Guolin Feng

Evaporation variability under climate warming in five reanalyses and its association with pan evaporation over China

With the motivation to identify actual evapotranspiration (AE) variability under climate warming over China, an assessment is made from five sets of reanalysis data sets [National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR), NCEP-Department of Energy (NCEP-DOE), Modern-Era Retrospective Analysis for Research and Applications (MERRA), Interim Reanalysis, and Japanese 55-year Reanalysis (JRA-55)]. Based on comparison with AE estimates calculated using the Budyko equation, all five reanalysis data sets reasonably reproduce the spatial patterns of AE over China, with a clearly southeast-northwest gradient. Overall, JRA-55 (NCEP-DOE) gives the lowest (highest) reanalysis evaporation (RE) values. From 1979 to 2013, dominant modes of RE among five reanalyses are extracted using multivariate empirical orthogonal function analysis. Accordingly, the interdecadal variation of RE is likely driven by the change of temperature, and the interannual variation is constrained by the water supply conditions. Under climate warming, RE increase in the Northwest China, Yangtze-Huaihe river basin, and South China, while they decrease in Qinghai-Tibet Plateau, and northern and Northeast China. Moreover, the relationship between RE and pan evaporation (PE) are comprehensively evaluated in space-time. Negative correlations are generally confirmed in nonhumid environments, while positive correlations exist in the humid regions. Our analysis supports the interpretation that the relationship between PE and AE was complementary with water control and proportional with energy control. In view of data availability, important differences in spatial variability and the amount of RE can be found in Northwest China, the Qinghai-Tibet Plateau, and the Yangtze River Basin. Generally speaking, NCEP-NCAR and MERRA have substantial problems on describing the long-term change of RE; however, there are some inaccuracies in the JRA-55 estimates when focusing on the year-to-year variation.

Forecast scheme and strategy for extended-range predictable components

Although extended-range forecasting has exceeded the limit of daily predictability of weather, there are still partially predictable characteristics of meteorological fields in such forecasts. A targeted forecast scheme and strategy for extended-range predictable components is proposed. Based on chaotic characteristics of the atmosphere, predictable components and unpredictable random components are separated by using the standpoint of error growth in a numerical model. The predictable components are defined as those with slow error growth at a given range, which are not sensitive to small errors in initial conditions. A numerical model for predictable components (NMPC) is established, by filtering random components with poor predictability. The aim is to maintain predictable components and avoid the influence of rapidly growing forecast errors on small scales. Meanwhile, the analogue-dynamical approach (ADA) is used to correct forecast errors of predictable components, to decrease model error and statistically take into account the influence of random components. The scheme is applied to operational dynamical extended-range forecast (DERF) model of the National Climate Center of China Meteorological Administration (NCC/CMA). Prediction results show that the scheme can improve forecast skill of predictable components to some extent, especially in high predictability regions. Forecast skill at zonal wave zero is improved more than for ultra-long waves and synoptic-scale waves. Results show good agreement with predictability of spatial scale. As a result, the scheme can reduce forecast errors and improve forecast skill, which favors operational use.

Interactions of Multiple Atmospheric Circulation Drive the Drought in Tarim River Basin.

Global warming is likely to cause overall drying of land surfaces and aridity increasing leading to expansion of dry climate zones. There is an increased risk of extremely arid environment and large deserts developed progressively in the central Asia. However, the key factors causing the drying in mid-Asia remain inconclusive. Here, we analyzed the relationship among precipitation, water vapor transportation in Tarim River Basin (TRB) and Multiple Atmospheric Circulation (MAC) to explore the mechanism of MAC driving the drying in TRB, through comparing MAC between abundant and scarce precipitation years. We found that Westerly Circulation (WC) and Asian Summer Monsoon (ASM) are likely to promote the precipitation respectively. Whereas, they not only have their own influence but also restrict each other and facilitate the forming of peculiar water vapor transport channel for TRB, which is probably to restrain the precipitation and its distribution pattern and accelerate the drying in this region. Our results enrich the findings on mechanisms of wet places becoming wetter while dry areas getting drier under the global warming.

