A numerical weather prediction (NWP) system that represents continental Australia at a convection-permitting resolution presents both advantages and challenges. It must represent diverse weather regimes over areas of variable observation coverage, which complicates the land and atmospheric modelling and data assimilation. A single-domain prototype system, ACCESS-A (Australia Community Climate and Earth System Simulator – Australia), has been developed and tested. Compared with the current operational NWP system comprising seven small domains (ACCESS-C), ACCESS-A incorporates improvements to satellite, conventional and radar data assimilation and uses an upgraded model science configuration. ACCESS-A was extensively evaluated over two 3-month periods. Qualitative and quantitative precipitation verification indicates that ACCESS-A reproduces the seasons’ weather patterns and observed behaviour of convective precipitation. Objective verification of defined subdomains shows that forecast skill of near-surface weather is variable across the continent. It is found more skilful in better-observed regions that coincide with areas dominated by more predictable, synoptically driven weather systems. Regions with lower skill, particularly corresponding to areas not covered by ACCESS-C, suggest a focus for future research. A comparison with ACCESS-C confirmed the anticipated improved skill related to the forecast model’s upgraded land and atmospheric physics, and provides confidence in the combined impact of all upgrades implemented in ACCESS-A. ACCESS-A is demonstrated to be ready to prepare for operational NWP and ongoing research at the Australian Bureau of Meteorology.

We present the preliminary results of an information content study to assess the potential of a proposed hyperspectral microwave satellite instrument, the Bureau of Meteorology (the Bureau) microwave-sounding mission (MSM), to provide atmospheric temperature and humidity profile information under clear-sky conditions in the context of the Bureau's global numerical weather prediction (NWP) system. First, we conducted a user survey that allowed us to develop the requirements for the mission, which indicated a desire to prioritise 50-60-GHz band capability and noise performance. To examine trade-offs between band availability and spectral resolution versus noise, we used a one-dimensional optimal estimation technique to calculate the information content of clear-sky temperature and humidity vertical profiles from the hyperspectral MSM in the context of NWP. The degrees of freedom for signal (DFS) are computed for temperature and water vapour using the column representation of the Bureau's operational background error covariance matrix (B) for different combinations of observation errors (R) and spectral resolutions. The results show that the information content is highest when the instrument is configured to a 10-MHz bandwidth in the 50-60-GHz band, and with single-sideband 20-MHz and 40-MHz bandwidths in the 118- and 183-GHz bands respectively. The information content can be considerably improved by using a 3 x 3 averaging of adjacent footprints, similar to the preprocessing of the advanced technology microwave sounder (ATMS) in operational use at the Bureau, indicating that spatial oversampling is a desirable feature of such an instrument. The study indicates that the hyperspectral MSM would provide considerably increased information content relative to ATMS and other existing operational microwave sounders. This finding is consistent with other studies into the benefits of hyperspectral microwave instrumentation, and in line with the impact provided by operational hyperspectral infrared sounders such as the infrared atmospheric sounding interferometer (IASI).

This study presents a complete assessment of the use of different observation types and their impacts in the Met Office global data assimilation system. The data denial experiments are run over three months using the observations from 15 December 2022 to 15 March 2023 and use a configuration very close to the Met Office operational global numerical weather prediction (NWP) suite, with some exceptions, mainly for reducing the computational cost. The control run uses the complete observing system assimilated operationally at the time, and data from all the main observing systems (e.g., microwave sounders, hyperspectral infrared sounders, clear-sky geostationary sounders, and conventional data, etc.) are denied categorically for a series of experiments. Results indicate that microwave satellite radiance observations continue to be the most important data source overall, being as impactful as conventional observations in the Northern Hemisphere and, along with Global Navigation Satellite System radio occultation (GNSS-RO), the most important source in the Tropics and Southern Hemisphere. Conventional observations from aircraft and radiosonde ascents continue to play an essential role, particularly in the Northern Hemisphere. Our investigation also suggests that GNSS-RO provides significant benefits, especially in the upper troposphere and lower stratosphere for key atmospheric variables, especially temperature. Hyperspectral sounders show less impact than in previous studies, though still show strong positive impact in the Southern Hemisphere. Atmospheric motion vectors show impact on tropospheric wind, particularly in the Tropics. In general, all observation types show a positive impact on the mean forecast skill in at least some aspects of the global NWP system.

Radiance observations are typically affected by biases that come mainly from instrument error (scanning or calibration) and inaccuracies of the radiative transfer model. These biases need to be removed for successful assimilation, so a bias correction scheme is crucial in the Numerical Weather Prediction (NWP) system. Today, most NWP centres, including the Bureau of Meteorology (hereafter, “the Bureau”), correct the biases through variational bias correction (VarBC) schemes, which were originally developed for global models. However, there are difficulties in estimating the biases in a limited-area model (LAM) domain. As a result, the Bureau’s regional NWP system, ACCESS-C (Australian Community Climate and Earth System Simulator-City), uses variational bias coefficients obtained directly from its global NWP system ACCESS-G (Global). This study investigates independent radiance bias correction in the data assimilation system for ACCESS-C. We assessed the impact of using independent bias correction for the LAM compared with the operational bias coefficients derived in ACCESS-G between February and April 2020. The results from our experiment show no significant difference between the control and test, suggesting a neutral impact on the forecast. Our findings point out that the VarBC-LAM strategy should be further explored with different settings of predictors and adaptivity for a more extended period and over additional domains.

