyses reproduce some basic features of the SAO. However, there are also large differences, depending on the model setup. Particularly, MERRA-2 seems to benefit from dedicated tuning of the gravity wave drag parameterization and assimilation of MLS observations. To study the interaction of gravity waves with the background wind, absolute values of gravity wave momentum fluxes and a proxy for absolute gravity wave drag derived from SABER satellite observations are compared with different wind data sets: the SPARC wind climatology; data sets combining ERA-Interim at low altitudes and MLS or SABER quasi-geostrophic winds at high altitudes; and data sets that combine ERA-Interim, SABER quasi-geostrophic winds, and direct wind observations by the TIMED Doppler Interferometer (TIDI). In the lower and middle mesosphere the SABER absolute gravity wave drag proxy correlates well with positive vertical gradients of the background wind, indicating that gravity waves contribute mainly to the driving of the SAO eastward wind phases and their downward propagation with time. At altitudes 75-85 km, the SABER absolute gravity wave drag proxy correlates better with absolute values of the background wind, suggesting a more direct forcing of the SAO winds by gravity wave amplitude saturation. Above about 80 km SABER gravity wave drag is mainly governed by tides rather than by the SAO. The reanalyses reproduce some basic features of the SAO gravity wave driving: all reanalyses show stronger gravity wave driving of the SAO eastward phase in the stratopause region. For the higher-top models ERA-5 and MERRA-2, this is also the case in the lower mesosphere. However, all reanalyses are limited by model-inherent damping in the upper model levels, leading to unrealistic features near the model top. Our analysis of the SABER and reanalysis gravity wave drag suggests that the magnitude of SAO gravity wave forcing is often too weak in the free-running general circulation models; therefore, a more realistic representation is needed.