The Impact of Horizontal Resolution on the CNOP and on Its Identified Sensitive Areas for Tropical Cyclone Predictions

In this study, the ilnpacts of horizontal resolution on the conditional nonlinear optimal perturbation （CNOP） and on its identified sensitive areas were investigated for tropical cyclone predictions. Three resolutions, 30 km, 60 km, and 120 kin, were studied for three tropical cyclones, TC Mindulle （2004）, TC Meari （2004）, and TC Matsa （2005）. Results show that CNOP may present different structures with different resolutions, and the major parts of CNOP become increasingly localized with increased horizontal resolution. CNOP produces spiral and baroclinic structures, which partially account for its rapid amplification. The differences in CNOP structures result in different sensitive areas, but there are common areas for the CNOP-identified sensitive areas at various resolutions, and the size of the common areas is different from case to case. Generally, the forecasts benefit more from the reduction of the initial errors in the sensitive areas identified using higher resolutions than those using lower resolutions. However, the largest improvement of the forecast can be obtained at the resolution that is not the highest for some cases. In addition, the sensitive areas identified at lower resolutions are also helpful for improving the forecast with a finer resolution, but the sensitive areas identified at the same resolution as the forecast would be the most beneficial.

An Effective Configuration of Ensemble Size and Horizontal Resolution for the NCEP GEFS

Two important questions are addressed in this paper using the Global Ensemble Forecast System （GEFS） from the National Centers for Environmental Prediction （NCEP）： （1） How many ensemble members are needed to better represent forecast uncertainties with limited computational resources？ （2） What is tile relative impact on forecast skill of increasing model resolution and ensemble size？ Two-month experiments at T126L28 resolution were used to test the impact of varying the ensemble size from 5 to 80 members at the 500- hPa geopotential height. Results indicate that increasing the ensemble size leads to significant improvements in the performance for all forecast ranges when measured by probabilistic metrics, but these improvements are not significant beyond 20 members for long forecast ranges when measured by deterministic metrics. An ensemble of 20 to 30 members is the most effective configuration of ensemble sizes by quantifying the tradeoff between ensemble performance and the cost of computational resources. Two representative configurations of the GEFS the T126L28 model with 70 members and the T190L28 model with 20 members, which have equivalent computing costs--were compared. Results confirm that, for the NCEP GEFS, increasing the model resolution is more （less） beneficial than increasing the ensemble size for a short （long） forecast range.

INFLUENCE OF MODEL HORIZONTAL RESOLUTION ON THE INTENSITY AND STRUCTURE OF RAMMASUN

We use the WRF(V3.4) model as the experimental model and select three horizontal resolutions of 15, 9,and 3 km to research the influence of the model's horizontal resolution on the intensity and structure of the super-strong typhoon Rammasun(1409) in 2014. The results indicate that the horizontal resolution has a very large impact on the intensity and structure of Rammasun. The Rammasun intensity increases as the horizontal resolution increases. When the horizontal resolution increases from 9 km to 3 km, the enhancement of intensity is more obvious, but the strongest intensity simulated by 3 km horizontal resolution is still weaker than the observed strongest intensity. Along with the increase of horizontal resolution, the horizontal scale of the Rammasun vortex decreases, and the vortex gradually contracts toward its center. The vortex structure changes from loose to compact and deep. The maximum wind radius,thickness of the eye wall, and outward inclination of the eye wall with height decrease, and the wind in the inner core region, updraft motion along the eye wall, and strength of the warm core become stronger. Additionally, the pressure gradient and temperature gradient of the eye wall region increase, and the vortex intensity becomes stronger. When the horizontal resolution increases from 9 km to 3 km, the change in the Rammasun structure is much larger than the change when the horizontal resolution increases from 15 km to 9 km. When the model does not employ the method of convection parameterization, the Rammasun intensity simulated with 3 km horizontal resolution is slightly weaker than the intensity simulated with 3 km horizontal resolution when the Kain-Fritsch(KF) convection parameterization scheme is adopted, while the intensity simulated with 9 km horizontal resolution is much weaker than the intensity simulated with 9 km horizontal resolution when the KF scheme is adopted. The influence of the horizontal resolution on the intensity and structure of Rammasun is larger than the influence when the KF scheme is adopted.

