The Flexible Modelling Framework for the Met Office Unified Model (Flex-UM, part of the UM 12.1 release)

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 of 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 Dynamic Laboratory (GFDL) idealised moist physics aquaplanet 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 aquaplanet simulations using the Isca climate modelling framework (based on the GFDL moist physics aquaplanet 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.

Bedymo: a combined quasi-geostrophic and primitive equation model in sigma coordinates

This paper introduces the idealised atmospheric circulation model Bedymo, which combines the quasi-geostrophic approximation and the hydrostatic primitive equations in one modelling framework. The model is designed such that the two systems of equations are solved as similarly as possible, such that differences can be unambiguously attributed to the different approximations, rather than the model formulation or the numerics. As a consequence, but in contrast to most other quasi-geostrophic models, Bedymo is using sigma-coordinates in the vertical. In addition to the atmospheric core, Bedymo also includes a slab ocean model and passive tracer module that will provide the basis for future idealised parametrisation of moisture and latent heat release. Further, Bedymo has a graphical user interface, making it particularly useful for teaching. Bedymo is evaluated for three atmosphere-only test cases and one coupled test case including the slab ocean component. The atmosphere-only test cases comprise the growth of a cyclonic disturbance in a baroclinic environment and the excitation of Rossby waves by isolated orography, both in a mid-latitude channel, as well as the simulation of a mid-latitude storm track. The atmosphere-ocean coupled test case is based on an equatorial channel and evaluates the coupled response to an isolated equatorial temperature anomaly in the ocean mixed layer. For all test cases, results agree well with expectations from theory and results obtained with more complex models.

Impact of sub-seasonal atmosphere-ocean interactions on extreme event statistics

<p>Modified frequencies and magnitudes of extreme events due to climate change can have large impacts on societies and are therefore a key area of current research. Large model ensembles are required to quantify and attribute changes to extreme events. Until now, the large ensembles used for such studies are commonly atmosphere-only models forced with time-varying sea surface temperatures (SST) and sea ice. </p><p>This approach is very powerful but presents problems of internal physical consistency. In an SST-forced model, the ocean acts as having an infinite heat capacity whereas in the real-world SST emerges dynamically from the interaction of atmospheric and oceanic processes (Dong et al., 2020). This is particularly relevant for the North Atlantic where ocean processes, in particular meridional heat transport, are key drivers of air-sea coupling. A long-standing challenge, however, is the computational cost of spinning up fully coupled atmosphere-ocean models that hinders their application to large-ensemble, high-resolution simulations required to quantify changing hazard frequencies of low-probability events.  </p><p>In this work we combine the HadAM4 (Webb et al., 2001) atmospheric model at N144 resolution with a Slab ocean (Hewitt & Mitchell, 1997; Williams et al., 2003), which includes a simple sea ice model, to yield the atmosphere-Slab Ocean model HadSM4. The Slab Ocean is forced with diagnosed heat convergence (Q-Flux) and surface currents for sea ice advection (a useful model-development finding for this kind of experiment is that including sea ice velocity information from reanalyses in the surface current field yields a substantially improved spatial pattern of sea ice). We are therefore able to directly compare SST-forced atmosphere-only runs with Q-Flux-forced runs where SST is an emergent property of the model, specifically accounting for the passive response of SSTs in the North Atlantic. Using the distributed infrastructure of climateprediction.net (Guillod et al., 2017; Massey et al., 2015) we run large ensembles to compare extreme statistics and quantify the importance of fast ocean-atmosphere coupling for extreme event statistics.</p><p>We further use this large ensemble setup to investigate the dynamics that drive extreme events from the ocean through air-sea interaction to atmospheric processes. We address is whether and how the slope of a return-time plot (related to the scale parameter of a GEV distribution) is affected by atmosphere-ocean interactions, since this statistic plays a central role in determining relative-risk estimates in event attribution studies. We then investigate how a perturbation to the Q-Flux, representing a change in ocean heat transport, propagates through the system and alters the statistics of extreme events.</p><p>Dong et al., 2020, Climate Dynamics, 55(5–6), 1225–1245. </p><p>Guillod et al., 2017, Geoscientific Model Development, 10(5), 1849–1872. </p><p>Hewitt & Mitchell, 1997, Climate Dynamics, 13(11), 821–834.</p><p>Massey et al., 2015, Quarterly Journal of the Royal Meteorological Society, 141(690), 1528–1545. </p><p>Webb et al., 2001, Climate Dynamics, 17(12), 905–922. </p><p>Williams et al., 2003, Climate Dynamics, 20(7–8), 705–721. </p>

The seesaw response of the Intertropical and South Pacific convergence zones to hemispherically asymmetric thermal forcing

