Description: A high-order global shallow water model on Yin-Yang grid has been developed by using the multi-moment constrained finite volume (MCV) method. Different from the traditional finite volume method, more degrees of freedom (DOFs) which are the values at the solution points within each mesh element are defined and updated in time. The time evolution equations for these point values are derived from a set constraint conditions in terms of the so-called multi-moment quantities, such as the point value (PV), the volume-integrated average (VIA) and derivative (DV). Different moments use different forms of equations which are all consistent with the shallow water equations, among which the VIA moment is computed from a finite volume formulation of flux form that guarantees the rigorous numerical conservation. A fourth-order formulation is devised with the third-order reconstruction built over each element using the DOFs locally available. A simple and orthogonal overset grid, the Yin-Yang grid, is used to represent the spherical geometry with quasi-uniform grid spacing. The resulting global shallow water model is attractive in algorithmic simplicity and computational efficiency. The model has been validated by widely used benchmark tests. The numerical results of the present model are competitive to most existing advanced models.

Description: Since 2007 a large decline in Arctic sea ice has been observed. The large-scale atmospheric circulation response to this decline is investigated in ERA-Interim reanalyses and HadGEM3 climate model experiments. In winter, post-2007 observed circulation anomalies over the Arctic, North Atlantic and Eurasia are small compared to interannual variability. In summer, the post-2007 observed circulation is dominated by an anticyclonic anomaly over Greenland which has a large signal-to-noise ratio. Climate model experiments driven by observed SST and sea ice anomalies are able to capture the summertime pattern of observed circulation anomalies, although the magnitude is a third of that observed. The experiments suggest warm SSTs and reduced sea ice in the Labrador Sea lead to warm temperature anomalies in lower troposphere which weaken the westerlies over North America through thermal wind balance. The experiments also capture cyclonic anomalies over north-western Europe, which are consistent with downstream Rossby wave propagation.

Description: This paper analyses ubiquitous flow structures that affect the dynamics of stable atmospheric boundary layers. These structures introduce non-stationarity and intermittency to turbulent mixing, thus invalidating the usual scaling laws and numerical model parametrizations, but their characteristics and generating mechanisms are still generally unknown. Detecting these unknown events from time series requires techniques that do not assume particular geometries or amplitudes of the flow structures. We use a recently developed such method with some modifications to study the nighttime structures over a three-month period during the FLOSSII experiment. The structures cover about 26% of the dataset, and can be categorized using clustering into only three classes with similar characteristics. The largest class, including about 50% of the events, contains smooth structures, often with wave-like shapes, that occur in stronger winds and weak stability. The second class includes sharper structures with large kurtosis. It is characterized by weaker winds and stronger stability. The smallest class, including about 20% of the events, contains predominantly sharp step-like structures, or microfronts. They occur in weakest winds with strong stability. Sharper, and particularly shallower, structures are related to transient low-level wind maxima that create inflection points and may affect generation of turbulence. Furthermore, large wind directional shear, which is another source of transient inflection points, is generated even by deep coherent structures when the background wind is weaker than the structure intensity. These results show that the complexity of structures can be reduced for the purpose of further analysis using a proper classification. Mapping common characteristics of such events leads to their better understanding, which, if combined with similar analyses of other boundary layer data, could lead to improving their effects in numerical models.

Description: This paper describes results from a new four-year (2009–2012) radar-based precipitation climatology for the southeastern United States (SE US). The climatology shows that a size-based classification between mesoscale precipitation features (MPF) and isolated precipitation reveals distinct seasonal and diurnal variability of precipitation. On average, from 70%-90% of precipitation is associated with MPF, generally less in the summertime and in southern coastal regions. MPF precipitation has a relatively small seasonal cycle except in Florida and the warm offshore waters of the Gulf Stream. In contrast, isolated precipitation has a dramatic seasonal cycle that outlines the SE US coastline whereas the MPF precipitation does not, consistent with a thermodynamic mechanism for onshore isolated storms in coastal regions. In summer, the isolated precipitation preferentially forms offshore at night, and dramatically “flips” inland by early afternoon. In contrast, MPF precipitation has no clear diurnal variations except in the southern coastal region in the summer, likely associated with sea breeze convection organized on the mesoscale. These results suggest that the MPF versus isolated precipitation system framework provides a useful basis for future studies of large scale and local controls on precipitation and resulting implications for long-range predictability of precipitation.

