Met Office Unified Model Research Articles

Assessing the impact of a NWP warm‐start system on model spin‐up over tropical Africa

AbstractConvection‐permitting (CP) models have provided a step change in the ability to forecast impactful convective storms, which pose risks such as flash flooding and lightning. Despite CP models being routinely run over tropical Africa, they typically lack data assimilation and are initialised directly from a global analysis. The CP model therefore takes time to generate convective structures and consequently precipitation, rendering the early part of the forecast unusable. A ‘warm‐starting’ method developed within the Met Office Unified Model is tested over a large tropical Africa domain, leading to substantial improvements in rainfall predictions during both the spin‐up period but also later in the forecast. The spin‐up period is found to vary considerably, both geographically and with model initialisation time, due to the differing states of the atmosphere through the day. We quantify this impact through a spin‐up diagnostic, which shows that over central Africa the spin‐up period can be as large as 24 hr. In these regions, the warm starting substantially reduces spin‐up time. Additionally, the large tropical Africa domain allows us to examine the impact of the prevailing meteorology on both the spin‐up period and the warm‐start method. We find large‐scale positive vorticity filaments over West Africa, likely tied to African Easterly Waves, increase rainfall rates across all models. The substantial reduction in the spin‐up of rainfall and increased skill at longer lead times in the warm‐start demonstrate the importance of representing convective features in the analysis for a CP model. This is especially important in large tropical domains where the initial conditions are likely to have a lasting impact on the forecast. Improvements in model performance across the forecast will enhance the ability of forecasters to provide advice on high‐impact weather events, supporting increased resilience against weather extremes in Africa.

The impact of surface heterogeneity on the diurnal cycle of deep convection

AbstractDespite some recent improvements, major deficiencies remain in model simulations using parameterised convection in capturing both the phase and amplitude of the daily cycle of precipitation in tropical regions. The difficulties are particularly acute in regions of heterogeneous surface conditions, since the simulations need not only to respond appropriately to the local forcing from surface fluxes but also to capture the interactions with near‐surface mesoscale circulations. Here, we examine such a situation by means of idealised cloud‐resolving simulations of deep convection over a heterogeneous surface, performed using the cloud‐resolving simulation known as the Met Office NERC Cloud model (MONC). In these simulations, we show that precipitation forms preferentially over dry and warm patches (“DRY”) compared with wet and cold patches (“WET”), with the peak precipitation rates differing by a factor of approximately 4. The initiation of precipitation occurs approximately 1.5 hr earlier in the DRY patches compared with the WET patches. Moreover, within the WET and DRY patches there are marked differences in the spatial distribution of the precipitation. These cloud‐resolving simulations are then used as a benchmark to assess the behaviour of simulations using parameterised convection, performed using the idealised configuration of the Met Office Unified Model (MetUM). The MetUM simulations produce a response with some qualitative similarities to the cloud‐resolving simulations. In particular, although the simulations with parameterised convection initiate precipitation too early, they are capable of capturing the relative amounts of daily‐mean precipitation in the DRY and WET patches. We propose that the cloud‐resolving simulations could be used further to investigate the impact of fully interactive surface schemes and as benchmark simulations to evaluate new parameterisation schemes.

Lightning-induced chemistry on tidally-locked Earth-like exoplanets

ABSTRACT Determining the habitability and interpreting atmospheric spectra of exoplanets requires understanding their atmospheric physics and chemistry. We use a 3-D coupled climate-chemistry model, the Met Office Unified Model with the UK Chemistry and Aerosols framework, to study the emergence of lightning and its chemical impact on tidally-locked Earth-like exoplanets. We simulate the atmosphere of Proxima Centauri b orbiting in the Habitable Zone of its M-dwarf star, but the results apply to similar M-dwarf orbiting planets. Our chemical network includes the Chapman ozone reactions and hydrogen oxide (HOx = H + OH + HO2) and nitrogen oxide (NOx = NO + NO2) catalytic cycles. We find that photochemistry driven by stellar radiation (177–850 nm) supports a global ozone layer between 20–50 km. We parametrize lightning flashes as a function of cloud-top height and the resulting production of nitric oxide (NO) from the thermal decomposition of N2 and O2. Rapid dayside convection over and around the substellar point results in lightning flash rates of up to 0.16 flashes km−2 yr−1, enriching the dayside atmosphere below altitudes of 20 km in NOx. Changes in dayside ozone are determined mainly by UV irradiance and the HOx catalytic cycle. ∼45 per cent of the planetary dayside surface remains at habitable temperatures (Tsurf > 273.15K), and the ozone layer reduces surface UV radiation levels to 15 per cent. Dayside–nightside thermal gradients result in strong winds that subsequently advect NOx towards the nightside, where the absence of photochemistry allows NOx chemistry to involve reservoir species. Our study also emphasizes the need for accurate UV stellar spectra to understand the atmospheric chemistry of exoplanets.

