Publications

2023

1. Kilometer-Scale Simulations of Trade-Wind Cumulus Capture Processes of Mesoscale Organization

   Leo Saffin, Adrian Lock, Lorenzo Tomassini, Alan Blyth, Steven Böing, Leif Denby, and John Marsham

   Journal of Advances in Modeling Earth Systems, 15

   Abstract HTML PDF

   Abstract The EUREC4A (Elucidating the role of clouds?circulation coupling in climate) and ATOMIC (Atlantic Tradewind Ocean-Atmosphere Mesoscale Interaction Campaign) joint field campaign gathered observations to better understand the links between trade-wind cumulus clouds, their organization, and larger scales, a large source of uncertainty in climate projections. A recent large-eddy simulation study showed a cloud transition that occurred during this field campaign, where small shallow clouds developed into larger clouds with detrainment layers, was caused by an increase in mesoscale organization generated by a dynamical feedback in mesoscale vertical velocities. We show that kilometer-scale simulations with the Met Office’s Unified Model reproduce this increase in mesoscale organization and the process generating it, despite being much lower resolution. The timing of development is associated with large-scale convergence. Sensitivity tests with a shorter spin-up time, to reduce initial organization, still have the same timing of development and sensitivity tests with cold pools suppressed show only a small effect on mesoscale organization. Mesoscale organization and clouds are sensitive to resolution, with higher resolution simulations producing weaker organization and less cloud, which affects changes in net radiation. The clouds also have substantial differences to observations. Therefore, while kilometer-scale simulations can be useful for understanding processes of mesoscale organization and links with large scales, including responses to climate change, simulations will still suffer from significant errors and uncertainties in radiative budgets.

2022

1. Climate Modeling in Low Precision: Effects of Both Deterministic and Stochastic Rounding

   E. Adam Paxton, Matthew Chantry, Milan Klöwer,  Leo Saffin, and Tim Palmer

   Journal of Climate, 35, 1215-1229

   Abstract HTML

   Motivated by recent advances in operational weather forecasting, we study the efficacy of low-precision arithmetic for climate simulations. We develop a framework to measure rounding error in a climate model, which provides a stress test for a low-precision version of the model, and we apply our method to a variety of models including the Lorenz system, a shallow water approximation for flow over a ridge, and a coarse-resolution spectral global atmospheric model with simplified parameterizations (SPEEDY). Although double precision [52 significant bits (sbits)] is standard across operational climate models, in our experiments we find that single precision (23 sbits) is more than enough and that as low as half precision (10 sbits) is often sufficient. For example, SPEEDY can be run with 12 sbits across the code with negligible rounding error, and with 10 sbits if minor errors are accepted, amounting to less than 0.1 mm (6 h)-1 for average gridpoint precipitation, for example. Our test is based on the Wasserstein metric and this provides stringent nonparametric bounds on rounding error accounting for annual means as well as extreme weather events. In addition, by testing models using both round-to-nearest (RN) and stochastic rounding (SR) we find that SR can mitigate rounding error across a range of applications, and thus our results also provide some evidence that SR could be relevant to next-generation climate models. Further research is needed to test if our results can be generalized to higher resolutions and alternative numerical schemes. However, the results open a promising avenue toward the use of low-precision hardware for improved climate modeling.

2021

1. Circulation conservation in the outflow of warm conveyor belts and consequences for Rossby wave evolution

   Leo Saffin, John Methven, Jake Bland, Ben Harvey, and Claudio Sanchez

   Quarterly Journal of the Royal Meteorological Society, 147, 3587-3610

   Abstract HTML PDF

   Rossby waves on the jet stream are associated with meridional motions, displacing air and the strong potential vorticity (PV) gradient on isentropic surfaces. Poleward motion along sloping isentropic surfaces typically results in ascent and a ridge of air with low PV values. Latent heating in the ascending warm conveyor belt (WCB) enables air to cross isentropic surfaces so that the WCB outflow into a ridge occurs in a higher isentropic layer than the inflow. However, the PV impermeability theorem states that there can be no PV flux across isentropic surfaces, so how can heating alter the PV pattern of a Rossby wave? Here, the ways in which heating in WCBs can influence Rossby wave evolution at tropopause level are explained in the context of the PV impermeability theorem. First, a WCB outflow volume is defined by the upper tropospheric air in a ridge that has experienced net heating over the last few days, using a tracer within short global model forecasts. Second, the boundary of this outflow volume is tracked backwards using isentropic trajectories allowing quantification of the degree to which circulation is conserved, as predicted by theory, even though the WCB transports mass into the volume from lower isentropic layers. This diabatic flux of mass into the outflow volume results in an increase in density and expansion in the outflow area, the partition being determined approximately by PV inversion. The area expansion, combined with conservation of circulation, implies stronger anticyclonic vorticity. The relative vorticity change from divergent outflow can be as large as the decrease relative to the background planetary vorticity associated with poleward displacement of the circuit. The additional anticyclonic relative motion results in enhanced anticyclonic overturning of PV contours on the eastern flank of the ridge, altering qualitatively the nonlinear evolution of the Rossby wave.