Description: This study assesses the impact of urban land use on the climatological distribution of thunderstorm initiation occurrences in the humid subtropical region of the Southeast United States, which includes the Atlanta, Georgia metropolitan area. Initially, an automated technique is developed to extract the locations of isolated convective initiation (ICI) events from 17 years (1997–2013) of composite reflectivity radar data for the study area. Nearly 26,000 ICI points were detected during 85 warm-season months, providing the foundation for first long-term, systematic assessment of the influence of urban land use on thunderstorm development. Results reveal that ICI events occur more often over the urban area compared to its surrounding rural counterparts, confirming that anthropogenic-induced changes in land cover in moist tropical environments lead to more initiation events, resulting thunderstorms, and affiliated hazards over the developed area. The ICI risk for Atlanta is greatest during the late afternoon and early evening in July and August in synoptically benign conditions. Greater ICI counts downwind of Atlanta suggest that prevailing wind direction also influences the location of these events. Moreover, ICI occurrences over the city were significantly higher on weekdays compared to weekend days—a result that was not apparent in a rural control region located west of the city. This suggests that the weekly commuting cycle and associated aerosol levels of Atlanta may amplify ICI rates. The investigation provides a methodological framework for future studies that examine the effect of land use, land cover, and terrain discontinuities on the spatiotemporal character of ICI events.

Description: This is the second part of a study on the effect of topography on tropical cyclone (TC) tracks. In Part I, idealized simulations using Weather Research and Forecast (WRF) Model were conducted with artificial TCs inserted at a location so that they make landfall on the central part of Taiwan (TW) and the Luzon. In this second part, the effects of local and remote topographies are studied by examining the tracks of several TCs approaching TW at different latitudes. TCs approaching to the south of TW (remote effect) slow down and are first deflected southwestward and then northward. Moreover, a sharp northward deflection occurs for a TC passing further south of TW but the deflection is small for a TC passing further north of TW. In the presence of China terrain, an additional larger but weaker anticyclonic gyre is induced so that the TC is affected even when it is further away from the land. The flow associated with this gyre reduces the northward deflection caused by the TW terrain but enhances the southwestward shift of the TC. However, no cyclonic gyre is found over inland China if the TC is far away from the China coast probably because of the irregular elevated topography. For TCs making landfall on South China, a westward shift is found just prior to landfall, which is more significant if the height of China terrain is doubled. This westward shift can be explained by the horizontal advection, vertical advection and diabatic heating terms in the potential vorticity tendency equation.

Description: Aerosols are known to significantly affect cloud and precipitation patterns and intensity, but these interactions are ignored or very simplistically handled in climate and Numerical Weather Prediction models. A suite of one-way nested MetOffice UM runs, with a single-moment bulk microphysics scheme was used to study two convective cases with contrasting characteristics observed in southern England. The autoconversion process that converts cloud water to rain is directly controlled by the assumed droplet number. The impact of changing cloud droplet number concentration (CDNC), on cloud and precipitation evolution can be inferred through changes to the autoconversion rate. This was done for a range of resolutions ranging from regional NWP (1 km) to high resolution (up to 100 m grid spacing) to evaluate the uncertainties due to changing CDNC as a function of horizontal grid resolution. The first case is characterised by moderately intense convective showers forming below an upper-level PV anomaly, with a low freezing level. The second case, characterised by one persistent stronger storm, is warmer with a deeper boundary layer. The colder case is almost insensitive to even large changes in CDNC, while in the warmer case a change of a factor of 3 in assumed CDNC affects total surface rain rate by ~17%. In both cases the sensitivity to CDNC is similar at all grid-spacing 〈1 km. The contrasting sensitivities of these cases are induced by their contrasting ice-phase proportion. The ice processes in this model damp the precipitation sensitivity to CDNC. For this model the convection is sensitive to CDNC when the accretion process is more significant than the melting process and vice versa.