Variability in Antarctic surface climatology across regional climate models and reanalysis datasets

Abstract. Regional climate models (RCMs) and reanalysis datasets provide valuable information for assessing the vulnerability of ice shelves to collapse over Antarctica, which is important for future global sea level rise estimates. Within this context, this paper examines variability in snowfall, near-surface air temperature and melt across products from the Met Office Unified Model (MetUM), Regional Atmospheric Climate Model (RACMO) and Modèle Atmosphérique Régional (MAR) RCMs, as well as the ERA-Interim and ERA5 reanalysis datasets. Seasonal and trend decomposition using LOESS (STL) is applied to split the monthly time series at each model grid cell into trend, seasonal and residual components. Significant systematic differences between outputs are shown for all variables in the mean and in the seasonal and residual standard deviations, occurring at both large and fine spatial scales across Antarctica. Results imply that differences in the atmospheric dynamics, parametrisation, tuning and surface schemes between models together contribute more significantly to large-scale variability than differences in the driving data, resolution, domain specification, ice sheet mask, digital elevation model and boundary conditions. Despite significant systematic differences, high temporal correlations are found for snowfall and near-surface air temperature across all products at fine spatial scales. For melt, only moderate correlation exists at fine spatial scales between different RCMs and low correlation between RCM and reanalysis outputs. Root mean square deviations (RMSDs) between all outputs in the monthly time series for each variable are shown to be significant at fine spatial scales relative to the magnitude of annual deviations. Correcting for systematic differences results in significant reductions in RMSDs, suggesting the importance of observations and further development of bias-correction techniques.

Below-cloud scavenging of aerosol by rain: a review of numerical modelling approaches and sensitivity simulations with mineral dust in the Met Office's Unified Model

Abstract. Theoretical models of the below-cloud scavenging (BCS) of aerosol by rain yield scavenging rates that are 1–2 orders of magnitude smaller than observations and associated empirical schemes for submicron-sized aerosol. Even when augmented with processes which may explain this disparity, such as phoresis and rear capture in the raindrop wake, the theoretical BCS rates remain an order of magnitude less than observations. Despite this disparity, both theoretical and empirical BCS schemes remain in wide use within numerical aerosol models. BCS is an important sink for atmospheric aerosol, in particular for insoluble aerosol such as mineral dust, which is less likely to be scavenged by in-cloud processes than purely soluble aerosol. In this paper, various widely used theoretical and empirical BCS models are detailed and then applied to mineral dust in climate simulations with the Met Office's Unified Model in order the gauge the sensitivity of aerosol removal to the choice of BCS scheme. We show that the simulated accumulation-mode dust lifetime ranges from 5.4 d in using an empirical BCS scheme based on observations to 43.8 d using a theoretical scheme, while the coarse-mode dust lifetime ranges from 0.9 to 4 d, which highlights the high sensitivity of dust concentrations to BCS scheme. We also show that neglecting the processes of rear capture and phoresis may overestimate submicron-sized dust burdens by 83 %, while accounting for modal widths and mode merging in modal aerosol models alongside BCS is important for accurately reproducing observed aerosol size distributions and burdens. This study provides a new parameterisation for the rear capture of aerosol by rain and is the first to explicitly incorporate the rear-capture mechanism in climate model simulations. Additionally, we answer many outstanding questions pertaining to the numerical modelling of BCS of aerosol by rain and provide a computationally inexpensive BCS algorithm that can be readily incorporated into other aerosol models.

