Description: Observations of high spatio-temporal resolution from a precipitation network across Stord Island, located off the west coast of southern Norway, are compared to state-of-the-art numerical model simulations. The 12 week long observation period shows a clear orographic precipitation signal across the 10–15 km wide island (peak elevation 750 m). The model experiment designed to capture this signal is run with 9–3–1 km nested grid and results are compared with observations at different accumulation intervals. The total amount of precipitation over the 12 week period is underpredicted, even for the 1 km grid. The maximum precipitation intensity, however, is slightly overpredicted. Time-step (5 s) precipitation from the model is also compared with observed intensities at the highest possible temporal resolution permitted by the rain collection method. The observations indicate that most of the precipitation is formed at intensities from 5 to 20 mm h −1 . A smaller fraction of the precipitation is formed with intensities 〉20 mm h −1 . The simulated precipitation at the 3 km grid did not reproduce at the correct intensities. The 1 km grid showed an improved tendency to produce the precipitation at the right intensities, but had too high maximum intensities. A test simulation where the intermediate grid had no cumulus parametrization was performed. Even though effects such as undercatchment and unresolved terrain influenced, it was concluded that the test run performed better than the control run. The investigation concluded that, in general, a 1 km grid is sufficient for capturing the most intensive precipitation event in a satisfying way. Copyright © 2012 Royal Meteorological Society

Description: This article explores an ensemble strategy for evaluating the impact of different observing networks. The impact is represented by the relative ensemble spread increase, in model space, of data-denial ensemble simulations with respect to an ‘all-observation’ ensemble experiment, evaluated independently for each observing network. The forecast-error covariance intercomparison reduces to the ensemble spread intercomparison; thus, the method can be applied to any assimilation system and requires only the proper construction of an ensemble system, although the impact assessment results depend on the specific configuration of the investigated analysis system. Our approach allows us to determine the impact of the observing networks in model space (unlike Observing System Experiments) and for different forecast ranges of the ocean general circulation model. No tangent-linear and adjoint coding of the ocean model are required. The method is applied for demonstration to a large-scale global ocean variational analysis system. The ensemble members are generated by (i) perturbing the observations within the 3D-Var assimilation scheme, (ii) perturbing the surface forcing, and (iii) stochastically perturbing the ocean model parametrisation tendencies. The impact is calculated for CTDs, XBTs, moorings, Argo, sea-level anomaly observations and sea-surface temperature measurements from space-borne microwave instruments within the three-year period from January 2003 to December 2005. It turns out, on the global scale, that altimetry exhibits the largest impact on near-surface temperature and sea-surface height. In contrast, deep-ocean impacts are led by the Argo float network. As expected, space-borne observations (sea-level anomaly and sea-surface temperature observations) increase their impact in the Southern Ocean, due to the lack of a robust network of in situ observations. The results of the impact on the salinity indicate the great importance of Argo floats, especially in the northern Extratropics. Copyright © 2013 Royal Meteorological Society

Description: The performance of the high-resolution Advanced Research Weather Research and Forecasting (ARW) model for tropical cyclone prediction over the Bay of Bengal region of the northern Indian Ocean is assessed through study of 21 cyclone systems. Error metrics in the predicted fields of maximum sustained winds (MSW), central sea-level pressure (CSLP) and the vector track position are computed by comparison with corresponding tropical cyclone estimations from the India Meteorological Department (IMD). From 65 sensitivity experiments for five severe cyclones the combination of Kain–Fritsch (KF) convection, Yonsei University (YSU) planetary boundary layer (PBL), LIN explicit microphysics and NOAH land surface schemes are found to provide the best simulations for intensity and track prediction. It has been found that the KF convection scheme gives higher convective warming with stronger vertical motions relative to other tested cumulus schemes and that the YSU scheme simulates more realistic winds in the inflow region than other tested PBL schemes. Results of simulations with the best physics for all 21 cyclones reveal that the model had a tendency to overestimate the intensity, with mean errors ranging from −2 to 15 hPa for CSLP, 1 to 22 m s −1 for MSW corresponding to 24 to 72 h predictions. The mean vector landfall position errors are found to be 122 km at 12 h, 170 km at 24 h, 244 km at 48 h and 250 km at 72 h, and 67% of the landfall errors are less than 135 km, indicating fairly good forecasts. Further, the predictions are found to be best for northward moving cyclones followed by northwestward, westward and northeastward moving cyclones. Copyright © 2012 Royal Meteorological Society

