Tropospheric water vapor profiles obtained with FTIR: comparison with balloon-borne frost point hygrometers and influence on trace gas retrievals
Journal Article
Abstract. Retrievals of vertical profiles of key atmospheric gases provide a critical long-term data record from ground-based Fourier Transform InfraRed (FTIR) solar absorption measurements. However, the characterization of the retrieved vertical profile structure can be difficult to validate, especially for gases with large vertical gradients and spatial-temporal variability such as water vapor. In this work, we evaluate the accuracy of the most common water vapor isotope (H216O, hereafter WV) FTIR retrievals in the lower and upper troposphere – lower stratosphere. Coincident high-quality vertically resolved WV profile measurements obtained from 2010 to 2016 with balloon-borne NOAA Frost Point Hygrometers (FPH) are used as reference to evaluate the performance of the retrieved profiles at two sites: Boulder, Colorado and in the mountain top observatory of Mauna Loa, Hawaii. For a meaningful comparison, the spatial-temporal variability has been investigated. Additionally, we evaluate the quantitative impact of different a priori profiles in the retrieval of WV vertical profiles using un-smoothed comparisons. An orthogonal linear regression analysis shows the best correlation among all layers using ERA-Interim (ERA-I) a priori profiles. In Boulder, we found a negative bias of 0.02 ± 1.9 % and precision of 3.7 % (r = 0.95) for the 1.5–3 km layer. A larger negative bias of 11.1 ± 3.5 % and precision of 7.0 % was found in the lower free troposphere layer of 3–5 km (r = 0.97) attributed to rapid vertical change of WV, which is not always captured by the retrievals. The bias improves in the 5–7.5 km layer (1.0 ± 5.3 %) and the precision worsens to about 10 %. The bias remains at about 13 % and the precision remains to about 10 % for layers above 7.5 km but below 13.5 km. At MLO the spatial mismatch is significantly larger due to the launch of the sonde being farther from the FTIR location. Nevertheless, we estimate a negative biases of 5.9 ± 4.6 % for the 3.5–5.5 km layer (r = 0.93) and 9.9 ± 3.7 % for the 5.5–7.5 km layer (r = 0.93), and positive biases of 6.2 ± 3.6 % for the 7.5–10 km layer (r = 0.95), and 12.6 % and greater values above 10 km. The agreement for the first layer is significantly better at BLD likely that the air masses are similar for both FTIR and FPH. Furthermore, for the first time we study the influence of different sources of WV profiles in the retrieval of selected gas profiles. Using NDACC standard retrievals we present results for hydrogen cyanide (HCN), carbon monoxide (CO), and ethane (C2H6) by taking NOAA FPH profiles as the ground-truth and evaluate the impact of other WV profile sources. We show that the effect is minor for C2H6 (bias ;
