GEFSv12 reforecast dataset for supporting subseasonal and hydrometeorological applications

For the newly implemented Global Ensemble Forecast System version 12 (GEFSv12), a 31-year (1989-2019) ensemble reforecast dataset has been generated at the National Centers for Environmental Prediction (NCEP). The reforecast system is based on NCEP’s Global Forecast System version 15.1 and GEFSv12, which uses the Finite Volume 3 dynamical core. The resolution of the forecast system is ∼25 km with 64 vertical hybrid levels. The Climate Forecast System (CFS) reanalysis and GEFSv12 reanalysis serve as initial conditions for the Phase 1 (1989–1999) and Phase 2 (2000–2019) reforecasts, respectively. The perturbations were produced using breeding vectors and ensemble transforms with a rescaling technique for Phase 1 and ensemble Kalman filter 6-h forecasts for Phase 2. The reforecasts were initialized at 0000 (0300) UTC once per day out to 16 days with 5 ensemble members for Phase 1 (Phase 2), except on Wednesdays when the integrations were extended to 35 days with 11 members. The reforecast data set was produced on NOAA’s Weather and Climate Operational Supercomputing System at NCEP. This study summarizes the configuration and dataset of the GEFSv12 reforecast and presents some preliminary evaluations of 500hPa geopotential height, tropical storm track, precipitation, 2-meter temperature, and MJO forecasts. The results were also compared with GEFSv10 or GEFS Subseasonal Experiment reforecasts. In addition to supporting calibration and validation for the National Water Center, NCEP Climate Prediction Center, and other National Weather Service stakeholders, this high-resolution subseasonal dataset also serves as a useful tool for the broader research community in different applications.

A case study for cyclone ‘Aila’ for forecasting rainfall using satellite derived rain rate data

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Simulation of ENSO teleconnections to precipitation extremes over the US in the high resolution version of E3SM

We evaluate the simulated teleconnection of El Niño Southern Oscillation (ENSO) to winter season precipitation extremes over the United States in a long (98 years) 1950-control high resolution version (HR, 25 km nominal atmosphere model horizontal resolution) of US Department of Energy’s (DOE) Energy Exascale Earth System Model version 1 (E3SMv1). Model bias and spatial pattern of ENSO teleconnections to mean and extreme precipitation in HR overall are similar to the low-resolution model’s (LR, 110 km) historical simulation (4-member ensemble, 1925-1959). However, over the Southeast US (SE-US), HR produces stronger El Niño associated extremes, reducing upon LR’s model bias. Both LR and HR produce weaker than observed increase in storm track activity during El Niño events there. But, HR improves the ENSO associated variability of moisture transport over SE-US. During El Niño, stronger vertical velocities in HR produce stronger large-scale precipitation causing larger latent heating of the troposphere that pulls in more moisture from the Gulf of Mexico into the SE-US. This positive feedback also contributes to the stronger mean and extreme precipitation response in HR. Over the Pacific Northwest, LR’s bias of stronger than observed La Niña associated extremes is amplified in HR. Both models simulate stronger than observed moisture transport from the Pacific Ocean into the region during La Niña years. The amplified HR bias there is due to stronger orographically driven vertical updrafts that create stronger large scale precipitation, despite weaker La Niña induced storm track activity.

