Beschreibung: A new nudging method for data assimilation, delay-coordinate nudging , is presented. Delay-coordinate nudging makes explicit use of present and past observations in the formulation of the forcing driving the model evolution at each time-step. Numerical experiments with a low order chaotic system show that the new method systematically outperforms standard nudging in different model and observational scenarios, also when using an un-optimized formulation of the delay-nudging coefficients. A connection between the optimal delay and the dominant Lyapunov exponent of the dynamics is found based on heuristic arguments and is confirmed by the numerical results, providing a guideline for the practical implementation of the algorithm. Delay-coordinate nudging preserves the easiness of implementation, the intuitive functioning and the reduced computational cost of the standard nudging, making it a potential alternative especially in the field of seasonal-to-decadal predictions with large Earth system models that limit the use of more sophisticated data assimilation procedures.

Beschreibung: Numerical models of the atmosphere combine a dynamical core, which approximates solutions to the adiabatic, frictionless governing equations for fluid dynamics, with tendencies arising from the parametrization of other physical processes. Since potential vorticity (PV) is conserved following fluid flow in adiabatic, frictionless circumstances, it is possible to isolate the effects of non-conservative processes by accumulating PV changes in an air-mass relative framework. This “PV tracer technique” is used to accumulate separately the effects on PV of each of the different non-conservative processes represented in a numerical model of the atmosphere. Dynamical cores are not exactly conservative because they introduce, explicitly or implicitly, some level of dissipation and adjustment of prognostic model variables which acts to modify PV. Here, the PV tracers technique is extended to diagnose the cumulative effect of the non-conservation of PV by a dynamical core and its characteristics relative to the PV modification by parametrized physical processes. Quantification using the Met Office Unified Model reveals that the magnitude of the non-conservation of PV by the dynamical core is comparable to those from physical processes. Moreover, the residual of the PV budget, when tracing the effects of the dynamical core and physical processes, is at least an order of magnitude smaller than the PV tracers associated with the most active physical processes. The implication of this work is that the non-conservation of PV by a dynamical core can be assessed in case studies with a full suite of physics parametrizations and directly compared with the PV modification by parametrized physical processes. The nonconservation of PV by the dynamical core is shown to move the position of the extratropical tropopause while the parametrized physical processes have a lesser effect at the tropopause level.

Beschreibung: The characteristics and environmental properties of warm-season quasi-stationary convective clusters (QSCCs) in Japan were statistically investigated using operational weather radar and upper-air sounding data from May to October during 2005–2012. The characteristics of the environmental conditions for the development of QSCCs were described through a comparison with those for no-rain cases. We identified 4,133 QSCCs over the Japanese major islands. By compiling numerous QSCC samples, the horizontal scales of QSCCs on average and at the maximum with a circular shape are about 20 km and 72 km, respectively, indicating that warm-season QSCCs in Japan are meso- β -scale phenomena. The analyses of the environmental conditions for the QSCC and no-rain cases showed that the amount of moisture in the lower layer controls the stability condition for the development of the QSCCs, and that the magnitudes of the wind shear and the helicity in the lower troposphere distinguish the kinematic environments for the development of the QSCCs. An increase in the middle-level moisture leads to a larger amount of precipitable water vapor in the QSCC environments, suggesting that atmospheric moistening before the development stage of convection plays an important role in the development of the QSCCs. Additionally, the precipitation intensity has a higher correlation with the convective instability, whereas the precipitation area with the shear intensity. A comparison between slower- and the faster-moving CCs indicated that the precipitation intensity of the slower-moving CCs is stronger. This feature is related to a higher convective instability for the slower-moving ones.

