This paper argues that a global large eddy simulation can be achieved through the application of the superparametrization (SP) methodology on massively parallel computers. SP was proposed over 15 years ago to improve the representation of deep convection and accompanying small-scale processes in large-scale models for the weather and climate. The main idea was to embed in all columns of the large-scale model (featuring horizontal grid lengths of the order of 100 km) a two-dimensional (2D) convection-permitting small-scale model with approximately a 1-km horizontal grid length and periodic lateral boundaries. We propose to expand this methodology by applying a high-spatial-resolution three-dimensional (3D) large-eddy simulation (LES) model as the SP model and by embedding it in all columns of a large-scale model with a horizontal grid length in the range of 10 to 50 km. The outer model can apply hydrostatic equations as typical global numerical weather prediction and climate models today and can simulate atmospheric processes down to the mesoscale, including organized convection. Small-scale processes, such as boundary-layer turbulence and convective drafts, can be simulated by embedded nonhydrostatic (e.g., anelastic) LES models. Although significantly more expensive than the traditional SP, SP LES is ideally suited to take advantage of parallel computation because of the minimal communication between LES models when compared to traditional domain-decomposition methodologies in parallel simulation. Moreover, as illustrated through the idealized 2D mock-Hadley cell simulations, LES models can feature different horizontal and vertical grids in various columns of the large-scale model, and thus target dominant cloud regimes in various geographical regions. Such a system allows an unstructured grid simulation with no additional model development.
This article first presents an overview of the recent advances in the analysis and prediction of tropical cyclones through assimilating reconnaissance aircraft observations. Many of these advances have now been implemented in operational and experimental real-time hurricane prediction models. These advances are made possible through improved methodologies including more efficient quality control and data thinning, advanced data assimilation techniques that use ensembles to estimate flow-dependent error covariances, and improved numerical models running at convection-permitting resolutions, along with the availability of massively parallel computing. Impacts of aircraft reconnaissance observations on hurricane prediction are then exemplified using a continuously cycling regional-scale convection-permitting analysis and forecast system based on the Weather Research and Forecasting (WRF) model and the ensemble Kalman filter (EnKF). In comparison to the non-reconnaissance experiment that assimilates only conventional observations, as well as to the WRF forecasts directly initialized with the global operational analysis, the cycling WRF-EnKF system with assimilation of aircraft flight-level and dropsonde observations can considerably reduce both the mean position and intensity forecast errors for lead times from day 1 to day 5 averaged over a large number of forecast samples including the real-time implementation during the 2013 Atlantic hurricane season. These findings reaffirm the added value and need for maintaining and maybe expanding routine airborne reconnaissance missions for better tropical cyclone monitoring and prediction.
The potential influence of the developing phases of the two types of El Niño (i.e., EP El Niño and CP El Niño) on rainfall over southern China was investigated using observational data sets from 1979 to 2008. The developing phases of the CP El Niño events are associated with enhanced rainfall over southern China during summer, whereas the influence is not significant in autumn. During the developing phases of the EP El Niño events, rainfall significantly increases over southern China during autumn but no significant increases are observed in summer. These increases in rainfall are a result of the circulation anomalies associated with the two types of El Niño events. The western Pacific subtropical high (WPSH) shifts northeastward during developing phases of the CP El Niño events in summer and is accompanied by enhanced convection and ascending flow. The WPSH shifts westward during the developing phases of the EP El Niño events in autumn and is associated with anomalous southwesterlies over southern China. These circulation anomalies favor more rainfall in southern China. No significant circulation anomalies occur during the developing phases of the CP El Niño events in autumn or the EP El Niño events in summer. These results highlight the importance of considering their developing phases when investigating effects of these two types of El Niño events on climate.
Sensitivity of the tropical easterly jet (TEJ) to the distribution and magnitude of tropical latent heating was examined using an aqua-planet configuration of the Community Atmosphere Model, version 3.1 (CAM-3.1), of the National Center for Atmospheric Research. A series of aqua-planet simulations show that the wind speeds associated with the TEJ are directly controlled by the magnitude and location of tropical heating. Specifically, heating in the off-equator and higher tropical latitudes can generate strong easterlies. In contrast, a single heat source in the equatorial region, even with high magnitude, does not force strong easterlies. However, heating in the higher tropical latitudes is not relevant to explain the structure of the TEJ, whereas equatorial heating is crucial for the realistic vertical and meridional structures of the TEJ. The location of peak zonal wind is influenced by the off-equatorial heating near the TEJ. In a realistic model configuration, it is shown that a zonally elongated precipitation band in the Pacific Ocean warm pool is important for the TEJ to extend eastwards into this region.