Cloud heights from TOVS Path‐B: Evaluation using LITE observations and distributions of highest cloud layers

Abstract

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 high‐level 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.