Huw C. Davies

Limitations of Some Common Lateral Boundary Schemes used in Regional NWP Models

Abstract A brief critical assessment is presented of several lateral boundary schemes currently employed in regional weather prediction models. Simple flow models are used to determine the nature and cause of the primary shortcomings of each of the considered schemes. An awareness of these deficiencies can prove helpful in the implementation and further refinement of these schemes, and also in the interpretation of the resulting prediction errors.

Heavy precipitation processes in a warmer climate

Climate simulations have suggested that a greenhouse-gas induced global warming would also lead to a moistening of the atmosphere and an intensification of the mean hydrological cycle. Here we study possible attendant effects upon the frequency of heavy precipitation events. For this purpose simulations with a regional climate model are conducted, driven by observed and modified lateral boundary conditions and sea-surface temperature distributions. The modifications correspond to a uniform 2K temperature increase and an attendant 15% increase of the specific humidity (unchanged relative humidity). This strategy allows to isolate the effects of an increased atmospheric moisture content from changes in the atmospheric circulation. The numerical experiments, carried out over Europe and for the fall season, indicate a substantial shift towards more frequent events of strong precipitation. The magnitude of the response increases with the intensity of the event and reaches several 10s of percent for events exceeding 30 mm per day. These results appear to apply to all precipitation events dominated by sea-to-land moisture transport.

Heavy precipitation on the alpine southside: An upper‐level precursor

Analyses of four separate autumnal events of very heavy precipitation on the southern slopes of the European Alps indicate that each was accompanied by a strikingly similar anomaly at upper-tropospheric levels. It took the form of a narrow (∼500km), deep (∼4km) and elongated (∼2000 km) streamer of intruded stratospheric air [sic. high potential vorticity air] extending north-south from the British Isles to the western Mediterranean. In each case the streamer translated comparatively slowly eastward, and the storm event ensued as its forward flank approached the Alpine ridge. Empirical indicators and theoretical considerations support the thesis that the streamer is an ubiquitous, dynamically significant, and distinctive precursor of storms on the Alpine southside.

Surrogate climate-change scenarios for regional climate models

A methodology is presented for generating surrogate climate-change scenarios with a regional climate model. The procedure is simple to implement and dynamically consistent. It entails (i) adopting a realized or simulated atmospheric flow evolution and (ii) prescribing specific thermodynamic modifications of this realization to a regional models initial fields and externally-specified time-dependent lateral boundaries fields. The resulting scenarios can be used for process and parameterization studies, to calibrate the regional response to a putative global climate change, and to intercompare different models. The approach is illustrated with two month-long regional climate model simulations. The experiment is designed to explore the response within Europe to a pseudo-global warming of 2 K with an accompanying increase in atmospheric water vapor content. Analysis reveals that there is a spatially-differentiated preciptation increase consonant with the domain-averaged increase of about 16% in the water vapor content.

The Palette of Fronts and Cyclones within a Baroclinic Wave Development

Abstract A study is undertaken of the concurrent development of fronts and cyclones within a growing baroclinic wave. The dynamical framework for the study is that of the semigeostrophic, adiabatic flow of a uniform potential vorticity atmosphere that is sandwiched between two horizontal plates on an f plane. A three-dimensional spectral model is used to examine the growth to finite amplitude of the normal modes of this system for basic states that comprise of a symmetric jetlike baroclinic flow with (and without) a superposition of a uniform barotropic shear of either cyclonic or anticyclonic vorticity. It is demonstrated that the presence of such lateral shear in the ambient environment exerts a profound influence upon the ensuing palette of fronts and cyclones. On the synoptic scale this flow component influences the scale and strength of the individual surface highs and lows, and also modifies the extent of their lateral drift away from, and their relative movement along, the baroclinic zone. Aloft, t...

Breaking Waves at the Tropopause in the Wintertime Northern Hemisphere: Climatological Analyses of the Orientation and the Theoretical LC1/2 Classification

Abstract Breaking waves on the tropopause are viewed as potential vorticity (PV) streamers on middle-world isentropic levels. A Northern Hemisphere winter climatology of the streamers’ spatial distribution and meridional orientation is derived from the 40-yr ECMWF Re-Analysis (ERA-40) dataset, and used to assess the nature and frequency of occurrence of breaking synoptic-scale waves. The streamers are grouped into two classes related to the so-called cyclonic (LC2) and anticyclonic (LC1) patterns, and the ambient wind strength and wind shear is also noted. It is shown that the occurrence of cyclonic and anticyclonic PV streamers exhibits a distinct spatial variability in the horizontal and the vertical. The majority of cyclonic PV streamers are found on lower isentropic levels that intersect the tropopause at more poleward latitudes, whereas anticyclonic streamers predominate at higher elevations in the subtropics. An analysis of the streamer patterns for the two phases of the North Atlantic Oscillation (...