THE EFFECT OF MODEL HORIZONTAL RESOLUTION ON THE PRECIPITATION OF RAMMASUN

This paper investigates the effect of horizontal resolution on the precipitation of the super typhoon Rammasun(1409). The experiment uses WRF(V3.4) model with resolutions of 15 km, 9 km and 3 km. The results suggest that the simulated Rammasun rain band shapes and distributions at different horizontal resolutions are nearly the same. When the resolution is increased from 15 km to 9 km and then to 3 km, heavy precipitation is observed to spread in all directions from a concentrated distribution, especially when the resolution is increased from 9 km to 3 km. The 6 h and 1 h heavy precipitations also show a more significant comma-shape distribution. Moreover, the water vapor distribution shows the same characteristics as the heavy precipitation with a notably enhanced ascending movement and a decreased height of the strongest ascending movement. Of the three resolutions, the precipitation distribution simulated at 3 km resolution is the closest to the observed distribution; however, there is still a noticeable difference between the simulated precipitation and the actual observation. With the absence of the convection parameterization in the model, the precipitation distributions simulated at 9 km and 3 km resolutions demonstrate the same features as when the KF convection parameterization is applied. However, the simulated precipitations at these two resolutions are smaller than those obtained with the KF scheme. Meanwhile the difference between the simulated precipitations at these two resolutions is also smaller than that in the latter case. In general, when KF scheme is applied to the model, the simulation effect of Rammasun precipitation is better than that obtained without the convection parameterization scheme.

Characteristics of the Asian–Pacific Oscillation in Boreal Summer Simulated by BCC_CSM with Different Horizontal Resolutions

The summer Asian-Pacific Oscillation （APO） is a major teleconnection pattern that reflects the zonal thermal contrast between East Asia and the North Pacific in the upper troposphere. The performance of Beijing Climate Center Climate System Models （BCC_CSMs） with different horizontal resolutions, i.e., BCC_CSM1.1 and BCC_CSM1.1 （m）, in reproducing APO interannual variability, APO-related precipitation anomalies, and associated atmospheric circulation anomalies, is evaluated. The results show that BCC_CSMI.I（m） can successfully capture the interannual variability of the summer APO index. It is also more capable in reproducing the APO＇s spatial pattern, compared to BCC_CSMI.1, due to its higher horizontal resolution. Associated with a positive APO index, the northward-shifted and intensified South Asian high, strengthened extratropical westerly jet, and tropical easterly jet in the upper troposphere, as well as the southwesterly monsoonal flow over North Africa and the Indian Ocean in the lower troposphere, are realistically represented by BCC_CSM1.1 （m）, leading to an improvement in reproducing the increased precipitation over tropical North Africa, South Asia, and East Asia, as well as the decreased precipitation over subtropical North Africa, Japan, and North America. In contrast, these features are less consistent with observations when simulated by BCC_CSM1.1. Regression analysis further indicates that surface temperature anomalies over the North Pacific and the southern and western flanks of the Tibetan Plateau are reasonably reproduced by BCC_CSM 1.1 （m）, which contributes to the substantial improvement in the simulation of the characteristics of summer APO compared to that of BCC_CSM1.1.

Modeling Arctic Intermediate Water: The effects of Neptune parameterization and horizontal resolution

Arctic Intermediate Water （AIW）, advected from the North Atlantic Ocean, has a potential influence on climate in the Arctic region, but is poorly simulated in coarse resolution models. In this study, a coupled ice-ocean model is used to investigate features of AIW by conducting two sensitivity experiments based on Neptune parameterization and horizontal resolution. The re- suits show that both experiments improve the modeling of temperature profiles in the western Eurasian Basin, mainly as a result of more realistic volume and heat transport through the Fram Strait. Topographical flows are well reproduced using Neptune parame- terization or a finer horizontal resolution. In the eddy-permitting model with relatively higher resolution, the velocity field is more realistic than in the Neptune parameterization model, and complex inflow and outflow belts of barotropic structure are well repro- duced. The findings of this study suggest that increased model resolution, as provided by an eddy-resolving model, is needed to reproduce realistic circulation and thermohaline structure in the Arctic, since the Rossby radius of deformation is only several kilometers in the Arctic Ocean. This paper focuses on the external heat input rather than internal mixing process, and obtains a conclusion that the heat input from the Fram Strait is a main factor to reproduce AIW in the Eurasian Basin successfully, at least for the western part.

