March 2024:
new publication:
Savita, A., Kjellsson, J., Pilch Kedzierski, R., Latif, M., Rahm, T., Wahl, S., and Park, W.: Assessment of climate biases in OpenIFS version 43r3 across model horizontal resolutions and time steps, Geosci. Model Dev., 17, 1813–1829, https://doi.org/10.5194/gmd-17-1813-2024, 2024.
Summary: "We examine the impact of horizontal resolution and model time step on the climate of the OpenIFS version 43r3 atmospheric general circulation model. A series of sim- ulations for the period 1979–2019 are conducted with vari- ous horizontal resolutions (i.e. ∼ 100, ∼ 50, and ∼ 25 km) while maintaining the same time step (i.e. 15 min) and us- ing different time steps (i.e. 60, 30, and 15 min) at 100 km horizontal resolution. We find that the surface zonal wind bias is significantly reduced over certain regions such as the Southern Ocean and the Northern Hemisphere mid-latitudes and in tropical and subtropical regions at a high horizon- tal resolution (i.e. ∼ 25 km). Similar improvement is evident too when using a coarse-resolution model (∼ 100 km) with a smaller time step (i.e. 30 and 15 min). We also find im- provements in Rossby wave amplitude and phase speed, as well as in weather regime patterns, when a smaller time step or higher horizontal resolution is used. The improvement in the wind bias when using the shorter time step is mostly due to an increase in shallow and mid-level convection that enhances vertical mixing in the lower troposphere. The en- hanced mixing allows frictional effects to influence a deeper layer and reduces wind and wind speed throughout the tropo- sphere. However, precipitation biases generally increase with higher horizontal resolutions or smaller time steps, whereas the surface air temperature bias exhibits a small improvement over North America and the eastern Eurasian continent. We argue that the bias improvement in the highest-horizontal- resolution (i.e. ∼ 25 km) configuration benefits from a combination of both the enhanced horizontal resolution and the shorter time step. In summary, we demonstrate that, by re- ducing the time step in the coarse-resolution (∼ 100 km) OpenIFS model, one can alleviate some climate biases at a lower cost than by increasing the horizontal resolution."
(a) Annual mean ERA5 surface zonal wind [m s−1]. (b–d) Annual mean zonal wind [m s−1] bias for different model time steps (1 h (b), 30 m (c), and 15 m (d)) using ∼ 100 km resolution and (e, f) with different horizontal resolutions (∼ 50 (e) and ∼ 25 km (f)). Biases are computed with respect to ERA5 over the period 1979–2019.
February 2024:
new publication:
Nnamchi, H.C., Diallo, I. Inconsistent Atlantic Links to Precipitation Extremes over the Humid Tropics. Earth Systems Environment (2024). https://doi.org/10.1007/s41748-023-00370-0
Abstract:
This study investigates extreme wet and dry conditions over the humid tropics and their connections to the variability of the tropical ocean basins using observations and a multi-model ensemble of 24 state-of-the-art coupled climate models, for the 1930–2014 period. The extreme wet (dry) conditions are consistently linked to Central Pacific La Niña (Eastern Pacific El Niño), the weakest being the Congo basin, and homogeneous patterns of sea surface temperature (SST) variability in the tropical Indian Ocean. The Atlantic exhibits markedly varying configurations of SST anomalies, including the Atlantic Niño and pan-Atlantic decadal oscillation, with non-symmetrical patterns between the wet and dry conditions. The oceanic influences are associated with anomalous convection and diabatic heating partly related to variations in the strength of the Walker Circulation. The observed connection between the Amazon basin, as well as the Maritime continent, and the Indo-Pacific variability are better simulated than that of the Congo basin. The observed signs of the Pacific and Indian SST anomalies are reversed for the modelled Congo basin extreme conditions which are, instead, tied to the Atlantic Niño/Niña variability. This Atlantic–Congo basin connection is related to a too southerly location of the simulated inter-tropical convergence zone that is associated with warm SST biases over the Atlantic cold tongue. This study highlights important teleconnections and model improvements necessary for the skillful prediction of extreme precipitation over the humid tropics.
a Mean annual precipitation from GPCC 1930–2014, whereas the contours show the monthly-mean precipitation rates in mm day–1 from the GPCP satellite-derived dataset 1979–2014 (only values of 3 mm day–1 are contoured at the interval of 1 mm day–1). b–d (left axis) annual cycle of land precipitation (bar plots) averaged over the Amazon basin (5° N–10° S, 50–80° W), Congo basin (5° N–10° S, 10–30° E), and Maritime continent (5° N–10° S, 100–150° E) shown by the red boxes in a. The right axis in b–d shows the annual cycle of 2-m air temperature represented by the dashed line. e Number of rain gauges within a grid-cell averaged over the Amazon basin (red), Congo basin (blue), and Maritime continent (grey). Dashed vertical lines in e delineate the study period 1930–2014. Note that for the area-average over the humid tropic regions, only land grid points included to the boxes are considered