Interannual variability and regional climate simulations

SummaryAn assessment is made of a regional climate models skill in simulating the mean climatology and the interannual variability experienced in a specific region. To this end two ensembles comprising three realizations of month-long January and July simulations are undertaken with a limited are a operational NWP model. The modelling suite is driven at its lateral boundaries by analysed meteorological fields and the computational domain covers Europe and the North-western Atlantic with a horizontal resolution of 56 km.Validation is performed against both operational ECMWF analyses and objectively analysed precipitation fields from a network of ~ 1400 SYNOP rain gauge stations. Analysis of the simulated ensemble-mean climatology indicates that the model successfully reproduces both the winter and summer distributions of the primary dynamical and thermodynamical field, and also provides a reasonable representation of the measured precipitation over most of Europe. Typically the domain averaged model-biases are below 0.5 K for temperature and 0.1 g/kg for specific humidity. Analysis of the interannual variability reveals that the model captures the wintertime changes including that of the precipitation distribution, but in contrast the summertime precipitation totals for the individual years is not simulated satisfactorily and only partially reproduces the observed regional interannual variability.The latter shortcomings are related to the following factors. Firstly the model bias in the dynamical fields is somewhat larger for summer than winter, while at the same time summertime interannual variability is associated with weaker effects in the dynamical fields. Secondly the summertime precipitation distribution is more substantially affected by small-scale moist convection and surface hydrological processes. Together these two factors suggest that summertime precipitation over continental extratropical land masses might be intrinsically less predictable than wintertime synoptic scale precipitation.

A Multifaceted Climatology of Atmospheric Blocking and Its Recent Linear Trend

Abstract A dynamically based climatology is derived for Northern Hemisphere atmospheric blocking events. Blocks are viewed as large amplitude, long-lasting, and negative potential vorticity (PV) anomalies located beneath the dynamical tropopause. The derived climatology [based on the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40)] provides a concise, coherent, and illuminating description of the main physical characteristics of blocks and the accompanying linear trends. The latitude–longitude distribution of blocking frequency captures the standard bimodal geographical distribution with major peaks over the North Atlantic and eastern North Pacific in all four seasons. The accompanying pattern for the age distribution, the genesis–lysis regions, and the track of blocks reveals that 1) younger blocks (1–4 days) are more prevalent at lower latitudes whereas significantly older blocks (up to 12 days) are located at higher latitudes; 2) genesis is confined predominantly t...

Influence of Upstream Diabatic Heating upon an Alpine Event of Heavy Precipitation

Abstract Narrow extended troughs associated with elongated streamers of potential vorticity (PV) are a frequent feature of upper-tropospheric flow and they can instigate or modify surface weather systems. In this study consideration is given to the generation of one particular PV streamer that was itself a dynamical precursor of a heavy precipitation event in the Alpine region. It is shown that the streamers parturition over the eastern North Atlantic is linked to a prior event of cyclo- and frontogenesis upstream over the western North Atlantic. Diagnostic trajectory analysis and heuristic simulations with a limited-area NWP model suggest that the streamers origin is influenced crucially by the following chain of physical processes: cloud-diabatic heating associated with the upstream cyclogenesis enhances a downstream negative PV anomaly in the upper troposphere, and the flow associated with this anomaly subsequently helps transform a farther-downstream preexisting broad positive PV anomaly into an elo...

Forced Waves on a Zonally Aligned Jet Stream

Abstract The potential vorticity (PV) pattern in the vicinity of the jet stream takes the form of a narrow tube of enhanced PV gradient on the in situ isentropic surfaces. It is asserted that this distinctive structure can serve as a waveguide and a seat for trapped Rossby waves and that a neighboring vortexlike anomaly can trigger such waves and/or interact strongly with the jet. These conjectures are examined theoretically in an idealized setting comprising a finite-scale vortex forcing of a zonally aligned PV discontinuity. The quintessential dynamics of the vortexs influence upon the PV interface are first elucidated in the linear barotropic β-plane limit, and thereafter other aspects of the jet–vortex interaction are examined in a hemispheric primitive equation setting using a nonlinear numerical model. It is shown that for the selected setting the interface can sustain trapped waves, a strong response is favored by larger-scale forcing, and a quasi-resonant response can prevail for some ambient flo...