ALBERT

Description: The IMILAST project (‘Intercomparison of Mid-Latitude Storm Diagnostics’) was set up to compare low-level cyclone climatologies derived from a number of objective identification algorithms. This paper is a contribution to that effort where we determine the sensitivity of three key aspects of Northern Hemisphere cyclone behaviour [namely the number of cyclones, their intensity (defined here in terms of the central pressure) and their deepening rates] to specific features in the automatic cyclone identification. The sensitivity is assessed with respect to three such features which may be thought to influence the ultimate climatology produced (namely performance in areas of complicated orography, time of the detection of a cyclone, and the representation of rapidly propagating cyclones). We make use of 13 tracking methods in this analysis. We find that the filtering of cyclones in regions where the topography exceeds 1500 m can significantly change the total number of cyclones detected by a scheme, but has little impact on the cyclone intensity distribution. More dramatically, late identification of cyclones (simulated by the truncation of the first 12 hours of cyclone life cycle) leads to a large reduction in cyclone numbers over the both continents and oceans (up to 80 and 40%, respectively). Finally, the potential splitting of the trajectories at times of the fastest propagation has a negligible climatological effect on geographical distribution of cyclone numbers. Overall, it has been found that the averaged deepening rates and averaged cyclone central pressure are rather insensitive to the specifics of the tracking procedure, being more sensitive to the data set used (as shown in previous studies) and the geographical location of a cyclone. Keywords: cyclone identification, IMILAST, cyclone life cycle, rapidly intensifying cyclones, synoptic climatology, reanalysis (Published: 12 December 2014) Citation: Tellus A 2014, 66 , 24961, http://dx.doi.org/10.3402/tellusa.v66.24961 This publication is part of a Thematic Cluster entitled " Intercomparison of Mid-Latitude Storm Diagnostics ". Read the other papers from this thematic cluster here

Description: The impact of assimilating temperature, salinity, oxygen, phosphate and nitrate observations on marine ecosystem modelling is assessed. For this purpose, two 10-yr (1970–1979) reanalyses of the Baltic Sea are carried out using the ensemble optimal interpolation (EnOI) method and a coupled physical-biogeochemical model of the Baltic Sea. To evaluate the reanalyses, climatological data and available biogeochemical and physical in situ observations at monitoring stations are compared with results from simulations with and without data assimilation. In the first reanalysis, only observed temperature and salinity profiles are assimilated, whereas biogeochemical observations are unused. Although simulated temperature and salinity improve considerably as expected, the quality of simulated biogeochemical variables does not improve and deep water nitrate concentrations even worsen. This unexpected behaviour is explained by a lowering of the halocline in the Baltic proper due to the assimilation causing increased oxygen concentrations in the deep water and consequently altered nutrient fluxes. In the second reanalysis, both physical and biogeochemical observations are assimilated and good quality in all variables is found. Hence, we conclude that if a data assimilation method like the EnOI is applied, all available observations should be used to perform reanalyses of high quality for the Baltic Sea biogeochemical state estimates. Keywords: reanalysis, data assimilation, numerical modelling, Baltic Sea, biogeochemical simulation (Published: 10 December 2014) Citation: Tellus A 2014, 66 , 24908, http://dx.doi.org/10.3402/tellusa.v66.24908

Description: At the end of January 2012, a low-level cloud from partly ice-free Lake Ladoga caused very variable 2-m temperatures in Eastern Finland. The sensitivity of the High Resolution Limited Area Model (HIRLAM) to the lake surface conditions was tested in this winter anticyclonic situation. The lake appeared to be (incorrectly) totally covered by ice when the lake surface was described with its climatology. Both parametrisation of the lake surface state by using a lake model integrated to the NWP system and objective analysis based on satellite observations independently resulted in a correct description of the partly ice-free Lake Ladoga. In these cases, HIRLAM model forecasts were able to predict cloud formation and its movement as well as 2-m temperature variations in a realistic way. Three main conclusions were drawn. First, HIRLAM could predict the effect of Lake Ladoga on local weather, when the lake surface state was known. Second, the current parametrisation methods of air–surface interactions led to a reliable result in conditions where the different physical processes (local surface processes, radiation and turbulence) were not strong, but their combined effect was important. Third, these results encourage work for a better description of the lake surface state in NWP models by fully utilising satellite observations, combined with advanced lake parametrisation and data assimilation methods. Keywords: lake-effect, data assimilation, lake ice, NWP, cold outbreaks, stable boundary layer, lake–atmosphere interaction (Published: 8 December 2014) Citation: Tellus A 2014, 66 , 23929, http://dx.doi.org/10.3402/tellusa.v66.23929 This publication is part of a Thematic Cluster entitled "Parameterization of lakes in numerical weather prediction and climate models". Read the other papers from this thematic cluster here

