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

D Dionisi,F Congeduti,C Hoareau,P Keckhut,N Montoux

doi:10.5194/amt-6-457-2013

Abstract

Abstract. Cirrus ice particle sedimentation velocity (vs) 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 vs (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.

Highlights

Sciences atic measurements for several years and are along a typical direction of an air path
The objective of this study was to describe the first observations obtained with two ground-based lidar, investigating existing data archives from both stations in order to assess the feasibility of a possible future observation strategy that couples lidar measurements to a Lagrangian tracking method to study cirrus cloud formation, air mass history and, in particular here, mean cirrus fall speed
Using the GDAS (Global Data Assimilation System) dataset for five years (2006–2010), the 24-h backward trajectories for three different heights (8, 10 and 12 km, respectively) have been calculated through the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT, Draxler and Rolph, 2003) model, in order to determine the frequency of occurrence of the air masses that passed over a 0.7◦ × 0.7◦ grid centred at OHP before arriving at Rome-Tor Vergata (RTV)

Summary

Introduction

Sciences atic measurements for several years and are along a typical direction of an air path. Have shown that the top of a cirrus cloud is mainly composed of small but numerous ice crystals, whereas the bottom consists more of few large crystals. Such vertical distributions can have some local cooling or warming radiative effects (Khvorostyanov lated climate sensitivity atondatSmSaososseplnhi,de2ri0cE0g2al)o.rbItanhlaclsitmudaytethmaot dree-l (GCM) parameters, Sanderson et al (2008) identified the ice crystal fall speed as the second most influential parameter to climate sensitivity, and a decrease in fall speed was related to an increase in cirrus cloud coverage, humidity and long-wave cLlaogurdanfogriacninngu. Such vertical distributions can have some local cooling or warming radiative effects (Khvorostyanov lated climate sensitivity atondatSmSaososseplnhi,de2ri0cE0g2al)o.rbItanhlaclsitmudaytethmaot dree-l (GCM) parameters, Sanderson et al (2008) identified the ice crystal fall speed as the second most influential parameter to climate sensitivity, and a decrease in fall speed was related to an increase in cirrus cloud coverage, humidity and long-wave cLlaogurdanfogriacninngu. mSiemriiclaaTrl cshiomnecullCuastiirooynnsos (hsMapvoehnbteoeuerxneeotbatal.i,n2e0d1u0s)i.nIgt has been demonstrated (Jakob, 2002) that, in the weather

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