Connecting spatial and temporal scales of tropical precipitation in observations and the MetUM-GA6

Abstract

Abstract. This study analyses tropical rainfall variability (on a range of temporal and spatial scales) in a set of parallel Met Office Unified Model (MetUM) simulations at a range of horizontal resolutions, which are compared with two satellitederived rainfall datasets. We focus on the shorter scales, i.e. from the native grid and time step of the model through subdaily to seasonal, since previous studies have paid relatively little attention to sub-daily rainfall variability and how this feeds through to longer scales. We find that the behaviour of the deep convection parametrization in this model on the native grid and time step is largely independent of the grid-box size and time step length over which it operates. There is also little difference in the rainfall variability on larger/longer spatial/temporal scales. Tropical convection in the model on the native grid/time step is spatially and temporally intermittent, producing very large rainfall amounts interspersed with grid boxes/time steps of little or no rain. In contrast, switching off the deep convection parametrization, albeit at an unrealistic resolution for resolving tropical convection, results in very persistent (for limited periods), but very sporadic, rainfall. In both cases, spatial and temporal averaging smoothes out this intermittency. On the ∼ 100 km scale, for oceanic regions, the spectra of 3-hourly and daily mean rainfall in the configurations with parametrized convection agree fairly well with those from satellite-derived rainfall estimates, while at ∼ 10-day timescales the averages are overestimated, indicating a lack of intra-seasonal variability. Over tropical land the results are more varied, but the model often underestimates the daily mean rainfall (partly as a result of a poor diurnal cycle) but still lacks variability on intra-seasonal timescales. Ultimately, such work will shed light on how uncertainties in modelling small-/short-scale processes relate to uncertainty in climate change projections of rainfall distribution and variability, with a view to reducing such uncertainty through improved modelling of small-/short-scale processes.

The Impact of Hybrid Usage of a Cumulus Parameterization Scheme on Tropical Convection and Large‐Scale Circulations in a Global Cloud‐System Resolving Model

“…Such forcing will normally be compensated for by subsidence of the surrounding grid cells, resulting in "spotty," disorganized convection. A similar effect was reported by Martin et al (2017), who turned off the cumulus parameterization scheme in a 20 km × 14 km grid model. Conversely, the relatively slow increase in tropospheric mositure and updraft area that occurred 1-2 months after initalization likely reflect the model preference upon forming a near-balanced state over larger horizontal scales.…”

“…Optimization of CS and cloud micorphysics scheme parameters should further decrease the model errors, but it remains to be seen whether the schene outperforms NoCS overall after careful tuning. The spottiness of convectivion can be reduced, as suggested by Martin et al (2017). Moreover, it is implied that the sensitivity of convection to environmental moisture, and thus, the model preference to convect over regions rich in mositure supply, can be controlled to a certain extent by changing the parameters of the CS scheme.…”

“…A known discrepancy of the nonhydrostatic icosahedral atmospheric model (NICAM; Satoh et al, 2008;Tomita & Satoh, 2004) at O(10)-km resolution is that it underestimates congestus clouds in the tropics. The mesh is too coarse to represent detailed structures of congestus clouds, and the model tends to produce intermittent, sporadic convection, as reported for other models (Martin et al, 2017), instead of a realistic gradual or stepwise transition from shallow convection to congestus to deep convection (e.g., Kikuchi & Takayabu, 2004;Mapes et al, 2006;Waite & Khouider, 2010). The underestimation of congestus results in a significant dry/wet bias in the mid/lower troposphere of the intertropical convergence zone (ITCZ) and adjacent areas, as shown in Figure 1.…”

CAM6 simulation of mean and extreme precipitation over Asia: sensitivity to upgraded physical parameterizations and higher horizontal resolution

“…The Pacific Decadal Oscillation (PDO) represents variability in the tropical and extratropical North Pacific at interdecadal timescales that significantly impact climate (Mantua et al, 1997;Meehl et al, 2013). Figure 6 shows a regres- Figure 5.…”

CAM6 simulation of mean and extreme precipitation over Asia: Sensitivity to upgraded physical parameterizations and higher horizontal resolution

“…GA6 includes a new dynamical core and updates various physical parameterizations [Walters et al, 2017]. Similarly, Martin et al [2017] analyzed tropical precipitation in GA6 with a range of horizontal resolutions and found that the behaviour of the deep convection parameterization in GA6 is largely independent of the grid-box size and time step length over which it operates. In addition to model physics upgrade, another area of growth in global climate model development is the enhancement of horizontal spatial resolution.…”