Beschreibung: During the first HyMeX Special Observation Period (SOP1) field campaign, the target site of North Eastern Italy (NEI) experienced a large amount of precipitation, locally exceeding the climatological values and distributed among several heavy rainfall episodes. In particular, two events that occurred during the last period of the campaign drew our attention. These events had common large-scale patterns and a similar mesoscale setting, characterised by southerly low-level flow interacting with the Alpine orography, but the precipitation distribution was very different. During IOP18 (31 October – 1 November 2012), convective systems were responsible for intense rainfall mainly located over a flat area of the eastern Po Valley, well upstream of the orography. Conversely, during IOP19 (4 – 5 November 2012), heavy precipitation affected only the Alpine area. In addition to IOP18 and IOP19, the present study analyses other heavy-precipitation episodes that display similar characteristics and which occurred over NEI during the autumn of recent years. A high-resolution (2-km grid spacing) non-hydrostatic NWP model and available observations are used for this purpose. The two different observed precipitation patterns are explained in terms of interaction between the impinging flow and the Alps. Depending on the thermodynamic profile, convection can be triggered when the impinging flow is forced to rise over a pre-existing cold-air layer at the base of the orography. In this situation persisting blocked-flow condition and upstream convergence are responsible for heavy rain localized over the plain. Conversely, if convection does not develop, flow-over conditions establish and heavy rain affects the Alps. Numerical parameters proposed in the literature are used to support the analysis. Finally, the role of evaporative cooling beneath the convective systems is evaluated. It turns out that the stationarity of the systems upstream of the Alps is mainly attributable to persisting blocked-flow conditions, while convective outflow slightly modifies the location of precipitation.

Beschreibung: The Rauch-Tung-Striebel (RTS) smoother is a linear-Gaussian smoothing algorithm that is popular in the engineering community. This note is a study of its ensemble formulation (EnRTS). An on-line expression is derived and discussed. In particular, it is used to show that the EnRTS is equivalent to the ensemble Kalman smoother (EnKS), even in the non-linear, non-Gaussian case. The theory is revisited under practical considerations and equability is illustrated by numerical experiments, even though equivalence is broken by inflation and localisation.

Beschreibung: The onset/progression phase of Indian summer monsoon (ISM) is very crucial for the agricultural sector of the country as it has strong bearing on the sowing of kharif crops, which in turn affects the overall food grain production and hence food security. The recent ISMs of 2013 and 2014 exhibited quite distinct progression phase. While 2013 had one of the fastest advancement in the last 70 years, 2014 witnessed comparatively lethargic progression phase. The major difference was felt in the early monsoon month of June, with 2013 (2014) monthly rainfall being +34% (−43%) of its long period average. Observational investigations reveal that during June 2013, the monsoon trough was very active in its normal position favouring low level positive vorticity generation and moisture convergence, whereas the absence of monsoon trough facilitated the prevalence of a strong low level anticyclonic circulation over central India during June 2014 hampering the northward progression of ISM. It is found that June 2013 (2014) was associated with (a) stronger (weaker) north–south tropospheric temperature (TT) gradient with positive (negative) TT anomalies over Eurasia and north of 60°N, (b) colder (warmer) SST anomalies over equatorial Indian Ocean, northwestern Arabian Sea and equatorial eastern Pacific, (c) stronger (weaker) monsoonal Hadley circulation, and (d) stronger (weaker) Walker circulation in response to the colder (warmer) SST anomalies over equatorial Pacific. The study also examines the skill of an Ensemble Prediction System (EPS) in predicting the observed contrasting behaviour during June 2013/2014 on extended range (~15-20 days in advance) in real time. The EPS not only forecasted the observed discrepancy, but also predicted the influential role of large scale meteorological conditions that were prevalent during June 2013 (2014), thus advocating the remarkable skill of the EPS in predicting June extremes.

Beschreibung: An extensive set of measurements (May-August 2012 and June-November 2013) taken at the Amundsen-Nobile Climate Change Tower located at Ny-Ålesund, Svalbard, allowed to investigate some features of the vertical structure of the atmospheric stable boundary layer (SBL) at high latitudes. The main sensors are 3 sonic anemometers and 4 low frequency thermo-hygrometers and anemometers. The momentum flux τ , the sensible heat flux Q and the turbulent kinetic energy K data at different levels evidenced the occurrence of both the traditional and upside-down SBL cases, according to the classification proposed in the literature. Based on a linear approximation of the profiles, the vertical scales were found to be different for the different second order moments. In the traditional case, the scales h τ , h Q and h K , defined as levels for which fluxes became zero, can be considered to approximate the boundary layer depth. While the distributions of h τ and h Q were found to be similar, being the ratio h Q / h τ equally distributed around 1 even if with a positive skewness, those of h τ and h K look quite different, being the ratio h K / h τ 〉 1 in about 90% of the cases and with a median value 〉 2. In the upside-down case, the scales s τ , s Q and s K , defined as the height at which the value of the considered quantity doubles that at the surface, give a measure of the vertical variations of the moments. Comparing the distributions of these three scales, similar results to those pertaining to the traditional case were found, even with slightly different statistics. The values of the ratio between scales affect the vertical profile of the local Obukhov length in both the traditional and upside-down SBL cases.

