Vertical cloud structure of warm conveyor belts – a comparison and evaluation of ERA5 reanalysis, CloudSat and CALIPSO data

Abstract

Abstract. Warm conveyor belts (WCBs) are important cyclone-related airstreams that are responsible for most of the cloud and precipitation formation in the extratropics. They can also substantially influence the dynamics of cyclones and the upper-level flow. So far, most of the knowledge about WCBs is based on model data from analyses, reanalyses and forecast data with only a few observational studies available. The aim of this work is to gain a detailed observational perspective on the vertical cloud and precipitation structure of WCBs during their inflow, ascent and outflow and to evaluate their representation in the new ERA5 reanalysis dataset. To this end, satellite observations from the CloudSat radar and the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) lidar are combined with an ERA5-based WCB climatology for nine Northern Hemisphere winters. Based on a case study and a composite analysis, the main findings can be summarized as follows. (i) WCB air masses are part of deep, strongly precipitating clouds, with cloud-top heights at 9–10 km during their ascent and an about 2–3 km deep layer with supercooled liquid water co-existing with ice above the melting layer. The maximum surface precipitation occurs when the WCB is at about 2–4 km height. (ii) Convective clouds can be observed above the inflow and during the ascent. (iii) At upper levels, the WCB outflow is typically located near the top of a 3 km deep cirrus layer. (iv) There is a large variability between WCBs in terms of cloud structure, peak reflectivity and associated surface precipitation. (v) The WCB trajectories with the highest radar reflectivities are mainly located over the North Atlantic and North Pacific, and – apart from the inflow – they occur at relatively low latitudes. They are associated with particularly deep and strongly precipitating clouds that occur not only during the ascent but also in the inflow and outflow regions. (vi) ERA5 represents the WCB clouds remarkably well in terms of position, thermodynamic phase and frozen hydrometeor distribution, although it underestimates the high ice and snow values in the mixed-phase clouds near the melting layer. (vii) In the lower troposphere, high potential vorticity is diabatically produced along the WCB in areas with high reflectivities and hydrometeor contents, and at upper levels, low potential vorticity prevails in the cirrus layer in the WCB outflow. The study provides important observational insight into the internal cloud structure of WCBs and emphasizes the ability of ERA5 to essentially capture the observed pattern but also reveals many small- and mesoscale structures observed by the remote sensing instruments but not captured by ERA5.

Warm conveyor belts in present-day and future climate simulations – Part 2: Role of potential vorticity production for cyclone intensification

“…WCBs are coherent airstreams that originate in the cyclone's warm sector and rapidly move poleward while ascending to the upper troposphere. They form elongated cloud bands with warm clouds in their inflow, mixed-phase clouds at mid-levels, and ice clouds in the upper troposphere (Joos and Wernli, 2012;Wernli et al, 2016;Binder et al, 2020). The intense cloud-diabatic processes within the ascending airstreams modify potential vorticity (PV) in the atmosphere (Wernli and Davies, 1997;Madonna et al, 2014).…”

Warm conveyor belts in present-day and future climate simulations. Part II: Role of potential vorticity production for cyclone intensification

“…WCBs are coherent airstreams that originate in the cyclone's warm sector and rapidly move poleward while ascending to the upper troposphere. They form elongated cloud bands with warm clouds in their inflow, mixed-phase clouds at mid-levels and ice clouds in the upper troposphere (Joos and Wernli, 2012;Wernli et al, 2016;Binder et al, 2020). The intense cloud-diabatic processes within the ascending airstreams modify potential vorticity (PV) in the atmosphere (Wernli and Davies, 1997;Madonna et al, 2014).…”

Lagrangian matches between observations from aircraft, lidar and radar in a warm conveyor belt crossing orography

“…The modelling study by Joos and Forbes (2016) showed that the vertical cloud structure of a WCB is affected by seemingly small changes in the implementation of the microphysics, with direct implications for the detailed shape and amplitude of the upper-tropospheric ridge influenced by the WCB outflow. More recently, Binder et al (2020) systematically compared the vertical structure of WCB-related clouds in the latest European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis dataset, ERA5 (Hersbach et al, 2020), with profiles derived from satellite-based lidar and radar observations and concluded that the model reproduces the overall cloud structure quite well but underestimates ice and snow water content in the mixed-phase layer in WCBs above the melting layer. Such weaknesses in models might arise from various assumptions made in microphysical parameterisations that account for the subgrid-scale nature of the cloud processes and often lead to uncertainties in NWP (Illingworth et al, 2007;Rodwell et al, 2018).…”

“…Their sensitivity experiments with the ECMWF forecast model revealed that with the corrected low-level moisture in the initial conditions the WCB outflow would have occurred at a lower potential temperature level and produced a less developed upper-level ridge. The humidity in the WCB inflow, however, is not only determined by boundary layer ventilation (Boutle et al, 2011;Pfahl et al, 2014). It can also be affected by a recycling of moisture within the WCB, which occurs, e.g., when raindrops from an elevated layer of the WCB fall into a sub-saturated lower layer of the WCB inflow where they evaporate (Crezee et al, 2017;Attinger et al, 2019;Spreitzer, 2020).…”

“…Most observational studies of WCBs so far, e.g. based on surface radar measurements (Browning, 1971) or more recently on aircraft and satellite data (Oertel et al, 2019;Binder et al, 2020), provide limited information about the variability of the WCB characteristics along the ascent, which typically covers spatial and temporal dimensions of > 1000 km and 1 d, respectively. To better address the Lagrangian nature of these airstreams, a few pioneering field campaign studies attempted to follow the pathway of a WCB with an aircraft and to measure the same WCB air parcels multiple times (e.g.…”