Dynamic-statistics combined forecast scheme based on the abrupt decadal change component of summer precipitation in East Asia

Based on the 1983∼2011 CMAP data, the precipitation anomaly in East Asia and its nearby sea regions (hereafter called East Asia for short) demonstrates the “+−+” pattern before 1999 and the “−+−” pattern afterwards; this decadal change is contained principally in the corresponding EOF3 component. However, the NCC_CGCM forecast results are quite different, which reveal the “+−+−” pattern before 1999 and the “−+−+” pattern afterwards. Meanwhile, the probability of improving NCC_CGCM’s forecast accuracy based on these key SST areas is discussed, and the dynamic-statistics combined forecast scheme is constructed for increasing the information of decadal change contained in the summer precipitation in East Asia. The independent sample forecast results indicate that this forecasting scheme can effectively modify the NCC_CGCM’s decadal change information contained in the summer precipitation in East Asia (especially in the area of 30°N–55°N). The ACC is 0.25 and ACR is 61% for the forecasting result based on the V SST area, and the mean ACC is 0.03 and ACR is 51% for the seven key areas, which are better than NCC_CGCM’s system error correction results (ACC is −0.01 and ACR is 49%). Besides, the modified forecast results also provide the information that the precipitation anomaly in East Asia mainly shows the “+−+” pattern before 1999 and the “−+−” pattern afterwards.

A regional extreme low temperature event and its main atmospheric contributing factors

The regional extreme low temperature event from December 30, 2010 to February 2, 2011 was a very rare and protracted cold event with the largest integrated index (Z) since 1979. Two meteorological factors could be responsible for this extreme winter event. First, a persistent blocking pattern existed in the mid-latitudes. This not only allowed cold air to persist in southern China but also enabled each perturbation from the west propagating around the blocking high to trigger downstream cold air intrusions from the north. Second, the consistently downward negative Arctic Oscillation (AO) was favorable for the eastward moving of Rossby waves in middle latitudes, which made the upper reaches positive center in SLP and negative center in Z500 move to East Asia. This stable and consistent situation favored the polar area cold air invasion to the mid-latitude region. Of these two factors, the blocking pattern was likely to be the direct cause, the co-effects of consistently strong downward negative AO from the stratosphere, and the corresponding eastward moving wave train in Z500 and SLP might be the prophase teleconnection culprit.

Analysis of stable components for extended-range (10–30 days) weather forecast: A case study of continuous overcast-rainy process in early 2009 over the mid-lower reaches of the Yangtze River

A continuous overcast-rainy weather (CORW) process occurred over the mid-lower reaches of the Yangtze River (MLRYR) in China from February 14 to March 9 in 2009, with a large stretch and long duration that was rarely seen in historical records. Using the empirical orthogonal function (EOF), we analyzed the geopotential height anomaly field of the NCEP-DOE Reanalysis II in the same period, and defined the stable components of extended-range (10–30 days) weather forecast (ERWF). Furthermore, we defined anomalous and climatic stable components based on the variation characteristics of the variance contribution ratio of EOF components. The climatic stable components were able to explain the impact of climatically averaged information on the ERWF, and the anomalous stable components revealed the abnormal characteristics of the continuous overcast-rainy days. Our results show that the stable components, especially the anomalous stable components, can maintain the stability for a longer time (more than 10 days) and manifest as monthly scale low-frequency variation and ultra-long-wave activities. They also behave as ultra-long waves of planetary scale with a stable and vertically coherent structure, reflect the variation of general circulation in mid-high latitudes, display the cycle of the zonal circulation and the movement and adjustment of the ultra-long waves, and are closely linked to the surface CORW process.