A 20‐Year Study of Melt Processes Over Larsen C Ice Shelf Using a High‐Resolution Regional Atmospheric Model: 1. Model Configuration and Validation

AbstractFollowing collapses of the neighboring Larsen A and B ice shelves, Larsen C has become a focus of increased attention. Determining how the prevailing meteorological conditions influence its surface melt regime is of paramount importance for understanding the dominant processes causing melt and ultimately for predicting its future. To this end, a new, high‐resolution (4 km grid spacing) Met Office Unified Model (MetUM) hindcast of atmospheric conditions and surface melt processes over the central Antarctic Peninsula is introduced. The hindcast is capable of simulating observed near‐surface meteorology and surface melt conditions over Larsen C. In contrast with previous model simulations, the MetUM captures the observed east‐west gradient in surface melting associated with foehn winds, as well as the interannual variability in melt shown in previous observational studies. As exemplars, we focus on two case studies—the months preceding the collapse of the Larsen B ice shelf in March 2002 and the high foehn, high melt period of March‐May 2016—to test the hindcast's ability to reproduce the atmospheric effects that contributed to considerable melting during those periods. The results suggest that the MetUM hindcast is a useful tool with which to explore the dominant causes of surface melting on Larsen C.

A 20‐Year Study of Melt Processes Over Larsen C Ice Shelf Using a High‐Resolution Regional Atmospheric Model: 2. Drivers of Surface Melting

AbstractQuantifying the relative importance of the atmospheric drivers of surface melting on the Larsen C ice shelf is critical in the context of recent and future climate change. Here, we present analysis of a new multidecadal, high‐resolution model hindcast using the Met Office Unified Model, described in Part 1 of this study. We evaluate the contribution of various atmospheric conditions in order to identify and rank, for the first time, the most significant causes of melting over the recent past. We find the primary driver of surface melting on Larsen C is solar radiation. Foehn events are the second most important contributor to surface melting, especially in nonsummer seasons when less solar radiation is received at the surface of the ice shelf. Third, cloud influences surface melting via its impact on the surface energy balance (SEB); when the surface temperature is warm enough, cloud can initiate or prolong periods of melting. Lastly, large‐scale circulation patterns such as the Southern Annular Mode (SAM), El Niño Southern Oscillation, and Amundsen Sea Low control surface melting on Larsen C by influencing the local meteorological conditions and SEB. These drivers of melting interact and overlap, e.g., the SAM influences the frequency of foehn, commonly associated with leeside cloud clearances and sunnier conditions. Ultimately, these drivers matter because sustained surface melting on Larsen C could destabilize the ice shelf via hydrofracturing, which would have consequences for the fate of the ice shelf and sea levels worldwide.

Observed and Modeled Mountain Waves from the Surface to the Mesosphere near the Drake Passage

Abstract Four state-of-the-science numerical weather prediction (NWP) models were used to perform mountain wave (MW)-resolving hindcasts over the Drake Passage of a 10-day period in 2010 with numerous observed MW cases. The Integrated Forecast System (IFS) and the Icosahedral Nonhydrostatic (ICON) model were run at Δx ≈ 9 and 13 km globally. The Weather Research and Forecasting (WRF) Model and the Met Office Unified Model (UM) were both configured with a Δx = 3-km regional domain. All domains had tops near 1 Pa (z ≈ 80 km). These deep domains allowed quantitative validation against Atmospheric Infrared Sounder (AIRS) observations, accounting for observation time, viewing geometry, and radiative transfer. All models reproduced observed middle-atmosphere MWs with remarkable skill. Increased horizontal resolution improved validations. Still, all models underrepresented observed MW amplitudes, even after accounting for model effective resolution and instrument noise, suggesting even at Δx ≈ 3-km resolution, small-scale MWs are underresolved and/or overdiffused. MW drag parameterizations are still necessary in NWP models at current operational resolutions of Δx ≈ 10 km. Upper GW sponge layers in the operationally configured models significantly, artificially reduced MW amplitudes in the upper stratosphere and mesosphere. In the IFS, parameterized GW drags partly compensated this deficiency, but still, total drags were ≈6 times smaller than that resolved at Δx ≈ 3 km. Meridionally propagating MWs significantly enhance zonal drag over the Drake Passage. Interestingly, drag associated with meridional fluxes of zonal momentum (i.e., ) were important; not accounting for these terms results in a drag in the wrong direction at and below the polar night jet. Significance Statement This study had three purposes: to quantitatively evaluate how well four state-of-the-science weather models could reproduce observed mountain waves (MWs) in the middle atmosphere, to compare the simulated MWs within the models, and to quantitatively evaluate two MW parameterizations in a widely used climate model. These models reproduced observed MWs with remarkable skill. Still, MW parameterizations are necessary in current Δx ≈ 10-km resolution global weather models. Even Δx ≈ 3-km resolution does not appear to be high enough to represent all momentum-fluxing MW scales. Meridionally propagating MWs can significantly influence zonal winds over the Drake Passage. Parameterizations that handle horizontal propagation may need to consider horizontal fluxes of horizontal momentum in order to get the direction of their forcing correct.

