Published by Copernicus Publications on behalf of the European Geosciences Union. constructed from averaged ACE-FTS data over China and completed with TES data over the 5 same area below 6 km. The vertical sensitivity of each CO sounding type of instrument is also 6 reported on the right-hand side of this plot. MW and TIR refer to millimeter-wave and 7 thermal infrared spectral regions, respectively. 8 Fig. 1. Schematic plot of a standard atmospheric CO profile, with the different sources of production (blue) and destruction/sinks (red) as a function of altitude. The CO profile was constructed from averaged ACE-FTS data over China and completed with TES data over the same area below 6 km. The vertical sensitivity of each CO sounding type of instrument is also reported on the right-hand side of this plot. MW and TIR refer to millimeter-wave and thermal infrared spectral regions, respectively.Abstract. The Atmospheric Chemistry Experiment (ACE) mission was launched in August 2003 to sound the atmosphere by solar occultation. Carbon monoxide (CO), a good tracer of pollution plumes and atmospheric dynamics, is one of the key species provided by the primary instrument, the ACE-Fourier Transform Spectrometer (ACE-FTS). This instrument performs measurements in both the CO 1-0 and 2-0 ro-vibrational bands, from which vertically resolved CO concentration profiles are retrieved, from the mid-troposphere to the thermosphere. This paper presents an updated description of the ACE-FTS version 2.2 CO data product, along with a comprehensive validation of these profiles using available observations (February 2004 to December 2006. We have compared the CO partial columns with ground-based measurements using Fourier transform infrared spectroscopy and millimeter wave radiometry, and the volume mixing ratio profiles with airborne (both high-altitude balloon flight and airplane) observations. CO satellite observations provided by nadir-looking instruments (MOPITT and TES) as well as limb-viewing remote sensors (MIPAS, SMR and MLS) were also compared with the ACE-FTS CO products. We show that the ACE-FTS measurements provide CO profiles with small retrieval errors (better than 5% from the upper troposphere to 40 km, and better than 10% above). These observations agree well with the correlative measurements, considering the rather loose coincidence criteria in some cases. Based on the validation exercise we assess the following uncertainties to the ACE-FTS measurement data: better than 15% in the upper troposphere (8-12 km), than 30% in the lower stratosphere (12-30 km), and than 25% from 30 to 100 km.

[1] Cloud height from the TIROS-N Operational Vertical Sounder (TOVS) Path-B climate data set has been evaluated by using vertical profiles of the backscattered radiation at 532 nm from quasi-simultaneous Lidar In-space Technology Experiment (LITE) observations. Two averaging methods for the LITE inversion have been studied. Because of the LITE noise level and the difficulty in determining the vertical structure of thick clouds, we have chosen to apply the inversion on the average backscatter signal of the LITE spots over regions of 1°latitude  1°longitude, which is also the spatial resolution of the TOVS Path-B data set. The cloud height determined by TOVS corresponds well in general to the height of the ''apparent middle'' of the cloud system with coincidences for 53% of TOVS Path-B low-level clouds within 1 km and for 49% of TOVS Path-B highlevel clouds within 1.5 km. In addition, 22.5% of TOVS Path-B low-level clouds are covered by a very thin high cloud layer not detectable by TOVS. Comparing for these cases the TOVS cloud height with the second LITE cloud layer increases the overall agreement for low-level clouds to about 64%. High-level clouds appear more often in multilayer systems (about 75%) and are also vertically more extended. Differences in average cloud height of high-level clouds appear only to be significant (13.3 km from LITE compared to 11.3 km from TOVS Path-B) in the tropics with a large extent of laminar cirrus situated near the tropopause. The height of maximum backscatter of most thick clouds is several hundred meters above ''apparent cloud midlevel,'' whereas thin high-level clouds with underlying lower clouds provide a backscatter signal nearer to apparent cloud midlevel. In the latter case, the retrieved TOVS Path-B cloud height is on average 280 m underestimated. Pressure distributions of the highest cloud layer weighted by effective cloud amount confirmed that high clouds have the lowest pressure in the tropics because of a higher tropopause, and in these regions there are nearly no cloud systems with the highest cloud layer in the middle troposphere. The Southern Hemisphere midlatitudes are mostly covered by low-level clouds. Seasonal differences in the Northern Hemisphere (midlatitudes, with more equally distributed cloud altitudes in winter) are mostly caused by changes over land.