Long-Term Trends of Extra-Tropical Cyclones in the Extended ERA5 Reanalysis

<p>Long-term reanalysis data sets are needed to determine the natural variability of extra-tropical cyclone tracks and for the assessment of the response to global warming. Using a systematic change-point analysis we provide evidence that the pre-satellite ERA5 data of the Backward Extension (ERA5-BE, covering 1950-1978) is highly compatible with the standard ERA5 (1979-2021) data sets. We observe that the joint ERA5 data from 1950 to 2021 is consistent in all storm-related quantities, allowing long-term studies. Despite the high inter-annual variability, a trend analysis suggests that the intensity of extra-tropical cyclones has increased significantly in the Northern Hemisphere from 1950 to 2021. The propagation speed of extra-tropical cyclones has notably decreased and the North Atlantic cyclone track, in particular, has shifted northward. Furthermore, the number of North Pacific storms increased significantly; these storms exhibit longer life cycles and travel larger distances, while they also grow more slowly. From 1979 to 2021 we find increases in wind gusts and cyclone-related precipitation. The central geopotential height, a measure for storminess, has decreased in both storm track areas. The observed changes originating from potential changes in the atmospheric circulation are the result of natural variability and anthropogenic global warming. Future storm adaptation planning should consider the observed increase in storm-related impacts.</p>

Sub-seasonal variations of the tropical storm track in the western north Pacific

With 20-year (1975-94) climatological data, we demonstrate that the tropical storm track over the western North Pacific (0° - 40°N, 100 - 180°E) exhibits prominent sub-seasonal variations on a time scale of about 40 days from May to November. The storm track variability is regulated by the conspicuous Climatological Intra Seasonal Oscillation (CISO) in the strength of the western North Pacific summer monsoon and the associated position of the western Pacific Sub-tropical High. The CISO cycle regulates the number of tropical storm formation during the Pre-Onset and Withdraw Cycles but not during the Onset and Peak Monsoon Cycles (from mid-June to mid-September).

Rapid increase of explosive cyclone activity over the midwinter North Pacific in the late 1980s

Long-term changes in the activity of explosively developing “Bomb” cyclones over the wintertime North Pacific are investigated by using a particular version of a global atmospheric reanalysis dataset into which only conventional observations have been assimilated. Bomb cyclones in January are found to increase rapidly around 1987 in the midlatitude central North Pacific. Some of the increased “Bomb” cyclones formed over the East China Sea and then moved along the southern coast of Japan before developing explosively in the central North Pacific. The enhanced cyclone activity is found to be concomitant with rapid warming and moistening over the subtropical western Pacific, the South and East China Seas under the weakened monsoonal northerlies, leading to the enhancement of lower-tropospheric Eady growth rate and equivalent potential temperature gradient, setting a condition favorable for cyclone formation in the upstream of the North Pacific storm track. Along the storm track, poleward moisture transport in the warm sector of a cyclone and associated precipitation along the warm and cold fronts tended to increase and thereby enhance its explosive development. After the transition around 1987, a Bomb cyclone has become more likely to develop without a strong upper-level cyclonic vortex propagating from Eurasia than in the earlier period. The increased Bomb cyclone activity in January is found to contribute to the diminished midwinter minimum of the North Pacific storm track activity after the mid-1980s.

Energetics of transient eddies related to the midwinter minimum of the North Pacific storm-track activity

Storm-track activity over the North Pacific climatologically exhibits a clear minimum in midwinter, when the westerly jet speed sharply maximizes. This counterintuitive phenomenon, referred to as the “midwinter minimum (MWM)”, has been investigated from various perspectives, but the mechanisms are still to be unrevealed. Toward better understanding of this phenomenon, the present study delineates the detailed seasonal evolution of climatological-mean Eulerian statistics and energetics of migratory eddies along the NP storm-track over 60 years. As a comprehensive investigation of the mechanisms for the MWM, this study has revealed that the net eddy conversion/generation rate normalized by the eddy total energy, which is independent of eddy amplitude, is indeed reduced in midwinter. The reduction from early winter occurs mainly due to the decreased effectiveness of the baroclinic energy conversion through seasonally weakened temperature fluctuations and the resultant poleward eddy heat flux. The reduced net normalized conversion/generation rate in midwinter is also found to arise in part from the seasonally enhanced kinetic energy conversion from eddies into the strongly diffluent Pacific jet around its exit. The seasonality of the net energy influx also contributes especially to the spring recovery of the net normalized conversion/generation rate. The midwinter reduction in the normalized rates of both the net energy conversion/generation and baroclinic energy conversion was more pronounced in the period before the late 1980s, during which the MWM of the storm-track activity was climatologically more prominent.

