Aircraft measurements of microphysical properties of subvisible cirrus in the tropical tropopause layer

Abstract

Abstract. Subvisible cirrus (SVC) clouds are often observed within the tropical tropopause layer (TTL). Some studies suggest that SVC has a significant impact on the earth radiation budget. The Costa Rica Aura Validation Experiment (CR-AVE) sponsored by the National Aeronautics and Space Administration (NASA) took place near San Jose, Costa Rica from 14 January–15 February 2006. The NASA WB-57F sampled SVC in the TTL from −75°C to −90°C with an improved set of cloud particle probes. The first digital images of ice particles in the TTL are compared with replicator images of ice particles collected in 1973 by a WB-57F in the TTL. The newer measurements reveal larger particles, on the order of 100 μm compared with <50 μm from the earlier measurements, and also different particle shapes. The 1973 particles were mainly columnar and trigonal, whereas the newer measurements are quasi-spherical and hexagonal plates. The WB-57F also measured very high water vapor contents with some instruments, up to 4 ppmv, and aerosols with mixed organics and sulfates. It is unknown whether these ambient conditions were present in the 1973 studies, and whether such conditions have an influence on particle shape and the development of the large particles. A companion paper (Jensen et al., 2008) presents crystal growth calculations that suggest that the high water vapor measurements are required to grow ice particles to the observed sizes of 100 μm and larger.

The Impact of Two Coupled Cirrus Microphysics–Radiation Parameterizations on the Temperature and Specific Humidity Biases in the Tropical Tropopause Layer in a Climate Model

“…The two particle size distributions shown in Jensen et al (2016) extend to maximum dimensions of about 50 mm and almost 200 mm, at temperatures of ,195 K and between about 195 and less than 210 K, respectively. Meanwhile, Lawson et al (2008) report particle distributions extending to 165 mm in the subvisual TTL cirrus they sampled, which contrasts with the study of McFarquhar et al (2000), who found that particle maximum dimensions did not extend to beyond 50 mm. Determining the habit mixtures and particle distributions in TTL cirrus is important, as the choice of assumed particle distributions in climate models will influence their predicted radiative effect of cirrus.…”

“…The balance between cirrus warming or cooling the upper troposphere depends on its visible optical depth, as shown by Hong and Liu (2015), who demonstrated that cirrus with visible optical depths less than unity leads to a net heating of the upper troposphere, while optically thicker cirrus results in a net cooling of the upper troposphere. Moreover, subvisual cirrus occurs in the TTL, as found by Lawson et al (2008); also, Lee et al (2009) showed that these clouds may exert a net radiative effect on the order of about 1.1 W m 22 .…”

“…There have been aircraft campaigns that have examined the ice microphysics composition of the TTL. In the study by Heymsfield (1986), the common occurrence of trigonal particles was reported but more recent studies by Lawson et al (2008) have found few occurrences of trigonal ice particles. In the study by McFarquhar et al (2000), they found habit mixtures comprising 50% of hexagonal columns and trigonal ice crystals in the subvisual TTL cirrus they studied.…”

“…In the study by McFarquhar et al (2000), they found habit mixtures comprising 50% of hexagonal columns and trigonal ice crystals in the subvisual TTL cirrus they studied. Meanwhile, Lawson et al (2008) found the occurrence of quasispherical ice particles to be the most common particle type for all crystal sizes. However, it is uncertain as to whether these particles are actually quasi-spherical due to the limiting resolving power of the microphysics instrumentation used at the time; therefore, the appearance of quasi-sphericity could be due to diffractive and optical effects, as noted by Cotton et al (2010) and references therein.…”

“…On the other hand, these particles could be quasispherical, but instruments are required that can adequately resolve these ice crystals of an uncertain shape, such as the small ice detector described in Ulanowski et al (2006). For crystal sizes greater than 65 mm, Lawson et al (2008) found habit mixtures comprising mostly hexagonal plates and irregular ice crystals. In the study by Jensen et al (2008) they noted the occurrence of hexagonal plate aspect ratios of 6:1 of nearly 100 mm in size near the TTL, which were reported to be unassociated with deep tropical convection.…”