Interaction of microphysics and dynamics in a warm conveyor belt simulated with the ICOsahedral Nonhydrostatic (ICON) model

Warm conveyor belts (WCBs) produce a major fraction of precipitation in extratropical cyclones and modulate the large-scale extratropical circulation. Diabatic processes, in particular associated with cloud formation, influence the cross-isentropic ascent of WCBs into the upper troposphere and additionally modify the potential vorticity (PV) distribution, which influences the larger-scale flow. In this study we investigate heating and PV rates from all diabatic processes, including microphysics, turbulence, convection, and radiation, in a case study that occurred during the North Atlantic Waveguide and Downstream Impact Experiment (NAWDEX) campaign using the Icosahedral Nonhydrostatic (ICON) modeling framework. In particular, we consider all individual microphysical process rates that are implemented in ICON\u27s two-moment microphysics scheme, which sheds light on (i) which microphysical processes dominate the diabatic heating and PV structure in the WCB and (ii) which microphysical processes are the most active during the ascent and influence cloud formation and characteristics, providing a basis for detailed sensitivity experiments. For this purpose, diabatic heating and PV rates are integrated for the first time along online trajectories across nested grids with different horizontal resolutions. The convection-permitting simulation setup also takes the reduced aerosol concentrations over the North Atlantic into account. Our results confirm that microphysical processes are the dominant diabatic heating source during ascent. Near the cloud top longwave radiation cools WCB air parcels. Radiative heating and corresponding PV modification in the upper troposphere are non-negligible due to the longevity of the WCB cloud band. In the WCB ascent region, the process rates from turbulent heating and microphysics partially counteract each other. From all microphysical processes condensational growth of cloud droplets and vapor deposition on frozen hydrometeors most strongly influence diabatic heating and PV, while below-cloud evaporation strongly cools WCB air parcels prior to their ascent and increases their PV value. PV production is the strongest near the surface with substantial contributions from condensation, melting, evaporation, and vapor deposition. In the upper troposphere, PV is reduced by diabatic heating from vapor deposition, condensation, and radiation. Activation of cloud droplets as well as homogeneous and heterogeneous freezing processes have a negligible diabatic heating contribution, but their detailed representation is important for, e.g., hydrometeor size distributions. Generally, faster-ascending WCB trajectories are heated markedly more than more slowly ascending WCB trajectories, which is linked to larger initial specific humidity content providing a thermodynamic constraint on total microphysical heating. Yet, the total diabatic heating contribution of convectively ascending trajectories is relatively small due to their small fraction in this case study. Our detailed case study documents the effect of different microphysical processes implemented in ICON\u27s two-moment scheme for heating and PV rates in a WCB from a joint Eulerian and Lagrangian perspective. It emphasizes the predominant role of microphysical processes and provides a framework for future experiments on cloud microphysical sensitivities in WCBs

Revisiting the latent heating contribution to foehn warming - Lagrangian analysis of two foehn events over the Swiss Alps

Foehn flows are typically associated with warm air temperatures. Though several theories for the so-called foehn air warming have been developed over the past century, no conclusion about the most important mechanism has been reached. The development of new methods to calculate accurate air-mass trajectories over complex topography has opened up a new perspective on this question. Air-mass-trajectories derived from wind-field data from COSMO model simulations with 20 s temporal resolution are used in this study to investigate the origin of the foehn air and the contribution of adiabatic and diabatic processes for two foehn events in the Swiss Alps, with a focus on the Rhine valley. The first foehn event investigated has no precipitation on the upstream side of the Alps. The majority of air parcels stem from upstream altitudes above 1.8 km and most of the foehn air warming is due to adiabatic descent (∼79%). In the second event investigated, significant upstream precipitation occurred. For this case, a significantly larger fraction of the foehn air parcels originate within the lowest 2 km of the upstream atmosphere (up to 70%). Adiabatic descent accounts for the largest part of the temperature change (∼70%), while moist diabatic processes explain about 60% of the potential temperature change. The vertical displacement across the Alpine range is correlated with the diabatic temperature change: parcels strongly heated by condensation, deposition and freezing are in general found at high altitudes above the foehn valley, while parcels affected by diabatic cooling through evaporation, sublimation and melting arrive closer to the valley floor. The high-resolution trajectories also indicate a much more complicated vertical and horizontal flow pattern than generally assumed, with several distinct air streams upstream of the mountain range and vertical ‘scrambling’ of air masses

An online trajectory module (version 1.0) for the nonhydrostatic numerical weather prediction model COSMO