The use of 100 m scale NWP models to understand differences between different measures of mixing height in a morning growing clear convective boundary layer over London

AbstractAn important practical problem is to understand the depth to which pollutants will be mixed in the boundary layer over cities. This is particularly important shortly after sunrise on clear days when the mixed layer is often shallow and there may be large emissions from rush‐hour traffic. The 100 m scale NWP models currently under development have the potential to represent the boundary‐layer structure over cities on neighbourhood scales for air quality and other applications. Here we describe work with Met Office Unified Model (MetUM) configurations with grid lengths of 100, 50 and 25 m to represent the morning growth of a clear convective boundary layer over London compared to observations. All three models explicitly represent the convective overturning although it is not as well resolved at 100 m and this is a particular problem early in the day. It is found that in both the observations and in the models that the depth to which aerosol is mixed up into the model from the surface is lower than the turbulence mixed‐layer depth as measured from the vertical velocity variance. We present an analysis in the 25 m model that shows that the reason for the discrepancy is that the aerosol is transported upwards from the surface by convective plumes whereas further up there is wave motion which does not transport aerosol. The signature for these two layers can be seen both in the 25 m model and in the observations. We discuss the representation of these effects in the coarser‐resolution 100 and 55 m models and also possible mechanisms for the generation of the waves.

Projected Changes in the East Asian Hydrological Cycle for Different Levels of Future Global Warming

Recent decades have shown significant changes to the hydrological cycle over East Asia (EA), and further changes are expected due to future global warming. This study evaluates projected seasonal changes in the EA hydrological cycle using simulations that are 1.5 °C, 2.0 °C and 3.0 ∘C warmer than pre-industrial, from the Met Office Unified Model (MetUM) Global Ocean Mixed Layer model version 2.0 (GOML2.0), compared against present-day conditions. The moisture sources of the warming-induced precipitation changes are identified over five hydrologically unique regions within EA. Precipitation over EA increases with warming (except over southeastern EA in the spring and autumn) due to the intensified hydrological cycle. The projected seasonal changes in the hydrological cycle are usually nonlinear, with the rate of change between 1.5 ∘C and 2.0 ∘C larger than the rate of change between 2.0 ∘C and 3.0 ∘C of warming. The warming-induced precipitation increases are mainly associated with an increase in remote moisture convergence rather than local moisture recycling, except over the Tibetan Plateau. Decomposition of the changes in moisture sources by direction and flux component indicate that changes from the west are dominated by changes to moisture and changes from the north are more circulation driven. The changes from the south are moisture driven over southern EA and driven by moisture and circulation change over northern EA. Our results highlight the regionally and seasonally diverse projected changes to the EA hydrological cycle due to global warming, which will be useful for region-specific climate mitigation policies and the implementation of seasonally varying adaptation methods.

Contrasting contributions of surface hydrological pathways in convection permitting and parameterised climate simulations over Africa and their feedbacks on the atmosphere

The partitioning of rainfall at the land surface into interception, infiltration and surface runoff plays an important role in the water cycle as it controls the time scale at which water returns to the atmosphere. Rainfall intensity is of crucial importance to this partition. High resolution convection permitting models significantly improve simulated sub-daily rainfall intensity distributions, in particular those associated with convective rainfall in the tropics. Here we compare the land surface hydrological response in a pair of 10-year simulations over an African domain performed using the Met Office Unified Model: a typical configuration using parameterised convection operating at 25 km and the second a high resolution convection permitting simulation at 4.5 km with the parametrized convection switched off. Overall pan-African interception in the convection permitting scheme is 70% lower, whilst surface runoff is 43% higher than the parameterized convection model. These changes are driven by less frequent, but more intense rainfall with a 25% increase in rainfall above 20 mm h−1 in the 4.5 km model. The parameterised scheme has a ~ 50% canopy water contribution to evaporative fraction which is negligible in the convection permitting scheme. Conversely, the convection permitting scheme has higher throughfall and infiltration leading to higher soil moisture in the weeks following rain resulting in a 30–50% decrease in the daytime sensible heat flux. We examine how important the sub-grid rainfall parameterisation in the model is for the differences between the two configurations. We show how, switching a convective parameterisation off can substantially impact land surface behaviour.

