Ice Crystal Sizes in High Ice Water Content Clouds. Part II: Statistics of Mass Diameter Percentiles in Tropical Convection Observed during the HAIC/HIWC Project

Abstract

High ice water content (IWC) regions in mesoscale convective systems (MCSs) are a potential threat to commercial aviation, as they are suspected to cause in-service engine power-loss events and air data probe malfunctions. To investigate this, the high-altitude ice crystals (HAIC)/high ice water content (HIWC) projects set up a first field campaign in Darwin (Australia) in 2014. The airborne instrumentation was selected to provide the most accurate measurements of both the bulk total water content (TWC), using a specially developed isokinetic evaporator, and the individual ice crystals properties, using particle imaging probes. This study focuses on determining the size ranges of ice crystals responsible for the mass in high IWC regions, defined here as cloud regions with IWC greater than 1.5 g m−3. It is shown that for high IWC areas in most of the encountered MCSs, median mass diameters (MMDs) of ice crystals range from 250 to 500 μm and decrease with increasing TWC and decreasing temperature. At the same time, the mass contribution of the smallest crystals (below 100 μm) remains generally low (below 15%). In contrast, data from two flight missions in a long-lasting quasi-stationary tropical storm reveal that high IWC values can also be associated with MMDs in the range 400–800 μm and peak values of up to 2 mm. Ice crystal images suggest a major growth contribution by vapor deposition (columns, capped columns) even for such larger MMD values.

Comparison of global observations and trends of total precipitable water derived from microwave radiometers and COSMIC radio occultation from 2006 to 2013

“…Ice water content values are smaller, typically 0.01-0.03 g m −3 . Heymsfield et al (2002) reported high ice water content values ranging from 0.1 to 0.5 g m −3 in tropical cirrus and stratiform precipitating clouds, although values rarely reach as high as 1.5 g m −3 in deep tropical convective clouds (Leroy et al, 2017).…”

Evaluation of radar reflectivity factor simulations of ice crystal populations from in situ observations for the retrieval of condensed water content in tropical mesoscale convective systems

“…The dataset includes more than 17 000 data points where all parameters are synchronized and averaged over 5 s. As one of the priority of the HAIC/HIWC projects was to measure high ice water content and the variability of CWC as a function of distance from the convective cores at typical altitudes flown by commercial aircraft, the flight strategy was to fly long legs at constant altitude at −50, −40, −30, and −10 • C and to get as close as possible to the most convective zone of the MCS (see Fig. 2 in Leroy et al, 2017), thereby avoiding red aircraft radar echoes at normal gain as commercial aircraft would also do. More details on HAIC/HIWC projects and implemented flight strategy can be found in Dezitter et al (2013), Leroy et al (2016Leroy et al ( , 2017, Protat et al (2016), and .…”

“…2 in Leroy et al, 2017), thereby avoiding red aircraft radar echoes at normal gain as commercial aircraft would also do. More details on HAIC/HIWC projects and implemented flight strategy can be found in Dezitter et al (2013), Leroy et al (2016Leroy et al ( , 2017, Protat et al (2016), and .…”

A ubiquitous ice size bias in simulations of tropical deep convection

“…In particular, mass-size distributions (MSDs) constructed from these measurements may be compared with simulated MSDs in microphysics schemes. L16 and Leroy et al (2017) (hereafter referenced as L17) perform extensive analyses on median mass diameters (MMDs) retrieved from HAIC-HIWC data, a characteristic size metric that provides the diameter at which half of the bulk mass resides in smaller diameters and half resides in larger diameters. By comparing hydrometeor sizes and investigating the model parameters and processes that control size distributions, possible sources for biases may be identified and further observational constraints on these parameters may be implemented.…”