Exploring the Diabatic Role of Ice Microphysical Processes in Two North Atlantic Summer Cyclones

Abstract

Numerical simulations are performed with the Weather Research and Forecasting Model to elucidate the diabatic effects of ice phase microphysical processes on the dynamics of two slow-moving summer cyclones that affected the United Kingdom during the summer of 2012. The first case is representative of a typical midlatitude storm for the time of year, while the second case is unusually deep. Sensitivity tests are performed with 5-km horizontal grid spacing and at lead times between 1 and 2 days using three different microphysics schemes, one of which is a new scheme whose development was informed by the latest in situ observations of midlatitude weather systems. The effects of latent heating and cooling associated with deposition growth, sublimation, and melting of ice are assessed in terms of the impact on both the synoptic scale and the frontal scale. The results show that, of these diabatic processes, deposition growth was the most important in both cases, affecting the depth and position of each of the low pressure systems and influencing the spatial distribution of the frontal precipitation. Cooling associated with sublimation and melting also played a role in determining the cyclone depth, but mainly in the more intense cyclone case. The effects of ice crystal habit and secondary ice production are also explored in the simulations, based on insight from in situ observations. However in these two cases, the ability to predict changes in crystal habit did not significantly impact the storm evolution, and the authors found no obvious need to parameterize secondary ice crystal production at the model resolutions considered.

Ice lollies: An ice particle generated in supercooled conveyor belts

“…Dearden et al [] used the WRF numerical model to simulate two summer cyclones to gain an understanding of sensitivities to ice microphysics, including the role of primary versus secondary ice formation. Primary ice formation describes the nucleation of new ice particles via homogenous freezing of liquid droplets at cold temperatures (< −37°C [ Koop et al , ]) or via heterogeneous processes involving ice nuclei [ Vali et al , ].…”

“…Browning and Roberts [1994] further developed the understanding of frontal cyclones using model output and satellite images and discussed the dynamical factors leading to the formation of various cloud features. Dearden et al [2016] used the WRF numerical model to simulate two summer cyclones to gain an understanding of sensitivities to ice microphysics, including the role of primary versus secondary ice formation. Primary ice formation describes the nucleation of new ice particles via homogenous freezing of liquid droplets at cold temperatures (< À37°C [Koop et al, 2000]) or via heterogeneous processes involving ice nuclei [Vali et al, 2015].…”

“…Secondary ice formation processes, often referred to as "ice multiplication processes," increase the number of ice particles in clouds by fragmenting existing ice particles in various ways, such as freezing and shattering of water drops [Johnson and Hallett, 1968;Pruppacher and Schlamp, 1975;Lawson et al, 2015], rime splintering [Mossop and Hallett, 1974] fragmentation following ice crystal collisions [Hobbs and Farber, 1972;Vardiman, 1978], and fragmentation of ice due to evaporation and melting [Knight, 1979]. Dearden et al [2016] found that in deep frontal systems, precipitation was dominated by primary ice production mechanisms, and that secondary processes had no significant impact.…”

Using satellite and reanalysis data to evaluate the representation of latent heating in extratropical cyclones in a climate model

“…It is well known that latent heating can play a significant role in the evolution of ETCs (e.g., Manabe 1956;Sanders and Gyakum 1980;Davis et al 1993;Zhu and Newell 1994;Stoelinga 1996;Pomroy and Thorpe 2000) though this can vary substantially on a case-to-case basis (e.g., Kuo and Low-Nam 1990;Smith 2000;Wernli et al 2002;Dacre and Gray 2009;Fink et al 2012;Dearden et al 2016). It has been shown that latent heating can significantly influence the evolution and deepening of some of the most damaging storms (e.g., Ulbrich et al 2001;Liberato et al 2011) and has been shown to influence frontal structure and propagation around ETCs (see Posselt and Martin 2004;Reeves and Lackmann 2004).…”

The Role of Secondary Ice Processes in Midlatitude Continental Clouds

“…This process is often called rime splintering or the Hallett-Mossop process (H-M process). Numerical models often use the Hallett-Mossop process to describe SIP (Field et al, 2017), using the same size threshold (droplets radius > 12 μm) and temperature range (À3 to À8°C; Connolly et al, 2006;Crawford et al, 2012;Dearden et al, 2016;Fridlind et al, 2007;Scott et al, 1977). Another study suggested that the concentration of cloud condensation nuclei (CCN) that affects the cloud droplet size distribution (Andreae et al, 2004;Braga et al, 2017;Mossop, 1985) has a greater influence on ice content in clouds via SIP than the actual INP concentration (Sullivan et al, 2018).…”