Cirrus crystal fall velocity estimates using the Match method with ground-based lidars: first investigation through a case study

[摘要] Cirrus ice particle sedimentation velocity (v_s) is one of the criticalvariables for the parameterization of cirrus properties in a global climatemodel (GCM). In this study a methodology to estimate cirrus properties, suchas crystal mean fall speed, through successive lidar measurements isevaluated. This "Match" technique has been applied on cirrus cloudobservations and then tested with measurements from two ground-based lidarslocated in the Mediterranean area. These systems, with similar instrumentalcharacteristics, are installed at the Observatory of HauteProvence (OHP, 43.9° N, 5.7° E) in France and at RomeTor Vergata (RTV, 41.8° N, 12.6° E) in Italy. At adistance of approximately 600 km, the two lidar stations have provided systematicmeasurements for several years and are along a typical direction of an airpath. A test case of an upper tropospheric cirrus, observed over both sitesduring the night between 13 and 14 March 2008, has been selected and thefeasibility of the Match-cirrus approach investigated through this case. Theanalysis through lidar principal parameters (vertical location, geometricalthickness and optical depth) reveals a case of a thin sub-visible cirrus(SVC) located around the tropopause. A first range of values for v_s(1.4–1.9 cm s⁻¹, consistent with simple-shaped small crystals) has beenretrieved with a simplified approach (adiabatic transport and "frozen"microphysical conditions inside the cirrus). The backward trajectoryanalysis suggests a type of cirrus formed by large-scale transport processes(adiabatic cooling of moist air masses coming from the subtropical areaaround Mexico gulf), which is characterized by a long atmospheric lifetimeand horizontal extension of several hundred km. The analysis of thiscase study reveals that many uncertainties reduce the confidence of theretrieved estimates of the crystal fall velocity. However, this paper allowsfor assessing the technique feasibility by identifying the main critical issuesfor future similar investigations.

This study shows that such approach is feasible; however, the methodologyshould be improved and some directions have been suggested for futurecampaigns.

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