A module to calculate online trajectories has been implemented into the nonhydrostatic limited-area weather prediction and climate model COSMO. Whereas offline trajectories are calculated with wind fields from model output, which is typically available every one to six hours, online trajectories use the simulated resolved wind field at every model time step (typically less than a minute) to solve the trajectory equation. As a consequence, online trajectories much better capture the short-term temporal fluctuations of the wind field, which is particularly important for mesoscale flows near topography and convective clouds, and they do not suffer from temporal interpolation errors between model output times. The numerical implementation of online trajectories in the COSMO-model is based upon an established offline trajectory tool and takes full account of the horizontal domain decomposition that is used for parallelization of the COSMO-model. Although a perfect workload balance cannot be achieved for the trajectory module (due to the fact that trajectory positions are not necessarily equally distributed over the model domain), the additional computational costs are found to be fairly small for the high-resolution simulations described in this paper. The computational costs may, however, vary strongly depending on the number of trajectories and trace variables. Various options have been implemented to initialize online trajectories at different locations and times during the model simulation. As a first application of the new COSMO-model module, an Alpine north foehn event in summer 1987 has been simulated with horizontal resolutions of 2.2, 7 and 14 km. It is shown that low-tropospheric trajectories calculated offline with one- to six-hourly wind fields can significantly deviate from trajectories calculated online. Deviations increase with decreasing model grid spacing and are particularly large in regions of deep convection and strong orographic flow distortion. On average, for this particular case study, horizontal and vertical positions between online and offline trajectories differed by 50–190 km and 150–750 m, respectively, after 24 h. This first application illustrates the potential for Lagrangian studies of mesoscale flows in high-resolution convection-resolving simulations using online trajectories.ISSN:1991-9603ISSN:1991-959

Secondary Ice Formation in Idealised Deep Convection—Source of Primary Ice and Impact on Glaciation

Secondary ice production via rime-splintering is considered to be an important process for rapid glaciation and high ice crystal numbers observed in mixed-phase convective clouds. An open question is how rime-splintering is triggered in the relatively short time between cloud formation and observations of high ice crystal numbers. We use idealised simulations of a deep convective cloud system to investigate the thermodynamic and cloud microphysical evolution of air parcels, in which the model predicts secondary ice formation. The Lagrangian analysis suggests that the “in-situ” formation of rimers either by growth of primary ice or rain freezing does not play a major role in triggering secondary ice formation. Instead, rimers are predominantly imported into air parcels through sedimentation form higher altitudes. While ice nucleating particles (INPs) initiating heterogeneous freezing of cloud droplets at temperatures warmer than −10 °C have no discernible impact of the occurrence of secondary ice formation, in a scenario with rain freezing secondary ice production is initiated slightly earlier in the cloud evolution and at slightly different places, although with no major impact on the abundance or spatial distribution of secondary ice in the cloud as a whole. These results suggest that for interpreting and analysing observational data and model experiments regarding cloud glaciation and ice formation it is vital to consider the complex vertical coupling of cloud microphysical processes in deep convective clouds via three-dimensional transport and sedimentation

A trajectory-based classification of ERA-Interim ice clouds in the region of the North Atlantic storm track

A two-type classification of ice clouds (cirrus) is introduced, based on the liquid and ice water content, LWC and IWC, along air parcel backward trajectories from the clouds. In situ cirrus has no LWC along the trajectory segment containing IWC; it forms via nucleation from the gas phase. In contrast, liquid-origin cirrus has both LWC and IWC along their backward trajectories; it forms via lifting from the lower troposphere and freezing of mixed-phase clouds. This classification is applied to 12 years of ERA-Interim ice clouds in the North Atlantic region. Between 400 and 500 hPa more than 50% are liquid-origin cirrus, whereas this frequency decreases strongly with altitude (<10% at 200 hPa). The relative frequencies of the two categories vary only weakly with season. More than 50% of in situ cirrus occur on top of liquid-origin cirrus, indicating that they often form in response to the strong lifting accompanying the formation of liquid-origin cirrus

A scaling relation for warm-phase orographic precipitation: a Lagrangian analysis for 2D mountains

The precipitation produced by orographic clouds is the result of the interaction between nonlinear dynamical and microphysical processes. Focusing on warm-phase stable clouds, we propose that the precipitation efficiency is essentially controlled by the bulk Damköhler number of the system, i.e. the ratio of the characteristic advective to the microphysical time-scale of the cloud. These time-scales are investigated thoroughly for single air parcels along temporally highly resolved trajectories from quasi-two-dimensional numerical simulations with vertical Froude numbers between 0.38 and 2.31. Based on these results, analytical formulations for the Damköhler number of single parcels and for a bulk Damköhler number for the entire cloud are developed, which depend only on the far upstream flow properties and the cloud droplet number density. Relating the bulk Damköhler numbers to the precipitation efficiencies from the numerical simulations results in a unique scaling relation between the two non-dimensional numbers: no precipitation is observed for very small Damköhler numbers, while for large Damköhler numbers the precipitation efficiency asymptotically approaches a maximum value of about 0.8. For intermediate Damköhler numbers, the precipitation efficiency depends strongly on the microphysical properties of the clouds