Estimation of Contrail Frequency and Radiative Effects over the Taiwan Area

Abstract

The radiative effect of aircraft contrails over the Taiwan area was studied using a climatological analysisi of contrail occurrence frequency and numerical simulations of contrail formation. Although the most suitable altitude for the formation of contrails is 14~16 km, most contrails appear at heights between 10 and 12 km due to the altitude constraint on flying. Winter and spring are the most favorable seasons for contrail formation, whereas summer is the most unfavorable. After formation, 86% of the contrails will dissipate while the remainder will grow and persist. For more detailed analysis, we classify contrails into 12 types according to the climatology of contrail-forming conditions. The microphysical and radiative properties of each type of contrails were simulated using a two-dimensional cirrus model. From the simulation we obtained a time-averaged top-of-atmosphere longwave radiative forcing of about 1.1 W m-2 and 16 W m-2 directly under a dissipating and persistent contrail, respectively; whereas the shortwave radiative forcing, with the sun directly overhead, were -2.6 W m-2 and -40 W m-2,respectively.

By considering the effect of upper-wind advection on the regional distribution, a climatological analysis of the upper-wind distribution was performed. The time evolutions of the radiative effects were then converted into spatial distributions along all flight corridors over the Taiwan area. On any particular day, the average longwave radiative forcing on a regional scale is about 0.08 W m-2 when the atmosphere is suitable for forming dissipating contrails. The average longwave radiative forcing for persistent contrails is about 0.68 W m-2. The shortwave forcing is typically about 2.4 times greater in manitude than the longwave forcing, thus the net effect is a cooling of surface temperatures. Taking into account the occurrence frequency of different contrail types and diurnal flight frequency variations, the annual mean effect is a cooling of -0.006 W m-2 over the entire region, but may reach about -0.03 W m-2 near the flight corridors.

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