Aircraft reports are an important source of information for numerical weather prediction (NWP). From March 2020, the COVID‐19 pandemic resulted in a large loss of aircraft data but despite this it is difficult to see any evidence of significant degradation in the forecast skill of global NWP systems. This apparent discrepancy is partly because forecast skill is very variable, showing both day‐to‐day noise and lower frequency dependence on the mean state of the atmosphere. The definitive way to cleanly assess aircraft impact is using a data denial experiment, which shows that the largest impact is in the upper troposphere. The method used by Chen (2020, https://doi.org/10.1029/2020gl088613 ) to estimate the impact of COVID‐19 is oversimplistic. Chen understates the huge importance of satellite data for modern weather forecasts and raises more alarm than necessary about a drop in forecast accuracy.

Recent developments in numerical weather prediction have led to the use of correlated observation‐error covariance (OEC) information in data assimilation and forecasting systems. However, diagnosed OEC matrices are often ill‐conditioned and may cause convergence problems for variational data assimilation procedures. Reconditioning methods are used to improve the conditioning of covariance matrices while retaining correlation information. In this article, we study the impact of using the “ridge regression” method of reconditioning to assimilate Infrared Atmospheric Sounding Interferometer (IASI) observations in the Met Office 1D‐Var system. This is the first systematic investigation of how changing target condition numbers affects convergence of a 1D‐Var routine. This procedure is used for quality control, and to estimate key variables (skin temperature, cloud‐top pressure, cloud fraction) that are not analysed by the main 4D‐Var data assimilation system. Our new results show that the current (uncorrelated) OEC matrix requires more iterations to reach convergence than any choice of correlated OEC matrix studied. This suggests that using a correlated OEC matrix in the 1D‐Var routine would have computational benefits for IASI observations. Using reconditioned correlated OEC matrices also increases the number of observations that pass quality control. However, the impact on skin temperature, cloud fraction, and cloud‐top pressure is less clear. As the reconditioning parameter is increased, differences between retrieved variables for correlated OEC matrices and the operational diagonal OEC matrix reduce. As correlated choices of OEC matrix yield faster convergence, using stricter convergence criteria along with these matrices may increase efficiency and improve quality control.

This article reports the results of a preliminary mission study to assess the potential of space‐borne laser heterodyne radiometry (LHR) for the remote sensing of temperature for assimilation in a numerical weather prediction (NWP) model. The LHR instruments are low cost and small in size, lending themselves to a wide variety of satellite platforms. The impact of different configurations of an idealized LHR instrument is assessed against the Infrared Atmospheric Sounding Interferometer (IASI), via single‐column linear information content analysis, using inputs consistent with the background errors of the Met Office 4D‐Var assimilation system. Multiplexed configurations give promising results, in particular for sounding of upper‐atmospheric temperatures.

An atmospheric diffusion model of a gaseous cloud released from an infinite cross-wind line source with surface deposition is considered as a steady state boundary value problem which is solved by the classical method. The solution is applicable to unstable lapse rates and light winds, i.e. the mean wind is assumed to be constant with height, but the eddy difiusivity in the vertical direction is assumed to be proportional to the square of height.

AbstractA series of records, totalling over 120 hr showing the vertical fluctuations of high‐response wind‐vanes positioned on a captive balloon cable, are analysed to give statistical information on turbulence at heights between 500 and 5,000 ft, for an eddy frequency range 0·003‐12 cycles min−1. The use of specially designed filter and smoothing circuits facilitates the determination of the angular standard deviation of the inclination of the vane, as a function of run of wind, and of spectra, length‐scales and other parameters of turbulence. 18 spectra are given which show well‐defined peaks. Except in very stable conditions, the high wave‐number forms of the spectra are generally consistent with the existence of an inertial sub‐range. Estimates of the length‐scale, again excluding very stable conditions, do not apparently vary with height and average about 300 m. Intensity is found to depend on stability and to decrease in a very marked way with increasing wind speed, a result peculiar to these heights. This variation, applicable to specified meteorological conditions, is shown to have value in the determination of the spread of both continuous‐source plumes and clusters of particles according to recent treatments.

AbstractIn this paper, the problem of the spatial expansion of a cluster of particles under the action of turbulence is investigated. The basic difficulty, that this form of diffusion depends on the Lagrangian properties of the turbulent field, is met by the assumption that the Lagrangian and Eulerian time‐correlograms are functionally similar. In consequence, it is possible to solve the equation governing the cluster's expansion to give prediction laws expressed only in terms of easily measured parameters of the turbulence. The application of the full solution is lengthy but a simple working approximation follows from the fact that over an important part of the expansion at which the size of the cluster is of the same order of magnitude as the Eulerian length‐scale, the rate of expansion is approximately constant and is proportional to the square of the intensity of turbulence whilst independent of the length scale itself. Experiments to test these formulae have been carried out on the diffusion of clusters of Lycopodium spores over a few hundred metres downwind travel near the ground, and the diffusion of long crosswind clouds of aircraft‐released fluorescent particles, sampled along the vertical, after several kilometres travel in the free atmosphere. Whilst the experiments are few in number and the conditions often did not strictly conform to the conditions of isotropy and homogeneity required by the theory, the results are satisfactorily consistent with the prediction formulae and imply an effectively constant ratio between the scales of the Lagrangian and Eulerian time‐correlograms.

AbstractAerological data for the British Isles, 1953, at nine pressure levels, are analysed, to give the geostrophic and ageostrophic components of the wind and the radius of curvature of the isobars. The mean total energy over unit surface area is found to be 1·71 × 109 erg cm−2.The equation of energy associated with the ageostrophic component is obtained, and the work done by the flow down the pressure gradient is identified by means of this equation with a Coriolis term and a centrifugal term. These are estimated from the data, giving a value for the rate of generation of kinetic energy of 8·8 × 103 erg cm−2 sec−1. The mean net mass‐flux across unit length of isobar is found to be 25 kg cm−1 sec−1. The order of magnitude of this quantity is checked in the final section in which a mathematical analysis is applied to a model depression filling by the influx of mass across its perimeter.