Methods for Improving the Prediction Skill of Summer Precipitation over East Asia–West Pacific

AbstractThe performance of summer precipitation prediction skill of the Beijing Climate Center Climate System Model (BCC_CSM1.1; hereafter CSM) over the East Asia–west Pacific (EA–WP) region indicates the need for further development in order to improve model prediction skill. Two methods, namely the statistical alone prediction and the statistical and model combined prediction, are proposed to improve prediction skill. For the former, more than 2000 combinations of precursory predictors are considered in the predictive model; both most similar and dissimilar information are considered in the prediction process. For the latter, both the statistical analog information and the CSM’s forecast are considered in the predictive model; this new prediction method is based on merging information from both a dynamical model and historical analogs. Cross validation of summer precipitation over EA–WP for the years 1991–2010 shows that the average spatial anomaly correlation coefficient (ACC) of the statistical alone ...

A new algorithm for seasonal precipitation forecast based on global atmospheric hydrological water budget

Precipitation forecast has been identified as one of the central issues in climate research. However, the underlying mechanisms of precipitation are far from being understood. In this paper, a new algorithm of forecasting precipitation based on law of conservation of mass in hydrological cycle is proposed and its feasibility is verified. The algorithm mainly include three steps: in the first step, the area we employ is divided into a number of sub-areas, the precipitation source and evaporation whereabouts equations for sub-regions are established, and the rationality of them can be verified by checking whether the precipitation source and evaporation equations meet a self-consistent relationship or not; in the second step, a conversion equation for sub-regional precipitation prediction will be established, which characterize the relationship between precipitation and evaporation in the sub-areas; in the last step, if the regional evaporation, precipitation and moisture divergence (convergence) function keep stable in a certain time scale, then precipitation forecast is achieved by evaporation anomalies and moisture divergence function, which can be predicted according to the prophase sea surface temperature and atmospheric circulation. Finally, the northern and southern hemispheres seasonal precipitation, evaporation and moisture divergence (convergence) weighting coefficients are calculated using this algorithm based on European centre for medium-range weather forecasts (ECMWF) interim re-analysis (ERA-Interim) dataset, which well verifies the feasibility of the algorithm. The obtained results may provide new insights for precipitation forecast in the future.

Impact of the December North Atlantic Oscillation on the following February East Asian trough

During winter, the December North Atlantic Oscillation (NAO) has an impact on the following February East Asian trough (EAT), and a significant positive correlation exists between them. It is shown that the circulation anomalies affected by the December NAO for December and for the following January are primarily confined to the Euro-Atlantic sector while they extend to East Asia during the following February, and this is related to anomalous wave trains originating from the southwestern Atlantic and spreading to the northeastern Atlantic, northern Europe, western Siberia, and East Asia. When the NAO is positive phase in December, the sea surface temperature (SST) tripole pattern is forced by persistence positive NAO from December to the following January, contributing to pronounced positive SST anomalies in midlatitude areas of the North Atlantic during the following February. The pronounced positive SST anomalies found during this period can generate feedback for atmospheric anomalies, and the westerly winds are enhanced (reduced) to the north (south) side of the positive SST anomalies, which result from strengthened (weakened) baroclinicity there. In addition, the Rossby wave source over the northeastern Atlantic shows a positive anomaly, establishing a link between the positive SST anomalies in midlatitude areas of the North Atlantic and the deepened EAT downstream.

The applicability of research on moving cut data-approximate entropy on abrupt climate change detection

In this study, the performance of moving cut data-approximate entropy (MC-ApEn) to detect abrupt dynamic changes was investigated. Numerical tests in a time series model indicate that the MC-ApEn method is suitable for the detection of abrupt dynamic changes for three types of meteorological data: daily maximum temperature, daily minimum temperature, and daily precipitation. Additionally, the MC-ApEn method was used to detect abrupt climate changes in daily precipitation data from Northwest China and the Pacific Decadal Oscillation (PDO) index. The results show an abrupt dynamic change in precipitation in 1980 and in the PDO index in 1976. The times indicated for the abrupt changes are identical to those from previous results. Application of the analysis to observational data further confirmed the performance of the MC-ApEn method. Moreover, MC-ApEn outperformed the moving t test (MTT) and the moving detrended fluctuation analysis (MDFA) methods for the detection of abrupt dynamic changes in a simulated 1000-point daily precipitation dataset.