Evaluation of the AROME model's ability to represent ice crystal icing using in situ observations from the HAIC 2015 field campaign

Abstract

Since pilots generally avoid intense convective areas, ice crystals icing (ICI) is an aeronautical weather incident that mainly occurs in the anvil of tropical deep convective clouds. Samples of favorable conditions for the occurrence of ICI and data from the High Altitude Ice Crystals (HAIC) 2015 field campaign in French Guiana are investigated and compared with simulations of the French operational mesoscale forecast system Application of Research to Operations at Mesoscales (AROME). To this end, a contextualization of convective systems into convective, stratiform, and cirriform regions is employed for both observations and AROME. General features of the microphysics of deep tropical convective systems are identified. The number concentration of crystals larger than 125 μm and total water content (TWC) are strongly correlated at each temperature level, and both decrease with increasing distance from convective cores. AROME can reproduce the general behavior of the observed microphysics, especially TWC, but seems unable to simulate extreme ICI events. Reasons are sought in the assumptions performed in the microphysical scheme ICE3, and guidelines are proposed to enhance its skills in the context of ICI. In particular, the representation of the snow particle size distribution is adjusted across observations using a generalized gamma shape. This shape is found to outperform the usual Marshall–Palmer and gamma shapes. Additionally, a temperature and snow content dependence of generalized gamma parameters is found. These changes are found to significantly improve the snow concentration diagnostic of ICE3, and these modifications open the way for improvements in the ICE3 scheme.

Improvements to the parametrization of snow in AROME in the context of ice crystal icing

“…In this Figure 8, dots in the lower-right region of each graph correspond to ICI-prone conditions, especially those with colder temperatures. Dots where median TWC is higher than 0.5 g⋅m −3 and where Contrary to what has been sampled during the 2015 HAIC field campaign, very few ICI-favourable conditions were simulated in OPER (Wurtz et al, 2021) and Vt (not shown). Nevertheless, in the HAIC experiment, many more ICI-prone conditions are simulated (Figure 8).…”

“…These aggregates are represented in ICE3 by the snow category and represent indeed the majority of ice content in simulated anvil clouds. Following Wurtz et al (2021), different experiments were launched using the modifications described hereafter and are summed up in Table 3. These modifications are based on diagnostics derived from observations.…”

“…The first tested (A, B) couple has been derived by Houze et al (1979) on midlatitude fronts for temperatures from 0 • C to about −43 • C and is set to (9.006, 0.022). Heymsfield (2003) proposed two couples of (A, B) values depending on the two temperature ranges: above and below −10 • C. Following this study, two specific (A, B) couples for the HAIC field campaign have been derived using the median 𝜆 of the three sampled temperature levels (−10, −30 and −45 • C) (Wurtz et al, 2021). The value of 1,000 m −1 is chosen as the lower limit for 𝜆 at 273 K (Lo and Passarelli, 1982;Lo, 1983), and the law can be extrapolated between 0 and −10 • C. As a result, the (A, B) couple is set to (14.554, 0.0423) for temperatures above 263 K and to (6.226, 0.0106) for temperatures below this threshold.…”

“…Nevertheless, using a single-moment bulk scheme in a large-eddy simulation, Proctor et al (2017) found that it was possible to simulate anvil clouds with high and sustained IWC with below 30 dBZ reflectivities. The skills of the operational mesoscale system Applications of Research to Operations at Mesoscale (AROME) have been assessed by Wurtz et al (2021) using the French-Guiana HAIC 2015 airborne field campaign. This study suggests that revisions to the microphysical scheme Intercity-Express 3 (ICE3) are necessary to enable the simulation of high and sustained IWC, favourable to ICI, in anvil clouds.…”

“…A detailed analysis of the resulting alteration in the convective, stratiform and cirriform regions of the cloud microphysical composition is performed on the HAIC field campaign period and a case-study is analysed to better understand the microphysical processes leading to such conditions. In this study, behaviours in the convective region and in the anvil cloud (stratiform and cirriform regions) of the MCS are analysed separately using the contextualization of convective systems based on dynamics and precipitation rates of Wurtz et al (2021). Convective regions are either identified by maximum vertical velocities higher than 2.5 m⋅s −1 or by rainfall rates higher than 15 mm⋅h −1 , stratiform regions are non-convective regions where rainfall rates exceed 1 mm⋅h −1 and cirriform regions are non-stratiform regions where ice water paths exceed 0.5 kg⋅m −2 .…”