2. EUREC4A

   Bjorn Stevens, Sandrine Bony, David Farrell, Felix Ament, Alan Blyth, Christopher Fairall, Johannes Karstensen, Patricia K. Quinn, Sabrina Speich, and etc.

   Earth System Science Data, 13, 4067-4119

   Abstract HTML

   The science guiding the EUREC4A campaign and its measurements is presented. EUREC4A comprised roughly 5 weeks of measurements in the downstream winter trades of the North Atlantic - eastward and southeastward of Barbados. Through its ability to characterize processes operating across a wide range of scales, EUREC4A marked a turning point in our ability to observationally study factors influencing clouds in the trades, how they will respond to warming, and their link to other components of the earth system, such as upper-ocean processes or the life cycle of particulate matter. This characterization was made possible by thousands (2500) of sondes distributed to measure circulations on meso- (200 km) and larger (500 km) scales, roughly 400 h of flight time by four heavily instrumented research aircraft; four global-class research vessels; an advanced ground-based cloud observatory; scores of autonomous observing platforms operating in the upper ocean (nearly 10 000 profiles), lower atmosphere (continuous profiling), and along the air-sea interface; a network of water stable isotopologue measurements; targeted tasking of satellite remote sensing; and modeling with a new generation of weather and climate models. In addition to providing an outline of the novel measurements and their composition into a unified and coordinated campaign, the six distinct scientific facets that EUREC4A explored - from North Brazil Current rings to turbulence-induced clustering of cloud droplets and its influence on warm-rain formation - are presented along with an overview of EUREC4A’s outreach activities, environmental impact, and guidelines for scientific practice. Track data for all platforms are standardized and accessible at 10.25326/165 , and a film documenting the campaign is provided as a video supplement.

2020

1. Reduced‐precision parametrization: lessons from an intermediate‐complexity atmospheric model

   Leo Saffin, Sam Hatfield, Peter Düben, and Tim Palmer

   Quarterly Journal of the Royal Meteorological Society, 146, 1590-1607

   Abstract HTML PDF

   Abstract Reducing numerical precision can save computational costs which can then be reinvested for more useful purposes. This study considers the effects of reducing precision in the parametrizations of an intermediate complexity atmospheric model (SPEEDY). We find that the difference between double-precision and reduced-precision parametrization tendencies is proportional to the expected machine rounding error if individual timesteps are considered. However, if reduced precision is used in simulations that are compared to double-precision simulations, a range of precision is found where differences are approximately the same for all simulations. Here, rounding errors are small enough to not directly perturb the model dynamics, but can perturb conditional statements in the parametrizations (such as convection active/inactive) leading to a similar error growth for all runs. For lower precision, simulations are perturbed significantly. Precision cannot be constrained without some quantification of the uncertainty. The inherent uncertainty in numerical weather and climate models is often explicitly considered in simulations by stochastic schemes that will randomly perturb the parametrizations. A commonly used scheme is stochastic perturbation of parametrization tendencies (SPPT). A strong test on whether a precision is acceptable is whether a low-precision ensemble produces the same probability distribution as a double-precision ensemble where the only difference between ensemble members is the model uncertainty (i.e., the random seed in SPPT). Tests with SPEEDY suggest a precision as low as 3.5 decimal places (equivalent to half precision) could be acceptable, which is surprisingly close to the lowest precision that produces similar error growth in the experiments without SPPT mentioned above. Minor changes to model code to express variables as anomalies rather than absolute values reduce rounding errors and low-precision biases, allowing even lower precision to be used. These results provide a pathway for implementing reduced-precision parametrizations in more complex weather and climate models.