Comparative Experiments on the Effect of Radar Data Assimilation and Increasing Horizontal Resolution on Short-term Numerical Weather Prediction

To examine the effect of radar data assimilation and increasing horizontal resolution on the short-term numerical weather prediction, comparative numerical experiments are conducted for a Huabei （North China） torrential rainfall event by using the Advanced Regional Prediction System （ARPS） and ARPS Data Anal- ysis System （ADAS）. The experiments use five different horizontal grid spacings, i.e., 18, 15, 9, 6, and 3 km,respectively, under the two different types of analyses： one with radar data, the other without. Results show that, when radar data are not used in the analysis （i.e., only using the conventional observation data）, increasing horizontal resolution can improve the short-term prediction of 6 h with better representation of the frontal structure and higher scores of the rainfall prediction, particularly for heavy rain situations. When radar data are assimilated, it significantly improves the rainfall prediction for the first 6 h, especially the locality and intensity of precipitation. Moreover, using radar data in the analysis is more effective in improving the short-term prediction than increasing horizontal resolution of the model alone, which is demonstrated by the fact that by using radar data in the analysis and a coarser resolution of the 18-km grid spacing, the predicted results are as good as that by using a higher resolution of the 3-km grid spacing without radar data. Further study of the results under the radar data assimilation with grid spacing of 18-3 km reveals that the rainfall prediction is more sensitive to the grid spacing in heavy rain situations （more than 40 mm） than in ordinary rain situations （less than 40 mm）. When the horizontal grid spacing reduces from 6 to 3 km, there is no obvious improvement to the prediction results. This suggests that there is a limit to how far increasing horizontal resolution can do for the improvement of the prediction. Therefore, an effective approach to improve the short-term numerical prediction is to combine the radar data assimilation with an optimal horizontal resolution.

Impacts of Horizontal and Vertical Resolutions on the Microphysical Structure and Boundary Layer Fluxes of Typhoon Hato(2017)

We set four sets of simulation experiments to explore the impacts of horizontal resolution(HR)and vertical resolution(VR)on the microphysical structure and boundary layer fluxes of tropical cyclone(TC)Hato(2017).The study shows that higher HR tends to strengthen TC.Increasing VR in the upper layers tends to weaken TC,while increasing VR in the lower layers tends to strengthen TC.Simulated amounts of all hydrometeors were larger with higher HR.Increasing VR at the upper level enhanced the mixing ratios of cloud ice and cloud snow,while increasing VR at the lower level elevated the mixing ratios of graupel and rainwater.HR has greater impact on the distributions of hydrometeors.Higher HR has a more complete ring structure of the eyewall and more concentrated hydrometeors along the cloud wall.Increasing VR at the lower level has little impact on the distribution of TC hydrometeors,while increasing VR at the upper level enhances the cloud thickness of the eyewall area.Surface latent heat flux(SLHF)is influenced greatly by resolution.Higher HR leads to larger water vapor fluxes and larger latent heat,which would result in a stronger TC.A large amount of false latent heat was generated when HR was too high,leading to an extremely strong TC,VR has a smaller impact on SLHF than HR.But increasing VR at the upper-level reduces the SLHF and weakens TC,and elevating VR at the lower-level increases the SLHF and strengthens TC.The changes in surface water vapor flux and SLHF were practically identical and the simulation results were improved when HR and VR were more coordinated.The friction velocity was greater with higher VR.Enhancing VR at the lower level increased the friction velocity,while increasing VR at the upper level reduced it.