Description: A new humidity analysis variable has been successfully introduced into the Met Office's variational data assimilation system. The new variable uses a transformation consisting of a nonlinear normalization and a link with temperature increments that is a function of background (forecast) humidity. The normalization (which is the more important aspect) makes the new variable's errors more symmetrical and thus better represented by the variational cost function. As in the previous operational system, water vapour and cloud are combined into a single total water variable for the analysis step. The transform is now operational in both global and limited-area systems. The forecast improvements—to the mass as well as the humidity fields—are largest in the Southern Hemisphere and there is a better fit of both background and analysis to humidity-sensitive satellite channels. The results suggest that the transformation is particularly beneficial for the use of satellite data. The transform reduces the problem of negative humidities in the analysis which were more prevalent over the ocean. Copyright © 2012 British Crown copyright, the Met office. Published by John Wiley & Sons, Ltd

Description: The mechanisms responsible for poleward atmospheric heat transport at low latitudes are examined in the European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) dataset. Deep meridional overturning circulations are found in regions experiencing frequent convection (i.e. the Indo-Pacific ‘warm pool’) and it is suggested that these are the primary reason why atmospheric heat transport seems dominated by axisymmetric motions or ‘Hadley cell heat transport’. In contrast, the more complex distribution of meridional mass transport by the circulations over the ‘cold tongue’ regions (such as the eastern Atlantic area) and the presence of a pronounced minimum in moist static energy at mid-levels constrain these regions to contribute little to poleward heat transport year round. Regions experiencing frequent convection do not, however, account for all the annual mean poleward atmospheric heat transport. At the Equator, an annual net southward transport of heat of about 0.4 PW (1 PW = 10 15 W) is found, but the deep overturning cells found in the most convectively active regions contribute only 25% of this. This small contribution of ∼0.1 PW is understood to reflect a seasonal cancellation between large (∼1 PW) northward and southward heat transports in boreal winter and summer respectively. During December-February, most of the cross-equatorial heat transport is attributable to the convective regions, whereas in June-August, only half is attributable to these regions. In this season, there is a significant amount of heat transport by the strong Somali jet associated with the Asian summer monsoon. Rather than attributing the net southward cross-equatorial heat transport to asymmetries in the climatology associated with the northerly position of the intertropical convection zones throughout the year over the eastern Pacific and Atlantic oceans, our diagnostics suggest that the Somali jet is a significant contributor to the annual net southward atmospheric heat transport at the Equator. Copyright © 2012 Royal Meteorological Society

Description: Persistent northerly-to-easterly cold-air outbreaks affected the UK during the winters of 2009–10 and 2010–11, with the resulting convection frequently organizing into snowbands over the English Channel and Irish Sea. Sounding data and composite radar reflectivity images from the Met Office Nimrod precipitation radar network reveal that these bands formed along the major axis of each body of water (or sea) when the boundary-layer flow was roughly parallel to each of those axes (along-channel). For both seas, a band was present the majority of times that the 850 hPa flow was along-channel. Of these times of along-channel flow, the 850 hPa wind speed and surface-to-850 hPa temperature difference were significantly greater when bands were present than when they were not. For the English Channel only, the land–sea temperature difference was also significantly greater when bands were present than when they were not. In a real-data Weather Research and Forecasting model (WRF) control simulation of a typical band over the English Channel, a trough develops over the water and offshore air streams from either side converge along it. In the absence of surface fluxes, the trough, convergence and organized precipitation fail to develop altogether. Orography and roughness-length variations are less important in band development, affecting only the location and morphology. Copyright © 2012 Royal Meteorological Society

Description: The thermally driven rotating annulus is a laboratory experiment important for the study of the dynamics of planetary atmospheres under controllable and reproducible conditions. We use the analysis correction method to assimilate laboratory data into an annulus model. We analyze the 2S and 3AV regular flow regimes between rotation rates of 0.75 and 0.875 rad s −1 and the 3SV chaotic flow regime between rotation rates of 2.2 and 3.1 rad s −1 . Our assimilated observations are irregularly distributed, which is more meteorologically realistic than gridded observations as used in recent applications of data assimilation to laboratory measurements. We demonstrate that data assimilation can be used successfully and accurately in this context. We examine a number of specific assimilation scenarios: a wave-number transition between two regimes, information propagation from data-rich to data-poor regions, the response of the assimilation to a strong disturbance to the flow, and a vortex-shedding instability phenomenon at high rotation rate. At the highest rotation rates we calculated the barotropic E -vectors using unobserved variables such as temperature and the vertical structure of the velocity field that are only available via the assimilation. These showed that the mean flow is weakened by the action of eddies, going some way towards explaining why vortices are shed at the very highest rotation rates but not at lower rotation. Rossby-wave stability theory suggests that the underlying instability leading to vortex shedding may be baroclinic in character. Copyright © 2012 Royal Meteorological Society