2017

1. Processes Maintaining Tropopause Sharpness in Numerical Models

   Leo Saffin, Suzanne L. Gray, John Methven, and Keith D. Williams

   Journal of Geophysical Research: Atmospheres, 122, 9611-9627

   Abstract HTML PDF

   Recent work has shown that the sharpness of the extratropical tropopause declines with lead time in numerical weather prediction models, indicating an imbalance between processes acting to sharpen and smooth the tropopause. In this study the systematic effects of processes contributing to the tropopause sharpness are investigated using daily initialized forecasts run with the Met Office Unified Model over a three-month winter period. Artificial tracers, each forced by the potential vorticity tendency due to a different model process, are used to separate the effects of such processes. The advection scheme is shown to result in an exponential decay of tropopause sharpness toward a finite value at short lead times with a time scale of 20–24 h. The systematic effect of nonconservative processes is to sharpen the tropopause, consistent with previous case studies. The decay of tropopause sharpness due to the advection scheme is stronger than the sharpening effect of nonconservative processes leading to a systematic decline in tropopause sharpness with forecast lead time. The systematic forecast errors in tropopause level potential vorticity are comparable to the integrated tendencies of the parametrized physical processes suggesting that the systematic error in tropopause sharpness could be significantly reduced through realistic adjustments to the model parametrization schemes.

2. Linking weather forecast errors with the processes responsible

   Leo Saffin

   PhD Thesis, University of Reading

   Abstract HTML PDF

   Progress in numerical weather prediction (NWP) is made through better understand¬ing of the physical processes represented in numerical models and their impacts on the dynamics of large-scal  weather systems. Here, potential vorticity (PV) tracer diagnostics are used to investigate the representation of processes in the Met Office Unified Model (MetG:l1). An exact budget of the PV tracers is derived and a "dynamics-tracer inconsistency" diagnostic implemented to quantify non-conservation of PV by the dynamical core which was not previously accounted for. It is shown that non-conservation of PV by the dy¬namical core can have comparable tendencies to the dominant physical processes implying that non-conservation of PV by a dynamical core can, and should, be quantified alongside PV modification by physical processes. Recent work has shown that the sharpness of the extratropical tropopause declines with lead time in KWP models. In the MetUM, the advection scheme is shown to result in an exponential decay of tropopause sharpness and non-conservative processes are shown to sharpen the tropopause. The systematic errors in tropopause-level PV are comparable to the tendencies associated with physical processes, suggesting that the systematic error in tropopause sharpness could be significantly rednced through realistic adjustments to the model physics. I’ Turbulent mixing within the boundary layer has been previously shown to produce positive PV anomalies that can be advected into cyclones and reduce growth rates through an increase in static stability; however, it is unclear whether N\VP models correctly represent this mechanism. In the MetUM, the generation of these positive PV anomalies is found to be less clear due to large cancellations with other physical processes in the cold sector. Front-relative compositing .is used to separate the cold and warm sectors, providing the basis for investigating PV generation in the boundary layer systematically by compositing over many fronts.

2016

1. The non‐conservation of potential vorticity by a dynamical core compared with the effects of parametrized physical processes

   Leo Saffin, John Methven, and Suzanne L. Gray

   Quarterly Journal of the Royal Meteorological Society, 142, 1265-1275

   Abstract HTML PDF

   Numerical models of the atmosphere combine a dynamical core, which approximates solutions to the adiabatic, frictionless governing equations for fluid dynamics, with tendencies arising from the parametrization of other physical processes. Since potential vorticity (PV) is conserved following fluid flow in adiabatic, frictionless circumstances, it is possible to isolate the effects of non-conservative processes by accumulating PV changes in an air-mass-relative framework. This ‘PV tracer technique’ is used to accumulate separately the effects on PV of each of the different non-conservative processes represented in a numerical model of the atmosphere. Dynamical cores are not exactly conservative because they introduce, explicitly or implicitly, some level of dissipation and adjustment of prognostic model variables which acts to modify PV. Here, the PV tracers technique is extended to diagnose the cumulative effect of the non-conservation of PV by a dynamical core and its characteristics relative to the PV modification by parametrized physical processes. Quantification using the Met Office Unified Model reveals that the magnitude of the non-conservation of PV by the dynamical core is comparable to those from physical processes. Moreover, the residual of the PV budget, when tracing the effects of the dynamical core and physical processes, is at least an order of magnitude smaller than the PV tracers associated with the most active physical processes. The implication of this work is that the non-conservation of PV by a dynamical core can be assessed in case-studies with a full suite of physics parametrizations and directly compared with the PV modification by parametrized physical processes. The non-conservation of PV by the dynamical core is shown to move the position of the extratropical tropopause while the parametrized physical processes have a lesser effect at the tropopause level.