Modelling atmospheric structure, cloud and their response to CCN in the central Arctic: ASCOS case studies

Abstract

Abstract.Observations made during late summer in the central Arctic Ocean, as part of the Arctic Summer Cloud Ocean Study (ASCOS), are used to evaluate cloud and vertical temperature structure in the Met Office Unified Model (MetUM). The observation period can be split into 5 regimes; the first two regimes had a large number of frontal systems, which were associated with deep cloud. During the remainder of the campaign a layer of low-level cloud occurred, typical of central Arctic summer conditions, along with two periods of greatly reduced cloud cover. The short-range operational NWP forecasts could not accurately reproduce the observed variations in near-surface temperature. A major source of this error was found to be the temperature-dependant surface albedo parameterisation scheme. The model reproduced the low-level cloud layer, though it was too thin, too shallow, and in a boundary-layer that was too frequently well-mixed. The model was also unable to reproduce the observed periods of reduced cloud cover, which were associated with very low cloud condensation nuclei (CCN) concentrations (<1 cm −3 ). As with most global NWP models, the MetUM does not have a prognostic aerosol/cloud scheme but uses a constant CCN concentration of 100 cm −3 over all marine environments. It is therefore unable to represent the low CCN number concentrations and the rapid variations in concentration frequently observed in the central Arctic during late summer. Experiments with a single-column model configuration of the MetUM show that reducing model CCN number concentrations to observed values reduces the amount of cloud, increases the near-surface stability, and improves the representation of both the surface radiation fluxes and the surface temperature. The model is shown to be sensitive to CCN only when number concentrations are less than 10-20 cm −3 .

Meteorological conditions in the central Arctic summer during the Arctic Summer Cloud Ocean Study (ASCOS)

“…Many studies of different types, using different instrumentation, show a general consensus that the central Arctic Ocean freeze onset often occurs between the 2nd week of August and early September (e.g. Rigor et al, 2000;Belchansky et al, 2004;Overland et al, 2008). One oftenused definition is the first time that a running-mean nearsurface air temperature falls below differently defined thresholds, e.g.…”

“…1 Operational meteorological analysis and forecasts for the expedition were supplied by the European Centre for Medium Range Weather Forecasts (ECMWF) for large-scale forecasts and the UK Met Office for detailed column forecasts. In this paper we use the ECMWF analyses while the UK Met Office Unified Model forecasts are presented and analyzed in Birch et al (2012). To compare ASCOS conditions to climatology we use the NCAR/NCEP reanalysis products available from the National Oceanographic and Atmospheric Administration/Earth System Research Laboratory (NOAA/ESRL; http://www.esrl.noaa.gov/psd).…”

The role of ice nuclei recycling in the maintenance of cloud ice in Arctic mixed-phase stratocumulus

“…AMPS are challenging to model due to uncertainties in ice microphysical processes that determine phase partitioning between ice and radiatively important cloud liquid water (Sandvik et al, 2007;Tjernström et al, 2008;Klein et al, 2009, Karlsson andSvensson, 2011;Barton et al, 2012;Birch et al, 2012;de Boer et al, 2012), which drives turbulence that maintains the system. Phase partitioning depends upon the number, shape, and size of ice crystals, since these determine the efficiency of water vapor uptake by ice and hence the availability of water vapor for droplet formation (Chen and Lamb, 1994;Sheridan et al, 2009;Ervens et al, 2011;Hoose and Möhler, 2012).…”

A seamless approach to understanding and predicting Arctic sea ice in Met Office modelling systems

“…Budget analysis of the atmospheric energy budget in the Arctic suggests that at least one part of the problem with the Arctic simulation in the Hadley Centre climate models may be the simulation of summer cloud. This was also seen in the short-range forecast simulations of Birch et al [41] and was improved via changes to the cloud scheme. At the Met Office, we are fortunate to use the same model across a range of time scales (so-called seamless prediction).…”

“…The Met Office global NWP version of the UM can provide some insights into this issue. The model was evaluated during Arctic summer against field experiment data from the Arctic Ocean Experiment (AOE 2001) [40] and Arctic Summer Cloud Ocean Study (ASCOS 2007) [41]. This analysis shows that the UM has reasonable skill in predicting the passage of large frontal systems on a 1-to 3-day time scale, with deep frontal cloud being well captured.…”