Description: Alpine heavy precipitation events often affect small catchments, although the circulation pattern leading to the event extends over the entire North Atlantic. The various scale interactions involved are particularly challenging for the numerical weather prediction of such events. Unlike previous studies focusing on the Southern Alps, here a comprehensive study of a heavy precipitation event in the Northern Alps in October 2011 is presented with particular focus on the role of the large-scale circulation in the North Atlantic European region. During the event exceptionally high amounts of total precipitable water occurred in and North of the Alps. This moisture was initially transported along the flanks of a blocking ridge over the North Atlantic. Subsequently, strong and persistent northerly flow established at the upstream flank of a trough over Europe and steered the moisture towards the Northern Alps. Lagrangian diagnostics reveal that a large fraction of the moisture emerged from the West African coast where a subtropical upper-level cut-off low served as an important moisture collector. Wave activity flux diagnostics show that the ridge was initiated as part of a low-frequency, large-scale Rossby wave train while convergence of fast-transients helped to amplify it locally in the North Atlantic. A novel diagnostic for advective potential vorticity tendencies sheds more light on this amplification and further emphasizes the role of the ridge in amplifying the trough over Europe. Operational forecasts misrepresented the amplitude and orientation of this trough. For the first time, this study documents an important pathway for Northern Alpine flooding, in which the interaction of synoptic-scale to large-scale weather systems and of long-range moisture transport from the Tropics are dominant. Moreover, the trapping of moisture in a subtropical cut-off near the West African coast is found to be a crucial precursor to the observed European high impact weather.

Description: Variational methods are widely used to solve geophysical inverse problems. Although gradient-based minimization algorithms are available for high-dimensional problems (dimension 〉10 6 ), they do not provide an estimate of the errors in the optimal solution. In this study, we assess the performance of several numerical methods to approximate the analysis error covariance matrix, assuming reasonably linear models. The evaluation is performed for a CO 2 flux estimation problem using synthetic remote-sensing observations of CO 2 columns. A low-dimensional experiment is considered in order to compare the analysis error approximations to a full-rank finite-difference inverse Hessian estimate, followed by a real-life high-dimensional application. Two stochastic approaches, a Monte-Carlo simulation and a method based on random gradients of the cost function, produced analysis error variances with a relative error 〈10%. The long-distance error correlations due to sampling noise are significantly less pronounced for the gradient-based randomization, which is also particularly attractive when implemented in parallel. Deterministic evaluations of the inverse Hessian using the Broyden-Fletcher-Goldfarb-Shanno (BFGS) algorithm are also tested. While existing BFGS preconditioning techniques yield poor approximations of the error variances (relative error 〉120%), a new preconditioner that efficiently accumulates information on the diagonal of the inverse Hessian dramatically improves the results (relative error 〈50%). Furthermore, performing several cycles of the BFGS algorithm using the same gradient and vector pairs enhances its performance (relative error 〈30%) and is necessary to obtain convergence. Leveraging those findings, we proposed a BFGS hybrid approach that combines the new preconditioner with several BFGS cycles using information from a few (3–5) Monte-Carlo simulations. Its performance is comparable to the stochastic approximations for the low-dimensional case, while good scalability is obtained for the high-dimensional experiment. Potential applications of these new BFGS methods range from characterizing the information content of high-dimensional inverse problems to improving the convergence rate of current minimization algorithms.

Description: This numerical study focuses on a dense fog event that occurred during the night of 21–22 January 2008 in the Grand Casablanca region, on the north-western coast of Morocco. This fog event, which lasted for 15 hours, is simulated by the mesoscale non-hydrostatic model Meso-NH and analysed using: conventional meteorological observations from two synoptic stations of the region, MSG satellite imagery and the ECMWF ERA-Interim reanalysis. Results demonstrate that this fog event included the formation of radiation fog over a continental zone and its extension to the coastal zone through the lowering of low-stratus clouds, which is in agreement with observations and is well captured by the Meso-NH model. Sensitivity experiments show that coastal fog prediction improves with improved sea surface temperature. Model skill also depends on the adjustment of microphysical parameters when a single-moment microphysical scheme is used, and on reliable initial conditions.