Meridional temperature gradient and the midlatitude storm track

<p>The southern hemisphere’s atmospheric circulation experiences several annual and seasonal changes that are well documented and studied. The teleconnections between different variables are verified and used to explain variability in everyday climate and weather. Theories using physics are taught and published in textbooks to help us understand the connectivity and complexity of such a system. One theory is the meridional temperature gradient has a direct impact on the storm track. This thesis investigates that theory using the ERA-Interim dataset. The temperature gradient is a direct result of the temperature field, and depending on the latitudes you decide in which to constrain your gradient, the gradient experiences several changes. In the high latitudes, the southern annual oscillation created a two peaked pattern; the mid-latitudes display the expected seasonal mono peak pattern. The strong correlations seen in the high latitudes means that the gradient is driven by the patterns experienced at higher latitudes. The independence of behaviours displayed by the ocean sectors led to the research investigating the influences, looking at not just the hemisphere, but also each basin separately. The Pacific and Indian Ocean showed in several results to act independently from one another, in temperature gradients, wind field, and storm track position. The strong correlations between the temperature gradient and the wind field, as well as the storm track field show that the two are connected, as the theory suggests. If temperatures rise in the tropics, or decrease in the poles, then the temperature gradient will steepen. The pressure gradient force increases which pushes the thermal wind balance poleward, shifting the position of the westerlies. The area with the largest variation in the wind speed becomes the storm track, which would also shift poleward. Climatic factors such as the southern oscillation index, southern annular mode or Indian Ocean dipole show slight correlations with the temperature field, but have little to no influence on the temperature gradient itself. Precipitation levels in New Zealand are highly variable due to the nature of the countries location and topography. What was found was little connection between the northern part of the country and the storm track. However, closer proximity to the storm track, such as the south of the country, do experience a small amount of variation due to the storm tracks influence.</p>

Meridional temperature gradient and the midlatitude storm track

<p>The southern hemisphere’s atmospheric circulation experiences several annual and seasonal changes that are well documented and studied. The teleconnections between different variables are verified and used to explain variability in everyday climate and weather. Theories using physics are taught and published in textbooks to help us understand the connectivity and complexity of such a system. One theory is the meridional temperature gradient has a direct impact on the storm track. This thesis investigates that theory using the ERA-Interim dataset. The temperature gradient is a direct result of the temperature field, and depending on the latitudes you decide in which to constrain your gradient, the gradient experiences several changes. In the high latitudes, the southern annual oscillation created a two peaked pattern; the mid-latitudes display the expected seasonal mono peak pattern. The strong correlations seen in the high latitudes means that the gradient is driven by the patterns experienced at higher latitudes. The independence of behaviours displayed by the ocean sectors led to the research investigating the influences, looking at not just the hemisphere, but also each basin separately. The Pacific and Indian Ocean showed in several results to act independently from one another, in temperature gradients, wind field, and storm track position. The strong correlations between the temperature gradient and the wind field, as well as the storm track field show that the two are connected, as the theory suggests. If temperatures rise in the tropics, or decrease in the poles, then the temperature gradient will steepen. The pressure gradient force increases which pushes the thermal wind balance poleward, shifting the position of the westerlies. The area with the largest variation in the wind speed becomes the storm track, which would also shift poleward. Climatic factors such as the southern oscillation index, southern annular mode or Indian Ocean dipole show slight correlations with the temperature field, but have little to no influence on the temperature gradient itself. Precipitation levels in New Zealand are highly variable due to the nature of the countries location and topography. What was found was little connection between the northern part of the country and the storm track. However, closer proximity to the storm track, such as the south of the country, do experience a small amount of variation due to the storm tracks influence.</p>