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 critical variables for the parameterization of cirrus properties in a global climate model (GCM). In this study a methodology to estimate cirrus properties, such as crystal mean fall speed, through successive lidar measurements is evaluated. This "Match" technique has been applied on cirrus cloud observations and then tested with measurements from two ground-based lidars located in the Mediterranean area. These systems, with similar instrumental characteristics, are installed at the Observatory of Haute Provence (OHP, 43.9° N, 5.7° E) in France and at Rome Tor Vergata (RTV, 41.8° N, 12.6° E) in Italy. At a distance of approximately 600 km, the two lidar stations have provided systematic measurements for several years and are along a typical direction of an air path. A test case of an upper tropospheric cirrus, observed over both sites during the night between 13 and 14 March 2008, has been selected and the feasibility of the Match-cirrus approach investigated through this case. The analysis through lidar principal parameters (vertical location, geometrical thickness 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 been retrieved with a simplified approach (adiabatic transport and "frozen" microphysical conditions inside the cirrus). The backward trajectory analysis suggests a type of cirrus formed by large-scale transport processes (adiabatic cooling of moist air masses coming from the subtropical area around Mexico gulf), which is characterized by a long atmospheric lifetime and horizontal extension of several hundred km. The analysis of this case study reveals that many uncertainties reduce the confidence of the retrieved estimates of the crystal fall velocity. However, this paper allows for assessing the technique feasibility by identifying the main critical issues for future similar investigations. This study shows that such approach is feasible; however, the methodology should be improved and some directions have been suggested for future campaigns.