The Quarterly Journal of the Royal Meteorological Society

Luo, Dehai; Cha, Jing; Zhong, Linhao; Dai, Aiguo

doi: 10.1002/qj.2337pmid: N/A

In this article, a nonlinear multiscale interaction (NMI) model is used to propose an eddy‐blocking matching (EBM) mechanism to account for how synoptic eddies reinforce or suppress a blocking flow. It is shown that the spatial structure of the eddy vorticity forcing (EVF) arising from upstream synoptic eddies determines whether an incipient block can grow into a meandering blocking flow through its interaction with the transient synoptic eddies from the west. Under certain conditions, the EVF exhibits a low‐frequency oscillation on time‐scales of 2–3 weeks. During the EVF phase with a negative‐over‐ positive dipole structure, a blocking event can be resonantly excited through the transport of eddy energy into the incipient block by the EVF. As the EVF changes into an opposite phase, the blocking decays. The NMI model produces life cycles of blocking events that resemble observations. Moreover, it is shown that the eddy north–south straining is a response of the eddies to a dipole‐ or Ω‐type block. In our model, as in observations, two synoptic anticyclones (cyclones) can attract and merge with one another as the blocking intensifies, but only when the feedback of the blocking on the eddies is included. Thus, we attribute the eddy straining and associated vortex interaction to the feedback of the intensified blocking on synoptic eddies. The results illustrate the concomitant nature of the eddy deformation, the role of which, as a potential vorticity source for the blocking flow, becomes important only during the mature stage of a block. Our EBM mechanism suggests that an incipient block flow is amplified (or suppressed) under certain conditions by the EVF coming from the upstream of the blocking region. This also suggests that weather and climate models need to be run with a grid size below 100 km in order to simulate the matching EVF and thus atmospheric blocking.

Martínez‐Alvarado, O.; Joos, H.; Chagnon, J.; Boettcher, M.; Gray, S. L.; Plant, R. S.; Methven, J.; Wernli, H.

doi: 10.1002/qj.2276pmid: N/A

The warm conveyor belt (WCB) of an extratropical cyclone generally splits into two branches. One branch (WCB1) turns anticyclonically into the downstream upper‐level tropospheric ridge, while the second branch (WCB2) wraps cyclonically around the cyclone centre. Here, the WCB split in a typical North Atlantic cold‐season cyclone is analysed using two numerical models: the Met Office Unified Model and the COSMO model. The WCB flow is defined using off‐line trajectory analysis. The two models represent the WCB split consistently. The split occurs early in the evolution of the WCB with WCB1 experiencing maximum ascent at lower latitudes and with higher moisture content than WCB2. WCB1 ascends abruptly along the cold front where the resolved ascent rates are greatest and there is also line convection. In contrast, WCB2 remains at lower levels for longer before undergoing saturated large‐scale ascent over the system's warm front. The greater moisture in WCB1 inflow results in greater net potential temperature change from latent heat release, which determines the final isentropic level of each branch. WCB1 also exhibits lower outflow potential vorticity values than WCB2.

Steinhoff, Daniel F.; Bromwich, David H.; Speirs, Johanna C.; McGowan, Hamish A.; Monaghan, Andrew J.

doi: 10.1002/qj.2278pmid: N/A

Foehn winds are a prominent feature of the McMurdo Dry Valleys (MDVs) climate, and are responsible for periods of strong winds and warming. The foehn mechanism determined from a case study presented in earlier work is shown here to be robust for a set of the MDVs summer foehn events over the 1994–2009 period using output from the Polar Weather Research and Forecasting Model (Polar WRF). Gap flow south of the MDVs is evidenced by the positive relationship between the pressure gradient and near‐surface wind speed along the gap. Subsequently, mountain waves are generated and result in adiabatic warming and the downward transport of warm air into the MDVs, and differences in mountain wave characteristics depend on the ambient wind direction and the degree of flow nonlinearity. Pressure‐driven channelling then brings warm foehn air downvalley.

