Changes in Atmospheric Constituents and in Radiative Forcing

The subject of inverse problems in differential equations is of enormous practical importance, and has also generated substantial mathematical and computational innovation. Typically some form of regularization is required to ameliorate ill-posed behaviour. In this article we review the Bayesian approach to regularization, developing a function space viewpoint on the subject. This approach allows for a full characterization of all possible solutions, and their relative probabilities, whilst simultaneously forcing significant modelling issues to be addressed in a clear and precise fashion. Although expensive to implement, this approach is starting to lie within the range of the available computational resources in many application areas. It also allows for the quantification of uncertainty and risk, something which is increasingly demanded by these applications. Furthermore, the approach is conceptually important for the understanding of simpler, computationally expedient approaches to inverse problems.

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Inverse problems: A Bayesian perspective

A new version of the atmosphere–ocean general circulation model cooperatively produced by the Japanese research community, known as the Model for Interdisciplinary Research on Climate (MIROC), has recently been developed. A century-long control experiment was performed using the new version (MIROC5) with the standard resolution of the T85 atmosphere and 1° ocean models. The climatological mean state and variability are then compared with observations and those in a previous version (MIROC3.2) with two different resolutions (medres, hires), coarser and finer than the resolution of MIROC5. A few aspects of the mean fields in MIROC5 are similar to or slightly worse than MIROC3.2, but otherwise the climatological features are considerably better. In particular, improvements are found in precipitation, zonal mean atmospheric fields, equatorial ocean subsurface fields, and the simulation of El Nino–Southern Oscillation. The difference between MIROC5 and the previous model is larger than that between th...

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Improved Climate Simulation by MIROC5: Mean States, Variability, and Climate Sensitivity

Monthly and 3-hourly precipitation data from twentieth-century climate simulations by the newest generation of 18 coupled climate system models are analyzed and compared with available observations. The characteristics examined include the mean spatial patterns, intraseasonal-to-interannual and ENSO-related variability, convective versus stratiform precipitation ratio, precipitation frequency and intensity for different precipitation categories, and diurnal cycle. Although most models reproduce the observed broad patterns of precipitation amount and year-to-year variability, models without flux corrections still show an unrealistic double-ITCZ pattern over the tropical Pacific, whereas the flux-corrected models, especially the Meteorological Research Institute (MRI) Coupled Global Climate Model (CGCM; version 2.3.2a), produce realistic rainfall patterns at low latitudes. As in previous generations of coupled models, the rainfall double ITCZs are related to westward expansion of the cold tongue of...

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Precipitation Characteristics in Eighteen Coupled Climate Models

The development of NWP models with grid spacing down to ∼1 km should produce more realistic forecasts of convective storms. However, greater realism does not necessarily mean more accurate precipitation forecasts. The rapid growth of errors on small scales in conjunction with preexisting errors on larger scales may limit the usefulness of such models. The purpose of this paper is to examine whether improved model resolution alone is able to produce more skillful precipitation forecasts on useful scales, and how the skill varies with spatial scale. A verification method will be described in which skill is determined from a comparison of rainfall forecasts with radar using fractional coverage over different sized areas. The Met Office Unified Model was run with grid spacings of 12, 4, and 1 km for 10 days in which convection occurred during the summers of 2003 and 2004. All forecasts were run from 12-km initial states for a clean comparison. The results show that the 1-km model was the most skillfu...

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Scale-Selective Verification of Rainfall Accumulations from High-Resolution Forecasts of Convective Events

Many numerical weather prediction models are moving towards more prognostic schemes for the prediction of ice and liquid water contents within clouds. This paper describes a large-scale cloud and precipitation scheme developed for the UK Meteorological Office's Unified Model. It uses physically based transfer equations to predict ice as a prognostic variable. We review similar schemes and then describe our new scheme, giving examples of its performance in mesoscale forecasts compared with the current operational scheme and with observations. the microphysical processes occurring in a frontal cloud are well modelled. the prediction of supercooled stratocumulus cloud is much improved.

