High ice water content at low radar reflectivity near deep convection – Part 2: Evaluation of microphysical pathways in updraft parcel simulations

Abstract

Abstract. The aeronautics industry has established that a threat to aircraft is posed by atmospheric conditions of substantial ice water content (IWC) where equivalent radar reflectivity (Ze) does not exceed 20–30 dBZ and supercooled water is not present; these conditions are encountered almost exclusively in the vicinity of deep convection. Part 1 (Fridlind et al., 2015) of this two-part study presents in situ measurements of such conditions sampled by Airbus in three tropical regions, commonly near 11 km and −43 °C, and concludes that the measured ice particle size distributions are broadly consistent with past literature with profiling radar measurements of Ze and mean Doppler velocity obtained within monsoonal deep convection in one of the regions sampled. In all three regions, the Airbus measurements generally indicate variable IWC that often exceeds 2 g m-3 with relatively uniform mass median area-equivalent diameter (MMDeq) of 200–300 μm. Here we use a parcel model with size-resolved microphysics to investigate microphysical pathways that could lead to such conditions. Our simulations indicate that homogeneous freezing of water drops produces a much smaller ice MMDeq than observed, and occurs only in the absence of hydrometeor gravitational collection for the conditions considered. Development of a mass mode of ice aloft that overlaps with the measurements requires a substantial source of small ice particles at temperatures of about −10 °C or warmer, which subsequently grow from water vapor. One conceivable source in our simulation framework is Hallett–Mossop ice production; another is abundant concentrations of heterogeneous ice freezing nuclei acting together with copious shattering of water drops upon freezing. Regardless of the production mechanism, the dominant mass modal diameter of vapor-grown ice is reduced as the ice-multiplication source strength increases and as competition for water vapor increases. Both mass and modal diameter are reduced by entrainment and by increasing aerosol concentrations. Weaker updrafts lead to greater mass and larger modal diameters of vapor-grown ice, the opposite of expectations regarding lofting of larger ice particles in stronger updrafts. While stronger updrafts do loft more dense ice particles produced primarily by raindrop freezing, we find that weaker updrafts allow the warm rain process to reduce competition for diffusional growth of the less dense ice expected to persist in convective outflow.

A ubiquitous ice size bias in simulations of tropical deep convection

“…Two bulk microphysics schemes and one explicit bin microphysics scheme are evaluated, providing insight into how the bias and its causes differ between fundamentally different approaches to microphysics parameterization. The extent to which a reflectivity bias exists in bin microphysics schemes has not been extensively explored, although Ackerman et al (2015) show that a CRM simulation using bin microphysics failed to reproduce observed low reflectivity values in high-IWC regions, suggesting that this bias may exist across both bulk and bin schemes. Observations are described in Sect.…”

Investigating the Impacts of Saharan Dust on Tropical Deep Convection Using Spectral Bin Microphysics, Part 1: Ice Formation and Cloud Properties

“…The more numerous particles allow for the particle interaction/collection related to ice phase precipitation formation to occur lower in the atmosphere and earlier in the simulation, delaying sedimentation and increasing vapor competition between particles, which in turn limits the maximum 5 particle size. In the weaker updraft regime of the stratiform region, transported hydrometeors are able to grow more efficiently compared to the stronger updrafts of the convective region, an occurrence which has also been reported in the observations of Ackerman et al (2015). The reduced near surface reflectivity values observed by both Min et al (2009) and Li and Min (2010) suggest that total surface precipitation will be affected by the effects of dusty conditions on cloud dynamics and precipitation PSD, most likely resulting in an overall reduction in surface rain rates.…”

“…While ice multiplication from the Hallett-Mossop scheme is active in both cases, removing the depositional-condensation nucleation is sufficient to reduce graupel formation and delay the glaciation of the clouds until temperatures near the homogeneous freezing level 25 where immersion freezing is most active. The observations/modeling studies of Lawson et al (2015) and Ackerman et al (2015) both suggest that this effect on cloud glaciation can be explained by sufficiently fast ice multiplication at warmer temperatures, creating small ice crystals similar to the effects of significantly large deposition-condensation freezing, although it should be noted that primary ice formation was not the focus of those studies and ice nucleation was treated simply, leaving a possible gap in the understanding of the interactions between the ice nucleation and ice multiplication 30 processes. In our simulation the removal of the deposition-condensation freezing greatly hinders the formation of midlevel ice, despite an unchanged IN concentration and active ice multiplication, showing the importance of even small number of midlevel ice formation to the overall vertical hydrometeor distribution.…”

“…As noted in Meyers et al (1992) it is difficult to distinguish the relative contributions of depositional and condensational freezing in a parcel, since both form similarly sized small ice crystals, despite the different mechanisms of vapor to ice in the former and condensation followed immediately by freezing in the latter case. However, studies suggest that small ice formed in the temperature range covered by this scheme can have a large impact on subsequent ice formation at higher altitudes (Ackerman et al 2015;Hiron and 25 Flossman, 2015;Lawson et al, 2015).To link depositional/condensational freezing with aerosols, we follow the implementation of van den Heever et al (2006), updated from the Meyers et al (1992) version. The number of ice crystals generated by depositional-condensational nucleation (N hen ) is proportional to the IN number concentration (N IN ; l -1 ) by Eq.…”

Intercomparison study and optical asphericity measurements of small ice particles in the CERN CLOUD experiment

“…Since the liquid water content that freezes does not vary with updraft strength, the freezing of more numerous droplets in the faster updrafts simply produces smaller ice particles. This is clearly shown by Ackerman et al (2015), in which ice particle mass distributions in homogeneous freezing for stronger updrafts produce substantially smaller ice particles. Schnaiter et al (2016) further showed that high ice particle growth rates also enhance the formation of small-scale complexity, such as ice particle surface roughness.…”