The Flexible Modelling Framework for the Met Office Unified Model (Flex-UM, using UM 12.0 release)

Abstract. The Met Office Unified Model (UM) is a world-leading atmospheric weather and climate model. In addition to comprehensive simulations of the atmosphere, the UM is capable of running idealised simulations, such as the dry physics Held–Suarez test case, radiative convective equilibrium and simulating planetary atmospheres other than Earth. However, there is a disconnect between the simplicity of the idealised UM model configurations and the full complexity of the UM. This gap inhibits the broad use of climate model hierarchy approaches within the UM. To fill this gap, we have developed the Flexible Modelling Framework for the UM – Flex-UM – which broadens the climate model hierarchy capabilities within the UM. Flex-UM was designed to replicate the atmospheric physics of the Geophysical Fluid Dynamics Laboratory (GFDL) idealised moist physics aqua-planet model. New parameterisations have been implemented in Flex-UM, including simplified schemes for convection, large-scale precipitation, radiation, boundary layer and sea surface temperature (SST) boundary conditions. These idealised parameterisations have been implemented in a modular way, so that each scheme is available for use in any model configuration. This has the advantage that we can incrementally add or remove complexity within the model hierarchy. We compare Flex-UM to ERA5 and aqua-planet simulations using the Isca climate modelling framework (based on the GFDL moist physics aqua-planet model) and comprehensive simulations of the UM (using the GA7.0 configuration). We also use two SST boundary conditions to compare the models (fixed SST and a slab ocean). We find the Flex-UM climatologies are similar to both Isca and GA7.0 (though Flex-UM is generally a little cooler, with higher relative humidity and a less pronounced storm track). Flex-UM has a single Intertropical Convergence Zone (ITCZ) in the slab-ocean simulation but a double-ITCZ in the fixed-SST simulation. Further work is needed to ensure that the atmospheric energy budget closes to within 1–2 W m−2, as the current configuration of Flex-UM gains 9–11 W m−2 (the range covers the two SST boundary conditions). Flex-UM greatly extends the modelling hierarchy capabilities of the UM and offers a simplified framework for developing, testing and evaluating parameterisations within the UM.

C‐POOL: A scheme for modelling convective cold pools in the Met Office Unified Model

AbstractA new parametrization scheme representing the lifecycle and effects of convective cold pools (C‐POOL) is presented. The scheme comprises a column‐crossing prognostic model of the initiation by downdraughts, interaction and decay of cold pools, based upon approximations taken from similarity modelling of gravity currents. The prognostic variables are drawn upon to feed back into the convection parametrization scheme. These feedbacks affect the convective diagnosis, initiation from the boundary layer, parcel ascent and entrainment. Tests of the model show that it improves representation of the diurnal cycle of tropical rainfall, previously well‐known as a weakness of parametrized convection.

Interactions Between the Nocturnal Low-Level Jets and the Urban Boundary Layer: A Case Study over London

Understanding the physical processes that affect the turbulent structure of the nocturnal urban boundary layer (UBL) is essential for improving forecasts of air quality and the air temperature in urban areas. Low-level jets (LLJs) have been shown to affect turbulence in the nocturnal UBL. We investigate the interaction of a mesoscale LLJ with the UBL during a 60-h case study. We use observations from two Doppler lidars and results from two high-resolution numerical-weather-prediction models (Weather Research and Forecasting model, and the Met Office Unified Model for limited-area forecasts for the U.K.) to study differences in the occurrence frequency, height, wind speed, and fall-off of LLJs between an urban (London, U.K.) and a rural (Chilbolton, U.K.) site. The LLJs are elevated (approx 70 m) over London, due to the deeper UBL, while the wind speed and fall-off are slightly reduced with respect to the rural LLJ. Utilizing two idealized experiments in the WRF model, we find that topography strongly affects LLJ characteristics, but there is still a substantial urban influence. Finally, we find that the increase in wind shear under the LLJ enhances the shear production of turbulent kinetic energy and helps to maintain the vertical mixing in the nocturnal UBL.