A microphysically based precipitation scheme for the UK Meteorological Office Unified Model

Convection-permitting models (CPMs) have provided weather forecasting centres with a step-change in capabilities for forecasting rainfall. They are now used operationally to forecast precipitation in many parts of the world, including the UK. CPMs are models in which the dynamics of atmospheric convection is treated with sufficient accuracy in order to make it viable to switch off convection parametrization. This review describes the current state-of-the-art in operational CPM-based numerical weather prediction (NWP), primarily within the UK, and the historical development of CPMs. The characteristics of CPM systems and forecasts are highlighted and placed in an international context to recognize similar trends and highlight some differences. It is shown that the realism of CPM-based forecasts can provide improved subjective guidance on convection, and, when measured on appropriate scales, can improve rainfall forecasting skill compared to coarser-resolution NWP. Data assimilation techniques used with operational CPMs are reviewed and given historical context. Examples of new types of observations that may increase the skill of forecasts from improved initial conditions are discussed. CPM-based nowcasting systems are shown to provide considerable improvements in short-range forecasts of rapidly developing, intense systems. As a result, these CPM-based systems provide a new forecasting capability. Finally, the development of CPMs has also required new techniques to verify forecasts and define their skill. These have revealed that the lack of predictability of the smallest scales involving convection means that ensemble techniques are required to represent forecast uncertainty, resulting in a new capability to provide objective forecast probabilities of local precipitation.

Convection-permitting models: a step-change in rainfall forecasting

The development of a hurricane-like vortex over the Mediterranean Sea was
 studied using (mainly) the UK Met. Office Unified Model. The Mediterranean
 cyclone formed in the morning of 15 January 1995 over the sea between Greece
 and Sicily. Strong convection was observed prior to its genesis. During the
 longest part of the cyclone's lifetime, strong surface fluxes and, as a
 result, deep convection existed in its vicinity. Its track was influenced
 by the surface fluxes and the flow in the wider region. The forecast of the
 mesoscale and limited-area models reproduced the general characteristics of
 the actual system as they appeared at the surface and upper-air charts and
 at the satellite imagery. The investigation of the cyclone's characteristics
 gave strong evidence (including an ‘eye’ and a warm core) to support the
 initial assertion that it was similar to tropical cyclones and some polar
 lows. Baroclinic instability does not seem particularly important, although
 the cyclone formed at the edge of a baroclinic zone. A numerical experiment
 showed the vortex did not develop in the absence of surface heat and
 moisture fluxes. Another experiment showed that sensible and latent heat
 fluxes were equally important in its development.

The hurricane‐like Mediterranean cyclone of January 1995

The Met Office developed and demonstrated an hourly cycling 1.5 km resolution NWP-based nowcast system (0–6 hour forecasts) using 4-dimensional variational data assimilation (4D-Var). This was known as the Nowcasting Demonstration Project (NDP), and was principally for prediction of convective storms for flood forecasting. The NDP was run in real-time from March 2012 to April 2013 to cover the period of the London Olympics 2012. The system was run on a domain covering southern England and Wales nested in the UK variable resolution model (UKV). The UKV used a UK-wide 1.5 km domain with 3-hourly cycling 3-dimensional variational data assimilation (3D-Var) and produced 36 hour forecasts every 6 hours. The NDP 4D-Var included standard observations, Doppler radar radial winds, humidity derived from a 3D cloud cover analysis and geostationary satellite upper-tropospheric water vapour radiances not contaminated by cloud. This was used in combination with latent heat nudging of radar-derived precipitation rates. Example case studies compare the NDP precipitation forecasts to both the operational extrapolation/merged nowcast system and the UKV forecasts. Objective comparison of fraction skill score for the period June to August 2012 shows that the NDP skill was greater than the latest UKV forecasts, available to forecasters at the same time as the NDP, for the whole 6 hour forecast period. The skill of the NDP was greater than the operational extrapolation/merged nowcast beyond T+2 hours.

Performance of 4D‐Var NWP‐based nowcasting of precipitation at the Met Office for summer 2012

This article describes the development and trials at the Met Office of the assimilation of radar Doppler radial winds in a pre-operational framework. It uses a high-resolution 1.5 km convective-scale version of the Unified Model for nowcasting, on a small domain covering southern England and Wales. The assimilation was performed using a version of the Met Office 3D-variational data assimilation system at 1.5 km resolution. The Doppler radial wind observations were provided by the original four Dopplerized C-band single-polarization radars in the UK radar network, all situated in the south of England. This article provides a description of the Doppler radial wind observation operator, pre-processing, observation errors and data assimilation configuration. The system is tested on and verified for four case studies using continuous hourly cycling with 7 h forecasts. A significant positive impact for rain and wind is found in the first half of the forecasts (nowcasting time-scale).

Susan P. Ballard

Papers

A microphysically based precipitation scheme for the UK Meteorological Office Unified Model

Convection-permitting models: a step-change in rainfall forecasting

The hurricane‐like Mediterranean cyclone of January 1995

Performance of 4D‐Var NWP‐based nowcasting of precipitation at the Met Office for summer 2012

Doppler radar radial wind assimilation using an hourly cycling 3D-Var with a 1.5 km resolution version of the Met Office Unified Model for nowcasting