Can Climate Models Capture the Structure of Extratropical Cyclones?

Abstract

Composites of wind speeds, equivalent potential temperature, mean sea level pressure, vertical velocity, and relative humidity have been produced for the 100 most intense extratropical cyclones in the Northern Hemisphere winter for the 40-yr ECMWF Re-Analysis (ERA-40) and the high resolution global environment model (HiGEM). Features of conceptual models of cyclone structure-the warm conveyor belt, cold conveyor belt, and dry intrusion-have been identified in the composites from ERA-40 and compared to HiGEM. Such features can be identified in the composite fields despite the smoothing that occurs in the compositing process. The surface features and the three-dimensional structure of the cyclones in HiGEM compare very well with those from ERA-40. The warm conveyor belt is identified in the temperature and wind fields as a mass of warm air undergoing moist isentropic uplift and is very similar in ERA-40 and HiGEM. The rate of ascent is lower in HiGEM, associated with a shallower slope of the moist isentropes in the warm sector. There are also differences in the relative humidity fields in the warm conveyor belt. In ERA-40, the high values of relative humidity are strongly associated with the moist isentropic uplift, whereas in HiGEM these are not so strongly associated. The cold conveyor belt is identified as rearward flowing air that undercuts the warm conveyor belt and produces a low-level jet, and is very similar in HiGEM and ERA-40. The dry intrusion is identified in the 500-hPa vertical velocity and relative humidity. The structure of the dry intrusion compares well between HiGEM and ERA-40 but the descent is weaker in HiGEM because of weaker alongisentrope flow behind the composite cyclone. HiGEM's ability to represent the key features of extratropical cyclone structure can give confidence in future predictions from this model.

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

“…The tracking algorithm used in this study (e.g., Hodges 1994Hodges , 1995Hodges , 1999, has previously been applied in studies of ETCs (e.g., Bengtsson et al 2007Bengtsson et al , 2009Catto et al 2010;Dacre et al 2012;Hawcroft et al 2012;Zappa et al 2014;Hawcroft et al 2015). Northern Hemisphere ETCs are identified as features exceeding 1 × 10 −5 s −1 in the 850 hPa relative vorticity field, truncated to T42, in 3-hourly data and combined into cyclone tracks.…”

“…HiGEM has previously been shown to be capable of capturing the large-scale features of composite ETCs when compared to reanalysis (ERA-40, Uppala et al 2005) though with differences in the finer scale details of the structure of the warm conveyor belt (Catto et al 2010). HiGEM has also been shown to reproduce estimates of precipitation around composite ETCs that are broadly comparable to the ERAI reanalysis and GPCP daily dataset (Hawcroft et al 2015), indicating that the total column integrated latent heating in the model is in agreement with observations.…”

“…Having established uncertainty in the fidelity with which these fields are reproduced in both model and reanalysis, latent heating and associated processes in the model are then directly compared to reanalysis. For the first time this study combines both remote sensing data (see e.g., Field et al 2008Field et al , 2011Govekar et al 2014;Booth et al 2013) and reanalysis (see e.g., Bengtsson et al 2006Bengtsson et al , 2009Catto et al 2010) to evaluate a climate model within a composite cyclone centred framework, allowing both the structure of cloud features to be evaluated and the underlying processes within the model which are responsible for any errors in the representation of those features to be investigated. The study focusses on winter (DJF), northern hemisphere ETCs since this is the season when the stormtracks and ETCs in the Atlantic and Pacific are at their most intense.…”

Quantifying the climatological relationship between extratropical cyclone intensity and atmospheric precursors

“…[6] Following the work by Catto et al [2010], an objective feature tracking algorithm [Hodges, 1994[Hodges, , 1995 has been applied to the six-hourly European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis, ERA-Interim [Dee et al, 2011]. The data covers the period from March 1989 to February 2009.…”

“…(Note that the data shown in the figures only extends to 15 in radius for clarity). Following Catto et al [2010], the precursor fields are rotated according to the true direction of travel of each cyclone such that the direction of travel becomes eastward.…”