Monday, 4 June 2018: 3:15 PM
Colorado B (Grand Hyatt Denver)
Over the last decade, a tremendous increase in computing power has facilitated the advent of global convection-permitting numerical weather prediction models. These models are able to simulate Earth's atmosphere with astonishing realism (Fig. 1) and allow the seamless prediction of weather from local to planetary scales. Not surprisingly, global convection-permitting models are often expected to revolutionize the weather prediction enterprise, for example by predicting high-impact weather up to two weeks ahead (ECMWF 2016). However, many aspects of the atmosphere's predictability are not well understood; consequently, it is not clear what forecast problems are potentially tractable. To address this issue, the intrinsic predictability of the atmosphere was explored through a set of global convection-permitting model simulations. The simulations were produced on a globally-uniform 4-km mesh using the Model for Prediction Across Scales. This resolution allowed the illumination of error growth from convective to global scales. During the first two days, errors grew through moist convection and other mesoscales processes, and the character of the error growth resembled the case of k-5/3 turbulence. Between 2--13 days, errors grew with the background baroclinic instability, and the character of the error growth mirrored the case of k-3 turbulence. The existence of an error growth regime with properties similar to k-5/3 turbulence confirmed the radical idea of E. N. Lorenz that the atmosphere has a finite limit of predictability, no matter how small the initial error. The global mean predictability limit of the troposphere is estimated to be around 2--3 weeks, which is in agreement with previous work. However, scale-dependent predictability limits differ between the divergent and rotational wind component and between vertical levels, indicating that atmospheric predictability is a more complex problem than commonly thought. The practical value of global cloud-resolving models, especially with regard to deterministic predictions of high-impact weather phenomena, will be discussed in light of the various aspects of atmospheric predictability.
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References:
ECMWF, 2016: ECMWF Strategy 2016--2025. Tech. Rep., European Centre for Medium-Range Weather Forecasts. URL https://www.ecmwf.int/sites/default/files/ECMWF_Strategy_2016-2025.pdf.
Fig. 1. Outgoing longwave radiation (left, in W/m2) and 2-m temperature (right, in ºC) from a Model for Prediction Across Scales simulation produced on a globally uniform 4-km mesh.
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