Beschreibung: During the first special observation period of the HyMeX program dedicated to heavy precipitation over the western Mediterranean, several Mesoscale Convective Systems (MCSs) formed over the sea and produced heavy precipitation over the coastal regions, as for example during IOP (Intensive Operation Period) 16a. On 26 October 2012, back-building MCSs formed and renewed over the northwestern Mediterranean Sea while producing heavy rain over the French coastal urbanized regions. This paper analyses the storm evolution along with the ambient flow and the initiation and maintenance mechanisms of the offshore deep convection observed during IOP16a. The suites of water vapour lidars, wind profilers, radiosoundings and boundary layer drifting balloons over and along the coast of the northwestern Mediterranean offer a unique framework for validating the convective processes over the sea investigated using kilometric-scale analyses and simulation. The high-resolution simulation shows clearly that the convective system is fed during its evolution over the sea by moist and conditionally unstable air carried by a southwesterly to southeasterly low-level jet. The low-level wind convergence in this southeasterly to southwesterly flow over the sea is the main triggering mechanism acting to continually initiate and maintain the renewal of training convective cells contributing to the back-building system. The convergence line appears when a secondary pressure low forms in the lee of the Iberian mountains. A sensitivity test turning off the evaporative cooling within the microphysical parametrisation shows that the exact location of the main convergence area focusing the heaviest precipitation is determined by small-scale feedback mechanisms of the convection to the environment.

Beschreibung: General expressions for the dynamical behavior of prediction errors of time-averaged observables involved in extended-range forecasts are derived and applied to systems giving rise to chaotic dynamics, systems close to criticalities and stochastically forced systems. The role of the initial conditions, of the parameters present and of the averaging period in the properties of the error and in particular in the predictability gain with respect to the error associated to the instantaneous observables are revealed, providing insights on optimal choices of this period depending on the intrinsic properties of the system of interest

Beschreibung: In recent years, many eddy-diffusivity mass flux (EDMF) planetary boundary layer (PBL) parameterizations have been introduced. Yet, most validations are based on idealized setups and/or single column models. To address this gap, this paper focused on the effect the mass flux part has on the performance in the Quasi-Normal Scale Elimination-EDMF (QNSE-EDMF) PBL scheme in the Weather Research and Forecasting (WRF) model by comparing the results to observations from the CASES-97 field campaign. In addition, two refined versions, one introducing the parameterized clouds to the WRF radiation scheme, and the second adding a different entrainment formulation, were evaluated. The introduction of mass flux reduced errors in the average moisture profile but virtual temperature and wind speed profiles did not change as much. The modeled mixed-layer depth, while still low compared to observations, was closer to observed values with the addition of mass flux. The major changes in the virtual potential temperature flux profiles were an increase in entrainment ratios and a slight decrease in surface values. Allowing for biases in the observed fluxes, modeled and observed flux profiles had a similar shape. However, the agreement among modeled and between modeled and observed water-vapor flux profiles and vertical flux divergence was often poor, a likely result of the model's failure to capture the timing of the morning boundary layer water-vapor maximum. Further, both virtual potential temperature and water-vapor flux profiles display spurious spikes attributed to the way the non-local and local terms interact in the model. Adding mass flux based clouds to the radiation calculation improved the time and space averaged modeled incoming shortwave flux. The choice of the representation for entrainment/detrainment often affected the results to the same extent as adding mass flux did.