On the Importance of High-Resolution in Large-Scale Ocean Models

Eddying global ocean models are now routinely used for ocean prediction,and the value-added of a better representation of the observed ocean variability and western boundary currents at that resolution is currently being evaluated in climate models.This overview article begins with a brief summary of the impact on ocean model biases of resolving eddies in several global ocean-sea ice numerical simulations.Then,a series of North and Equatorial Atlantic configurations are used to show that an increase of the horizontal resolution from eddy-resolving to submesoscale-enabled together with the inclusion of high-resolution bathymetry and tides significantly improve the models’abilities to represent the observed ocean variability and western boundary currents.However,the computational cost of these simulations is extremely large,and for these simulations to become routine,close collaborations with computer scientists are essential to ensure that numerical codes can take full advantage of the latest computing architecture.

Representation of the ENSO Combination Mode and its Asymmetric SST Response in Different Resolutions of HadGEM3

Previous studies have revealed a combination mode （C-mode） occurring in the Indo-Pacific region, arising from nonlinear interactions between ENSO and the western Pacific warm pool annual cycle. This paper evaluates the simulation of this C-mode and its asymmetric SST response in HadGEM3 and its resolution sensitivity using three sets of simulations at horizontal resolutions of N96, N216 and N512. The results show that HadGEM3 can capture well the spatial pattern of the C-mode associated surface wind anomalies, as well as the asymmetric response of SST in the tropical Pacific, but it strongly overestimates the explained variability of the C-mode compared to the ENSO mode. The model with the three resolutions is able to reproduce the distinct spectral peaks of the C-mode at the near annual combination frequencies, but the performance in simulating the longer periods is not satisfactory, presumably due to the unrealistic simulation of the ENSO mode. Increasing the horizontal resolution can improve the consistency between atmospheric and oceanic representations of the C-mode, but not necessarily enhance the accuracy of C-mode simulation compared with observation.

Decadal Methane Emission Trend Inferred from Proxy GOSAT XCH4 Retrievals:Impacts of Transport Model Spatial Resolution

In recent studies,proxy XCH_(4)retrievals from the Japanese Greenhouse gases Observing SATellite(GOSAT)have been used to constrain top-down estimation of CH_(4)emissions.Still,the resulting interannual variations often show significant discrepancies over some of the most important CH_(4)source regions,such as China and Tropical South America,by causes yet to be determined.This study compares monthly CH_(4)flux estimates from two parallel assimilations of GOSAT XCH_(4)retrievals from 2010 to 2019 based on the same Ensemble Kalman Filter(EnKF)framework but with the global chemistry transport model(GEOS-Chem v12.5)being run at two different spatial resolutions of 4°×5°(R4,lon×lat)and 2°×2.5°(R2,lon×lat)to investigate the effects of resolution-related model errors on the derived long-term global and regional CH_(4)emission trends.We found that the mean annual global methane emission for the 2010s is 573.04 Tg yr^(-1)for the inversion using the R4 model,which becomes about 4.4 Tg yr^(-1)less(568.63 Tg yr^(-1))when a finer R2 model is used,though both are well within the ensemble range of the 22 top-down results(2008-17)included in the current Global Carbon Project(from 550 Tg yr^(-1)to 594 Tg yr^(-1)).Compared to the R2 model,the inversion based on the R4 tends to overestimate tropical emissions(by 13.3 Tg yr^(-1)),which is accompanied by a general underestimation(by 8.9 Tg yr^(-1))in the extratropics.Such a dipole reflects differences in tropical-mid-latitude air exchange in relation to the model’s convective and advective schemes at different resolutions.The two inversions show a rather consistent long-term CH_(4)emission trend at the global scale and over most of the continents,suggesting that the observed rapid increase in atmospheric methane can largely be attributed to the emission growth from North Africa(1.79 Tg yr^(-2)for R4 and 1.29 Tg yr^(-2)for R2)and South America Temperate(1.08 Tg yr^(-2)for R4 and 1.21 Tg yr^(-2)for R2)during the first half of the 2010s,and from Eurasia Boreal(1.46 Tg yr^(-2)for R4 and 1.63 Tg yr^(-2)for R2)and Tropical South America(1.72 Tg yr-2 for R4 and 1.43 Tg yr^(-2)for R2)over 2015-19.In the meantime,emissions in Europe have shown a consistent decrease over the past decade.However,the growth rates by the two parallel inversions show significant discrepancies over Eurasia Temperate,South America Temperate,and South Africa,which are also the places where recent GOSAT inversions usually disagree with one other.