Radiative transfer simulations and observations of infrared spectra in the presence of polar stratospheric clouds: Detection and discrimination of cloud types
Radiative transfer simulations and observations of infrared spectra in the presence of polar stratospheric clouds: Detection and discrimination of cloud types
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DOI：
10.5194/amt-2020-144
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发表时间：
2020-07
期刊：
影响因子：
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通讯作者：
C. Kalicinsky;S. Griessbach;R. Spang
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文献类型：
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作者：
C. Kalicinsky;S. Griessbach;R. Spang
摘要。极地平流层云对极地涡旋内部微量气体的时空演化起着重要的作用，其过程包括氯活化和NOy再分布等。由于在模式模拟中PSCs的表示仍然存在不确定性，因此需要对PSCs进行详细观测及其类型（硝酸三水合物（NAT），过冷三元溶液（STS）和冰）的信息。在调和（为增强北极平流层臭氧损失及其气候相互作用的可预测性而对基本过程参数进行调和）飞行活动期间，由机载红外边缘探测器CRISTA-NF（大气低温红外光谱仪和望远镜-新前沿）在PSCs内部进行的测量显示，光谱峰值约为816 cm−1。与820 cm−1的峰值相比，这个峰值发生了移位，这是由小的NAT粒子引起的。为了研究这种光谱差异的原因，我们对不同psc （NAT、STS、冰和混合物）存在下的红外翼发射光谱进行了大量的辐射传输模拟，用于CRISTA-NF的空中观测几何形状。NAT粒子在810 ~ 820 cm−1区域产生不同的光谱特征。模拟结果表明，随着NAT粒径分布中值半径的增大，该特征的形貌发生变化，从820 cm−1处的峰值变为移峰，最后变为阶梯状特征。基于这种行为，我们定义了不同的颜色指数来检测含有NAT颗粒的psc，并将它们分为三种尺寸：小NAT(≤1.0 μm)，中等NAT （1.5-4.0 μm）和大NAT（≥3.5 μm）。此外，我们开发了一种通过使用云指数（CI）、指示光学厚度的颜色比和CI的梯度来检测云的底部高度的方法。最后，我们应用CRISTA-NF仪器在调和飞机运动的一次本地飞行中的观测方法，发现了STS和中型NAT。
Abstract. Polar stratospheric clouds (PSCs) play an important role for the spatial and temporal evolution of trace gases inside the polar vortex due to different processes, such as chlorine activation and NOy redistribution. As there are still uncertainties in the representation of PSCs in model simulations, detailed observations of PSCs and information on their type (nitric acid trihydrate (NAT), supercooled ternary solution (STS), and ice) are desirable. The measurements inside PSCs by the airborne infrared limb sounder CRISTA-NF (CRyogenic Infrared Spectrometers and Telescope for the Atmosphere – New Frontiers) during the RECONCILE (Reconciliation of essential process parameters for an enhanced predictability of Arctic stratospheric ozone loss and its climate interactions) aircraft campaign showed a spectral peak at about 816 cm−1. This peak is shifted compared to the peak at about 820 cm−1, which is known to be caused by small NAT particles. To investigate the reason for this spectral difference we performed a large set of radiative transfer simulations of infrared limb emission spectra in the presence of various PSCs (NAT, STS, ice, and mixtures) for the airborne viewing geometry of CRISTA-NF. NAT particles can cause different spectral features in the region 810–820 cm−1. The simulation results show that the appereance of the feature changes with increasing median radius of the NAT particle size distribution from a peak at 820 cm−1 to a shifted peak and, finally, to a step-like feature in the spectrum. Based on this behaviour we defined different colour indices to detect PSCs containing NAT particles and to subgroup them into three size regimes: small NAT (≤ 1.0 μm), medium NAT (1.5–4.0 μm), and large NAT (≥ 3.5 μm). Furthermore, we developed a method to detect the bottom altitude of a cloud by using the cloud index (CI), a colour ratio indicating the optical thickness, and the gradient of the CI. Finally, we applied the methods to observations of the CRISTA-NF instrument during one local flight of the RECONCILE aircraft campaign and found STS and medium sized NAT.