Description: The problem of variational data assimilation for a nonlinear evolution model is formulated as an optimal control problem to find the initial condition function (analysis). The data contain errors (observation and background errors); hence there is an error in the analysis. For mildly nonlinear dynamics the analysis error covariance can be approximated by the inverse Hessian of the cost functional in the auxiliary data assimilation problem, and for stronger nonlinearity by the ‘effective’ inverse Hessian. However, it has been noticed that the analysis error covariance is not the posterior covariance from the Bayesian perspective. While these two are equivalent in the linear case, the difference may become significant in practical terms with the nonlinearity level rising. For the proper Bayesian posterior covariance a new approximation via the Hessian is derived and its ‘effective’ counterpart is introduced. An approach for computing the mentioned estimates in the matrix-free environment using the Lanczos method with preconditioning is suggested. Numerical examples which validate the developed theory are presented for the model governed by Burgers equation with a nonlinear viscous term. Copyright © 2012 Royal Meteorological Society

Description: Investigating the influence of surface friction on the inertial oscillation (IO) of an extratropical, non-growing, convective boundary layer (CBL), we paid particular attention to the stability-dependent interactive coupling of shear-induced turbulence and turbulent friction, which leads to a nonlinear relationship between friction and velocity. We showed that in contrast to common perception, IO damping is controlled not only by friction but also by the dependence of friction on velocity. Furthermore, we found that surface friction not only causes damping but also modifies the restoring force. Using these basic principles, we studied the oscillatory properties (equilibrium, periodicity and damping) of the CBL by means of a model based on Monin–Obukhov surface-layer similarity (MOS) and the mixed-layer approximation. We found that the model complies with a quadratic surface stress–velocity relationship (QS) in the neutral limit, and a linear surface stress–velocity relationship (LS) in the proximity of the free-convective limit. Dynamically, the LS leads to a harmonic oscillation with a constant periodicity and exponential damping of the IO. However, the QS displays rather complex anharmonic behaviour; in comparison with the LS it produces a 50% stronger overall damping and a 100% larger contribution to the restoring force. Considering CBLs of arbitrary stability, we found that the MOS stress–velocity relation can be very well approximated by a much simpler linear combination of the LS and the QS which, respectively, represent the convective and the shear-induced contributions to friction. This enabled us to link the set of the external parameters (surface roughness, surface buoyancy flux and boundary layer depth) to a set of three effective parameters: the equilibrium velocity, the convective friction constant and the neutral friction constant. Together with the Coriolis coefficient, these parameters completely determine the IO. Copyright © 2012 Royal Meteorological Society

Description: A mesoscale convective system (MCS) case study was observed over northeast Mali as part of the African Monsoon Multidisciplinary Analysis (AMMA) on 31 July 2006. Observations of this case suggest that the soil-moisture heterogeneity and atmospheric gravity waves emitted from a ‘parent’ MCS were important trigger mechanisms for this system. This study uses high-resolution Met Office Unified Model (MetUM) simulations to assess the importance of the synoptic circulation, land-surface and gravity waves in the initiation and development of the MCS. During the early afternoon shallow convection developed over a region of dry soil within a synoptic-scale convergence zone, which was caused by the confluence of the southerly monsoon flow with winds associated with the circulation around the Saharan heat low. Two pronounced waves were emitted from a nearby ‘parent’ storm and propagated towards the convergence zone. When the second wave reached the location of the shallow convection, deep convection was immediately initiated. Further convective cells developed later in the afternoon over dry soil, many adjacent to strong soil moisture gradients; these aggregated with the main storm, which later developed into the case study MCS. A comparison of model simulations with/without the soil-moisture heterogeneity and gravity waves shows that the synoptic-scale circulation and convergence zones, specified by the atmospheric analysis, were the most important factors for the successful simulation of the MCS. If the location of the initiation of the system is to be forecast accurately, the land-surface, that is, the soil moisture, must be represented adequately. In order to reproduce the timing of the secondary initiation of convection correctly the model must be able to capture gravity waves that are emitted by existing systems. Copyright © 2012 Royal Meteorological Society