<p>Considerations based on atmospheric energetics and aqua-planet model simulations link the latitudinal position of the global intertropical convergence zone (ITCZ) to atmospheric cross-equatorial energy transport—a greater southward transport corresponds to a more northerly position of the ITCZ. This study, rather than concentrating of the zonally-averaged ITCZ, focuses on the tropical Pacific and looks separately at precipitation in the northern and southern hemispheres. Using numerical experiments, we show that in the tropical Pacific the response of the fully coupled ocean-atmosphere system to a hemispherically asymmetric thermal forcing, modulating atmospheric cross-equatorial energy transport, involves an interplay between the ITCZ and its counterpart in the South Pacific—the Southern Pacific convergence zone (SPCZ). This interplay leads to interhemispheric seesaw changes in tropical precipitation, such that the latitudinal position of each rain band remains largely fixed, but their intensities follow a robust inverse relationship. The seesaw behavior is also evident in the past and future coupled climate simulations of the Climate Model Intercomparison Project Phase 5 (CMIP5). We further show that the tropical Pacific precipitation response to thermal forcing is qualitatively different between the aquaplanet (without ocean heat transport), slab-ocean (with climatological ocean heat transport represented by a “Q-flux”) and fully-coupled model configurations. Specifically, the induced changes in the ITCZ latitudinal position successively decrease, while the seesaw precipitation intensity response becomes more prominent, from the aqua-planet to the slab-ocean to the fully-coupled configuration. The ITCZ/SPCZ seesaw can explain a precipitation dipole pattern observed in paleoclimate without invoking a too strong climate forcing and is relevant to future projections of tropical precipitation.</p>

Assessing spatio-temporal variability and biases of climate models

<div>Atmospheric spatial and temporal variability are closely related with the former being relatively well observed compared to the latter. The former is also regularly assessed in the validation of numerical weather prediction models while the latter is more difficult to estimate. Likewise, thermodynamical fields and circulation are closely coupled calling for an approach that considers them simultaneously.  </div> <div>In this contribution, spatio-temporal variability spectra of the four major reanalysis datasets are discussed and applied for the validation of a climate model prototype.  A relationship between deficiencies in simulated variability and model biases is derived. The underlying method includes dynamical regime decomposition thereby providing a better understanding of the role of tropical variability in global circulation. </div> <div>Results of numerical simulations are validated by a 20th century reanalysis. A climate model was forced either with the prescribed SST or with a slab ocean model that updates SST in each forecast step.  Scale-dependent validation shows that missing temporal variance in the model relative to verifying reanalysis increases as the spatial scale reduces that appears associated with an increasing lack of spatial variance at smaller scales. Similar to variability, bias is strongly scale dependent; the larger the scale, the greater the bias. Biases present in the SST-forced simulation increase in the simulation using the slab ocean. The comparison of biases computed as a systematic difference between the model and reanalysis and between the SST-forced model and slab-ocean model (a perfect-model scenario) suggests that improving the atmospheric model increases the variance in the model on synoptic and subsynoptic scales but large biases associated with a poor SST remain at planetary scales.</div> <p> </p>

Impact of interactive sea surface temperature on convective clustering in radiative convective equilibrium

<p><span>We present results of radiative convective equilibrium runs using the WRF model coupled to an interactive slab ocean model, for which a relaxation term removes energy to constrain the domain mean sea surface temperature to a target value over a given timescale. By using a short adjustment timescale of one minute, drift in the mean temperature is constrained and the impact of the slab ocean is only through the spatial heterogeneity in sea<span>  </span>surface flux. We show how thin slabs slow the onset of organization, and conduct sensitivity experiments to determine the relative contributions of the radiative, sensible and latent surface fluxes, with surface fluxes key. Once clustering starts, the surface feedback acts to aid organization onset due to the drying atmosphere, although the speed of clustering onset is not significantly changed, indicating that it could be determined by a water vapour diffusive timescale as suggested by Windmiller and Craig.<span>  </span>An additional set of experiments that permit the mean surface temperature to undergo a diurnal adjustment show how diurnal variations in SST oppose the atmospheric radiative forcing and also act to prevent clustering onset. We show the mechanism for this acts through the reduction of the diurnal variation of convective mass flux and the distance between updraft towers. </span></p>

The seesaw response of the Intertropical and South Pacific convergence zones to hemispherically asymmetric thermal forcing

<p>Arguments based on atmospheric energetics and aqua-planet model simulations link the latitudinal position of the Intertropical Convergence Zone (ITCZ) to atmospheric cross-equatorial energy transport –- a greater southward transport corresponds to a more northerly position of the ITCZ. This idea is often invoked to explain an interhemispheric dipole pattern of precipitation anomalies in paleoclimates. In contrast, here we demonstrate that in the tropical Pacific the response of the fully coupled ocean-atmosphere system to a hemispherically asymmetric thermal  forcing, modulating this energy transport, involves an interplay between the ITCZ and its counterpart in the South Pacific - the Southern Pacific Convergence Zone (SPCZ). This interplay leads to interhemispheric seesaw changes in tropical precipitation, such that the latitudinal position of each rain band remains largely fixed, but their intensities follow a robust inverse relationship. The seesaw behavior is also evident in the past and future coupled climate simulations of the Climate Model Intercomparison Project Phase 5 (CMIP5). We also show that the tropical Pacific precipitation response to thermal forcing is qualitatively different between the aqua-planet (without ocean heat transport), slab-ocean (with climatological ocean heat transport represented by a ``Q-flux'') and fully-coupled model configurations. Specifically, the induced changes in the ITCZ latitudinal position successively decrease, while the seesaw precipitation intensity response becomes more prominent, from the aqua-planet to the slab-ocean to the fully-coupled configuration. Thus, the ITCZ/SPCZ seesaw can explain the paleoclimate precipitation dipole pattern without invoking a too strong climate forcing and is relevant to future projections of tropical precipitation.</p>