Sukhatme, Jai

doi: 10.1002/qj.2264pmid: N/A

Motivated by observations of the mean state of tropical precipitable water (PW), a moist, first baroclinic mode, shallow‐water system on an equatorial β‐plane with a background saturation profile that depends on latitude and longitude is studied. In the presence of a latitudinal moisture gradient, linear analysis of the non‐rotating problem reveals large‐scale, symmetric, eastward and westward propagating unstable modes. The introduction of a zonal moisture gradient breaks the east–west symmetry of the unstable modes. The effects of rotation are then included by numerically solving the resulting eigenvalue problem on an equatorial β‐plane. With a purely meridional moisture gradient, the system supports large‐scale, low‐frequency, eastward and westward moving neutral modes. Some of the similarities, and some of the discrepancies of these modes with intraseasonal tropical waves are pointed out. Finally, a zonal moisture gradient in the presence of rotation renders some of the aforementioned neutral modes unstable. In particular, according to observations of large‐scale, low‐frequency tropical variability, it is seen that regions where the background saturation profile increases (decreases) to the east favour eastward (westward) moving moist modes.

Sun, Xiaoming; Barros, Ana P.

doi: 10.1002/qj.2255pmid: N/A

The Weather Research and Forecasting (WRF) model was used to simulate the evolution of tropical storm Ivan (2004) in the southeast United States using both the Yonsei University (YSU) and Mellor–Yamada–Janjić (MYJ) boundary‐layer parametrizations. In contrast to tropical cyclone (TC) simulations over the ocean, the effect of the surface layer becomes secondary for a dissipating hurricane along its terrestrial track. Although these two schemes can reproduce Ivan reasonably well, our results suggest that the mixing properties for damped mechanical turbulent conditions (weakly stable) are strongly underestimated by both parametrizations. This underestimation impacts the thermodynamic properties of the storm, leading to significant differences in the storm areal extent and the simulated precipitation fields. Suggestions for further improvements are provided.

Li, Yubin; Cheung, Kevin K. W.; Chan, Johnny C. L.

doi: 10.1002/qj.2259pmid: N/A

Idealized simulations on an f‐plane of tropical cyclone (TC) landfall under a quiescent environment in the Southern Hemisphere are performed to investigate the effects of land–sea surface contrast on precipitation. In the control simulation, with realistic roughness and moisture over land to the south of the TC, the simulated vortex moves toward land due to a land‐induced steering flow. The abrupt decrease (increase) of tangential wind at the surface leads to convergence (divergence) on the onshore (offshore) flow side. Enhanced convergence at the top of the planetary boundary layer is found on both onshore and offshore sides and is caused by advection from the surface and the enhanced offshore radial wind. The boundary‐layer top convergence pattern is consistent with the rainfall distribution. Vertical wind shear develops during the landfall process associated with the low‐ and upper‐level asymmetric flows across the model domain. The wavenumber‐1‐like low‐level asymmetric flow is introduced by an asymmetric geopotential height field that is generated by the large area of frictionally induced convergence on the onshore side and divergence on the offshore side. The upper‐level asymmetric flow is attributed to asymmetric convection and associated diabatic heating after landfall. Most rainfall is found in the down‐shear right quadrant, which is consistent with previous studies that focused on the effect of environmental shear. Although the existence of feedback from the vertical wind shear to rainfall remains an open question, the relation between maximum rainfall and vertical wind shear is robust, especially when the shear magnitude is large after landfall.

Hankinson, Mai C. N.; Reeder, Michael J.; Davidson, Noel E.; Puri, Kamal

doi: 10.1002/qj.2275pmid: N/A

In an earlier numerical study, Nguyen et al. found that as hurricane Katrina (2005) intensified it vacillated between highly asymmetric and relatively symmetric states. The characteristics of vacillation cycles produced in a series of high‐resolution simulations of hurricane Katrina are reported. As before, these simulations are made with the Australian Bureau of Meteorology's Tropical Cyclone Limited Area Prediction System (TCLAPS). It is found that the majority of the simulated vortices exhibit vacillation cycles during the 48 h integration from 0000 UTC on 27 August 2005. The dominant periods of vacillation range from 8–23 h with a median of about 13 h. In most simulations with vacillation cycles, the more asymmetric states are associated with lower intensification rates (as measured by the rate of change of the maximum azimuthally averaged tangential wind). Both barotropic and convective instabilities are shown to be important for the development of these vacillation cycles.

