Ice‐supersaturated regions and subvisible cirrus in the northern midlatitude upper troposphere

Abstract

Abstract. Humidity and temperature data from the Measurement of Ozone by Airbus in-service Aircraft (MOZAIC) project have been used to produce maps of probability for ice supersaturation in two 50 hPa thick layers centered around 200 and 250 hPa. As the MOZAIC data cover only international air routes, the resulting maps cover mainly the northern midlatitudes. The data of ice supersaturation have then been correlated with data of frequency of occurrence of subvisible cirrus from the Stratospheric Aerosol and Gas Experiment (SAGE II) satellite instrument. The correlation analysis provided strong indications that subvisible cirrus (SVC) is associated to ice-supersaturated regions (ISSRs), although processes are possible that can decouple SVC from ISSRs. A first trial to derive a global picture of ice supersaturation near the tropopause was performed using a measure of cirrus fractional coverage constructed from meteorological analyses of European Centre for Medium-Range Weather Forecasts and to correlate this with the supersaturation data. The correlation was only moderate (although significant), leading to the tentative conjecture that regions of frequent ice supersaturation are to be expected over the Indonesian archipelago, over the Amazonas basin, and over the northern Pacific between Japan and Canada. A final correlation analysis between the meteorological analysis data and the SVC data indicated that the formation of SVC is generally thermodynamically controlled, with the exception of the northern midlatitude SVC. The composition of the aerosol at the northern midlatitude tropopause is probably variable due to industrial emissions and air traffic. Hence the freezing properties of these particles may become important, which results in a weaker thermodynamic control of SVC formation in the northern midlatitudes.

Satellite observations of cirrus clouds in the Northern Hemisphere lowermost stratosphere

“…ISSRs have similar vertical and horizontal scales and occurrence distribution to SVC (e.g. Gierens et al, 2000;Spichtinger et al, 2003a) and are observed with various ground-, air-, and spaceborne sensors. Statistics in respect to the tropopause location are less frequently reported.…”

“…The limited vertical resolution (∼ 3 km) of MLS and integration along the line of sight (∼ 200 km) will reduce the detection sensitivity for ISSR by a factor of 0.2 compared to data from the Measurements of OZone and water vapour in Airbus In-service airCraft experiment (MOZAIC). Gierens et al (2000) found that the MOZAIC flights from 1995 to 1997 spent about 15 % of their time in ice supersaturated air masses. Stratospheric ISSR events were only rarely found in the MLS data, but only a crude tropopause determination was applied in the analyses and may create biases in the results (Spichtinger et al, 2003b).…”

Simulating the global atmospheric black carbon cycle: a revisit to the contribution of aircraft emissions

“…Persistent contrails form at or above ice saturation. Since the frequency of ice supersaturated regions mostly is smaller than 25% in the main flight areas (Gierens et al, 2000), the impact of contrail ice particles on BC from aviation is neglected here.…”

Evidence of impact of aviation on cirrus cloud formation

“…In the tropics, their formation is mainly triggered by deep convection (Jensen et al, 1996) while over the northern middle latitudes, they are associated to the existence of large ice supersaturated regions in the upper troposphere (Gierens et al, 2000). Cirrus cloud formation depends strongly on the existence of small particles that provide the nucleus for ice crystals (Jensen and Toon, 1997).…”