Description: This paper studies the question how the zonal mean potential vorticity ( PV ) distribution in potential temperature ( θ ) coordinates is established in the atmosphere by the interaction of diabatic processes (cross-isentropic transport of mass) with adiabatic dynamical processes (isentropic transport of mass and potential vorticity substance). As an aid in dissecting this interaction, a simplified model of the general circulation is constructed, which contains parametrisations of radiative transfer, wave drag and water cycle. This model reproduces the following four observed features of the atmosphere below 10 hPa: (1) a permanently present eastward subtropical jet, which in winter is separated from an eastward stratospheric jet by a zone (referred to as the ‘surf zone’), between θ =380 K and θ =550 K, where planetary wave drag reduces PV over the polar cap; (2) a stratospheric zonal wind reversal in spring or beginning of summer; (3) a tropical cold layer at 100 hPa, and (4) a realistic distribution of zonal mean cross-isentropic flow. The strength of the cross-isentropic flow depends on wave drag, latent heat release and the thermal inertia of both the atmosphere and the earth’s surface. Of special interest is the layer between θ =315 K and θ =370 K (the ‘Middleworld’), which lies in the troposphere in the tropics and in the stratosphere in the extratropics. Mass converges diabatically into this layer in the deep tropics, mainly due to latent heat release, and diverges out of this layer elsewhere due to radiation flux divergence. Meridional isentropic vorticity flux divergence in the tropical Middleworld, associated with the upper branch of the Hadley circulation, creates a region in the subtropics, at θ =350 K and adjacent isentropic levels, with a marked isentropic meridional PV -gradient, forming the isentropic dynamical tropopause. Keywords: potential vorticity, thermal inertia, diabatic circulation, zonal wind jets, tropopause, surf zone, planetary wave drag, radiation, water cycle (Published: 5 December 2014) Citation: Tellus A 2014, 66 , 24880, http://dx.doi.org/10.3402/tellusa.v66.24880 To access the supplementary material to this article, please see Supplementary files under Article Tools online.

Description: Orographic winds near a 914 m high mountain in Southwest-Iceland are explored using unique observations made aloft with a small remotely piloted aircraft, as well as with traditional observations and high-resolution atmospheric simulations. There was an inversion well above mountain top level at about 2 km with weak winds below. Observed winds in the lee of the mountain were indicative of flow locally enhanced by wave activity aloft. Winds descended along the lee slope with a prevailing direction away from the mountain. They were relatively strong and gusty at the surface close to the mountain, with a maximum at low levels, and weakening and becoming more diffuse a short distance further downstream. The winds weakened further aloft, with a minimum on average near mountain top level. This situation is reproduced in a high-resolution atmospheric simulation forced with atmospheric analysis as well as with the observed lee-side profiles of wind and temperature below 1.4 km. Without the additional observations consisting of the lee-side profiles, the model fails to reproduce the winds aloft as well as at the surface in a region in the lee of the mountain, as was also the case for the operational numerical models at that time. A sensitivity simulation indicates that this poor performance is a result of the poorly captured strength and sharpness of the inversion aloft. The study illustrates, firstly, that even at very low wind speed, in a close to neutral low-level flow, gravity waves may still be a dominating feature of the flow. Secondly, the study presents an example of the usefulness of lee-side atmospheric profiles, retrieved by simple model aircraft, for improving numerical simulations and short-term weather forecasting in the vicinity of mountains. Thirdly, the study confirms the sensitivity of downslope flow to only moderate change in the sharpness of an upstream inversion. Keywords: downslope flow, complex orography, small remotely piloted aircraft, observational nudging (Published: 28 November 2014) Citation: Tellus A 2014, 66 , 25421, http://dx.doi.org/10.3402/tellusa.v66.25421

Description: The meridional structures of stratospheric and tropospheric planetary wave variability (PWV) over the Northern Hemisphere (NH) extratropics were investigated and compared using reanalysis data. By performing the spherical double Fourier series expansion of geopotential height data, the horizontal structures of PWV at each vertical level could be examined in the two-dimensional (2D) wavenumber (zonal and meridional wavenumbers) space. Comparing the amplitudes of wave components during the last three decades, the results suggested that the structures of PWV in the NH troposphere significantly differ from the stratospheric counterparts. The PWV in the troposphere shows multiple meridional wave-like structures, most pronounced for the meridional dipole; while in contrast, PWV in the stratosphere mainly shows large-scale zonal wave patterns, dominated by zonal waves 1 and 2, and have little wave-like fluctuation in the latitudinal direction. The dominant patterns of the NH PWV also show contrasting features of meridional structure between the stratosphere and the troposphere. As represented in the 2D wavenumber space, the leading two empirical orthogonal functions of PWV in the stratosphere largely exhibit the zonal wave 1 pattern, while those in the troposphere clearly show meridional wave-like structures and are dominated by the dipole. The refractive index was derived based on the zonal mean basic state to qualitatively interpret the observational findings. The results suggested that the basic state in the NH troposphere is much more favourable for latitudinally propagating stationary waves than the stratosphere. The difference in meridional structure between stratospheric and tropospheric planetary waves can be well captured in a linear baroclinic model with the observed zonal mean basic state. Furthermore, both theoretical and modelling analyses demonstrated that the fact that zonal wave patterns are preferred in the NH stratosphere may be partly attributable to the vertical curvature of the stratospheric zonal mean basic state. Keywords: planetary waves, stratosphere, troposphere, spherical double Fourier series, two-dimensional wavenumber (Published: 28 November 2014) Citation: Tellus A 2014, 66 , 25303, http://dx.doi.org/10.3402/tellusa.v66.25303