Influence of Orography Upon Summertime Low‐Level Jet Dust Emission in the Central and Western Sahara

AbstractLow‐level jets (LLJs) drive frequent emission of mineral dust in the central and western Sahara in boreal summer. A major hotspot for this process is central Algeria, northern Mali and Mauritania, through which blow the dry near‐surface northeasterly Harmattan winds, with a peak in dust emission around the low‐lying Tidihelt region. North African orography is thought to contribute to the strength of the LLJ over the Bodele dust source in Chad, but its influence on erosivity over summertime source regions remains unquantified. In this paper, the contribution of central Saharan orography to the strength of Harmattan LLJs and associated dust emission frequency is tested. An idealized simulation with flattened Hoggar mountains is compared with a control using the Met Office Unified Model at 12 km horizontal resolution. In the absence of the Hoggar mountains, dust emission frequency estimated using an empirical relationship with surface wind speeds is found to decline across the entire northeasterly “LLJ alley,” including by 31% in the Tidihelt where composited jet surface winds drop from 9.0 to 7.3 m s−1 under a more easterly regime. The mountains are linked to a low‐level leeward geopotential height perturbation, with a northern limb reinforcing northeasterlies through the Tidihelt. Dome‐shaped elevated heating situated over the Hoggar mountains explains the difference between the simulated wind fields in the two experiments. These findings suggest that central Saharan orography plays an important role in sustaining erosive dusty conditions during boreal summer.

Exploring the sensitivity of atmospheric nitrate concentrations to nitric acid uptake rate using the Met Office's Unified Model

Abstract. Ammonium nitrate is a major aerosol constituent over many land regions and contributes to air pollution episodes, ecosystem destruction, regional haze, and aerosol-induced climate forcing. Many climate models that represent ammonium nitrate assume that the ammonium–sulfate–nitrate chemistry reaches thermodynamic equilibrium instantaneously without considering kinetic limitations on condensation rates. The Met Office's Unified Model (UM) is employed to investigate the sensitivity of ammonium nitrate concentrations to the nitric acid uptake coefficient (γ) in a newly developed nitrate scheme in which first-order condensation theory is utilised to limit the rate at which thermodynamic equilibrium is attained. Two values of γ representing fast (γ=0.193) and slow (γ=0.001) uptake rates are tested in 20-year global UM integrations. The global burden of nitrate associated with ammonium in the “fast” simulation (0.11 Tg[N]) is twice as great as in the “slow” simulation (0.05 Tg[N]), while the top-of-the-atmosphere radiative impact of representing nitrate is −0.19 W m−2 in the fast simulation and −0.07 W m−2 in the slow simulation. In general, the fast simulation exhibits better spatial correlation with observed nitrate concentrations, while the slow simulation better resolves the magnitude of concentrations. Local near-surface nitrate concentrations are found to be highly correlated with seasonal ammonia emissions, suggesting that ammonia is the predominant limiting factor controlling nitrate prevalence. This study highlights the high sensitivity of ammonium nitrate concentrations to nitric acid uptake rates and provides a novel mechanism for reducing nitrate concentration biases in climate model simulations. The new UM nitrate scheme represents a step change in aerosol modelling capability in the UK across weather and climate timescales.

The impact of weak environmental steering flow on tropical cyclone track predictability

AbstractTyphoonsHaiyan(2013) andHagupit(2014) are examples of two tropical cyclones (TCs) for which, despite similarities in the track and intensity, the predictabilities differed greatly. Both TCs made landfall over the Philippines having followed a similar track across the Pacific and both reached intensities in excess of 60 ms−1. Operational global ensemble forecasts showed large uncertainty in the track of , whereas the ensemble spread for was considerably less. Using the Met Office's Unified Model, 5‐day global ensemble forecasts were produced for both storms. Consistent with the operational forecasts produced at the time of the storms, the spread of tracks is greater in the forecasts produced for than . The latter was located on the southern periphery of the subtropical high and embedded in a strong easterly flow. In contrast, the position of between two anticyclones earlier in the forecast is key to the subsequent motion of the storm in determining whether would make landfall over the Philippines or turn to the north. Upper‐level winds contributed the most to the depth‐averaged steering flow. Statistically significant differences in the strength of the upper‐level anticyclone to the east of the storm, the strength and position of the upper‐level ridge downstream of the storm and the location of a detached potential vorticity (PV) streamer appear between two groups of ensemble members – those which turn to the north and those which make landfall. Positional differences of the TC in different ensemble members earlier in the forecasts, particularly in the east to west direction, are correlated to larger northeast to southwest position differences later in the forecast. Ensemble sensitivity analysis suggests that this initial east–west positional variance is linked to the upper‐level geopotential height directly south of the storm. Accurately representing both the steering flow and the position of is vital for an accurate forecast.