Vitart, Frédéric

doi: 10.1002/qj.2256pmid: N/A

Sub‐seasonal forecasts have been routinely produced at ECMWF since 2002 with reforecasts produced ‘on the fly’ to calibrate the real‐time sub‐seasonal forecasts. In this study, the skill of the reforecasts from April 2002 to March 2012 and covering a common set of years (1995 to 2001) has been evaluated. Results indicate that the skill of the ECMWF reforecasts to predict the Madden–Julian Oscillation (MJO) has improved significantly since 2002, with an average gain of about 1 day of prediction skill per year. The amplitude of the MJO has also become more realistic, although the model still tends to produce MJOs which are weaker than in the ECMWF re‐analysis. As a consequence, the ability of the ECMWF model to simulate realistic MJO teleconnections over the Northern and Southern Extratropics has improved dramatically over the 10‐year period. Forecast skill scores have also improved in the Extratropics. For instance, weekly mean forecasts of the North Atlantic Oscillation Index are more skilful in recent years than 10 years ago. A large part of this improvement seems to be linked to the improvements in the representation of the MJO. Skill to predict 2 m temperature anomalies over the Northern Extratropics has also improved almost continuously since 2002. Changes in the horizontal and vertical resolutions of the atmospheric model had only a small impact on the skill scores, suggesting that most of the improvements in the ECMWF sub‐seasonal forecasts were due to changes in model physics which were primarily designed to improve the model climate and medium‐range forecasts. The impact of changes in the data assimilation system and in the observing data has not been considered in this study, since all the reforecasts used for this study were initialized from the same re‐analysis over a common set of years.

Hally, A.; Richard, E.; Fresnay, S.; Lambert, D.

doi: 10.1002/qj.2257pmid: N/A

Heavy precipitation events (HPEs) affect the southeastern area of France frequently during the months of September–November. Very high amounts of rain can fall during these events, with the ensuing flash floods causing widespread damage. The cases of 6 September 2010 and 1–4 November 2011 represent the different large‐scale conditions under which these episodes can occur. These HPEs are forecast with differing levels of skill by the Méso‐NH model at 2.5 km resolution. The case of 6 September 2010 is used to test different methods of addressing cloud physics parametrization uncertainties. Three ensembles are constructed, where the warm‐process microphysical time tendencies are perturbed by different methods. Results are compared by examining the spatio‐temporal distribution of the precipitation field as well as looking at ensemble statistics. The ensemble methodology that induces the most dispersion in the rainfall field is deemed the most suitable. This method is then used to examine the sensitivity of four cases from November 2011 to errors in the microphysical and turbulent parametrizations. It appears that the sensitivity to microphysical perturbations varies according to the model skill for the HPE. Events where the model skill is high (low) show low (moderate) sensitivity. These cases show a stronger sensitivity to perturbations performed upon the turbulent tendencies, while perturbing the microphysical and turbulent tendencies together produces even greater dispersion. The results show the importance and usefulness of ensembles with perturbed physical parametrizations in the forecasting of HPEs.

Ferro, C. A. T.

doi: 10.1002/qj.2270pmid: N/A

The notion of fair scores for ensemble forecasts was introduced recently to reward ensembles with members that behave as though they and the verifying observation are sampled from the same distribution. In the case of forecasting binary outcomes, a characterization is given of a general class of fair scores for ensembles that are interpreted as random samples. This is also used to construct classes of fair scores for ensembles that forecast multicategory and continuous outcomes. The usual Brier, ranked probability and continuous ranked probability scores for ensemble forecasts are shown to be unfair, while adjusted versions of these scores are shown to be fair. A definition of fairness is also proposed for ensembles with members that are interpreted as being dependent and it is shown that fair scores exist only for some forms of dependence.