Characterising extratropical near‐tropopause analysis humidity biases and their radiative effects on temperature forecasts

AbstractA cold bias in the extratropical lowermost stratosphere in forecasts is one of the most prominent systematic temperature errors in numerical weather prediction models. Hypothesized causes of this bias include radiative effects from a collocated moist bias in model analyses. Such biases would be expected to affect extratropical dynamics and result in the misrepresentation of wave propagation at tropopause level. Here the extent to which these humidity and temperature biases are connected is quantified. Observations from radiosondes are compared to operational analyses and forecasts from the European Centre for Medium‐Range Weather Forecasts (ECMWF) Integrated Forecasting System (IFS) and Met Office Unified Model (MetUM) to determine the magnitude and vertical structure of these biases. Both operational models over‐estimate lowermost stratospheric specific humidity, with a maximum moist bias around 1 km above the tropopause where humidities are around of the observed values on average. This moist bias is already present in the initial conditions and changes little in forecasts over the first five days. Though temperatures are represented well in the analyses, the IFS forecasts anomalously cool in the lower stratosphere, relative to verifying radiosonde observations, by 0.2 K day. The IFS single column model is used to show this temperature change can be attributed to increased long‐wave radiative cooling due to the lowermost stratospheric moist bias in the initial conditions. However, the MetUM temperature biases cannot be entirely attributed to the moist bias, and another significant factor must be present. These results highlight the importance of improving the humidity analysis to reduce the extratropical lowermost stratospheric cold bias in forecast models and the need to understand and mitigate the causes of the moist bias in these models.

Contrasting Responses of Idealised and Realistic Simulations of Shallow Cumuli to Aerosol Perturbations

AbstractShallow clouds remain greatly significant in improving our understanding of the atmosphere. Using the Met Office Unified Model, we compare highly idealised simulations of shallow cumuli with those using more realistic domains, with open lateral boundaries and varying large‐scale forcing. We find that the realistic simulations are more capable of representing the cloud field on large spatial scales, and appear to limit the aerosol perturbations leading to impacts on the thermodynamic conditions. Aerosol perturbations lead to changes in the cloud vertical structure, and thermodynamic evolution of the idealised simulations; a central feature of behavior seen previously in idealised simulations. Modelling approaches with open boundaries and time‐varying forcing may allow for improved representation of shallow clouds in the atmosphere, and greater understanding of how they may respond to perturbations.

Towards a more “scale‐aware” orographic gravity wave drag parametrization: Description and initial testing

AbstractMany current orographic gravity wave drag parametrizations employed in numerical weather prediction and climate models assume idealised elliptical subgrid orography. As a result, they do not fully capture the spectrum of subgrid orographic scales and do not exhibit the desired behaviour when the model grid length is varied. This article describes the motivation, formulation, and testing of a new orographic gravity wave drag parametrization in the Met Office Unified Model that is more “scale‐aware”. The scheme circumvents the need to make assumptions about the shape and distribution of the subgrid orography, moving away from elliptical mountains, through the use of Fourier transforms. The reduction in gravity wave surface stress when orographic flow blocking is present is also accounted for. The result is a new scheme that represents subgrid mountain scales more faithfully and consistently, rather than assuming a single mountain scale for each grid box. Tests using both regional and global simulations, ranging in grid spacing from 2–130 km, demonstrate that the scale‐aware scheme provides a more constant total, resolved plus parametrized, orographic gravity wave momentum flux, when the grid length is varied over most regions of the globe. Global five‐day forecasts also show a significant reduction in the too‐strong zonal wind bias within the Northern and Southern Hemisphere upper troposphere to lower stratosphere at grid spacings of 130 and 40 km when the scale‐aware scheme is employed. Some discussion on future developments for the scheme is also presented, noting that improvements to the orographic flow‐blocking drag component are required before the scheme can be implemented operationally.