January Precipitation Research Articles

Spatio-temporal trends of hydrological components: the case of the Tafna basin (northwestern Algeria)

Abstract Climate change is expected to increase in the future in the Mediterranean region, including Algeria. The Tafna basin, vulnerable to drought, is one of the most important catchments ensuring water self-sufficiency in northwestern Algeria. The objective of this study is to estimate the evolution of hydrological components of the Tafna basin, throughout 2020–2099, compared to the period 1981–2000. The SWAT model (Soil and Water Assessment Tool), calibrated and validated on the Tafna basin with good Nash at the outlet 0.82, is applied to analyze the spatial and temporal evolution of hydrological components over the basin throughout 2020–2099. The application is produced using a precipitation and temperature minimum/maximum of an ensemble of climate model outputs obtained from a combination of eight global climate models and two regional climate models of the Coordinated Regional Climate Downscaling Experiment project. The results of this study show that the decrease of precipitation in January, on average −25%, ranged between −5% and −44% in the future period. This diminution affects all of the water components and fluxes of a watershed, namely, in descending order of impact: the river discharge causing a decrease −36%, the soil water available −31%, the evapotranspiration −30%, and the lateral flow −29%.

Dendrochronological potential of Prumnopitys andina (Podocarpaceae) at the southern limit of its range in the Chilean Andes

ABSTRACT Prumnopitys andina is a South American podocarp with a restricted distribution in southern Andean temperate forests, and is severely threatened due to habitat loss. Despite much dendrochronological work on other temperate South American conifers, little is known about the dendrochronological potential of P. andina. Here, we examine the main features of P. andina tree rings and evaluate the potential of this species for yielding tree-ring chronologies sensitive to climatic variability. In a population at the southern range limit of the species, we collected tree cores and cross-section samples of P. andina for analyses following standard dendrochronological techniques. 76 tree-ring series from 45 trees were examined, 32 (42%) of which were successfully cross-dated and included in a chronology. We found that the species forms well-defined tree rings with distinct boundaries delimiting annual growth. The chronology began in 1778, with notable periods of suppression and release occurring in the late nineteenth and early twentieth centuries, respectively. The chronology showed a high inter-series correlation value of 0.51, suggesting a strong common signal among trees. December to January precipitation showed a positive relationship with radial growth. Conversely, maximum and mean temperature showed negative relationships with radial growth during the austral summer and autumn. These significant relationships of climate variables with radial growth of P. andina indicate that tree-ring series of this species are suitable for dendroclimatological studies.

Analyzing the trend and extent of Sudan low pressure and its impact on precipitation in western and southwestern Iran

The goal of the study is to analyze the trend and extent of Sudan low pressure with respect to climate change and its relation to the upper atmosphere levels and the influences of the variation on precipitation in October, November, December, January, February, and March during the period from 1951 to 2018. This period was divided into seven 10-year periods for a deeper analysis. It was found that the average geopotential height (hgt) in the two studied levels (1000 and 500 hPa) and all the periods, except for the first (1951 to 1960), had an increasing rate, which points to the weakening of the Sudan low pressure. The mean hgt at 1000 hPa increased with a slope of 0.9, 2.64, and 3.2 hgt for October, November, and December, and 3.6, 1.8, and 1.3 hgt for January, February, and March, respectively. The results showed that December and January had the highest rate of increase in pressure at 1000 hPa. The coefficients of variation (CV) had a significant trend of − 1.4, − 1.4, 0.08, − 0.19, − 0.36, and − 1.8% per decade for October, November, December, January, February, and March, respectively. The minimum and maximum hgt showed a noticeable increasing trend in the levels. The low-pressure extend showed a significant decreasing trend of 62,8347.3 and 62,440.8 km2 in December and January, respectively, which was significant at 0.01. There was a negative and significant correlation between hgt at 1000 hPa and precipitation of western and southwestern Iran in winter regardless of periods, whereas there was a significant negative correlation in Ahvaz taking the periods into account.

Winter Precipitations of Northern Part of Farming-Pastoral Zone and Hulunbuir Grassland

In January 2018, a high record of monthly total precipitation in northern China drew our attention. The precipitation amount is 4 times more than that in normal winters over the past 30 years. Up to now, many researches over the world focus on the intensity of precipitations or the number of intense precipitations, however, no sufficient study is investigated in the continuous moderate precipitations. The abnormal precipitation in January 2018 in northern China is a typical case of the continuous moderate precipitation. The research region is composed by the northern part of the farming-pastoral zone and the Hulunbuir Grassland. The main method in this study consists of analyzing the abnormal precipitation month by identifying the precipitation types and locations, and exploring the atmospheric circulations at the moment of typical precipitations. The research found that the continuity is the key point for high precipitation amount in January 2018 in Northern China. Besides, this continuity is caused by frequent appearance of cyclones in the vicinity.

Connection between the November snow cover over northeast Asia and the following January precipitation in southern China

AbstractIn this paper, the connection between the November northeast Asian snow cover and the following winter precipitation over southern China is studied for the period of 1979–2014. The results indicate that, when there is more November snow cover over northeast Asia, the precipitation in southern China is depressed during the following January, and vice versa. Mechanism analysis suggests that the troposphere‐stratosphere interaction may play an important role in the delayed connection between the two. The more November snow cover over northeast Asia could enhance the vertical propagation of the wave activity, and it could also weaken the westerly winds over northern Asia in the troposphere by decreasing the meridional temperature gradient over the region. The persistent weakening of the tropospheric westerly winds is in favour of the upward propagation of the wave activity into the stratosphere in November and December, and the associated convergence anomalies of the E‐P flux in the stratosphere induce the weakened stratospheric polar vortex. Thereafter, the abnormal polar vortex propagates downward to the lower troposphere and produces a low pressure anomaly to the east of Lake Baikal during January. Such a low pressure anomaly tends to strengthen the southward invasion of the cold air to southern China, which is unfavourable for the precipitation there. This leading signal of the November snow cover over northeast Asia provides a potential source for the prediction of the January precipitation over southern China.

Interannual variations of monthly precipitation and associated mechanisms over the Three River Source region in China in winter months

AbstractThis study investigates the interannual variability of winter precipitation over the Three River Source (TRS) region in China based on precipitation observations collected at stations in the TRS region and reanalysis datasets for the period of 1980–2015. The results suggest that the TRS winter precipitation has distinct interannual variability with discordant trends in different months, that is, an increasing trend is found in November and February, a decreasing trend is found in other winter months, and interannual variations in precipitation are different in different months. The mechanisms for the interannual variation in monthly precipitation over the TRS region are significantly different in different winter months. The interannual variability of TRS precipitation in November is modulated by an anomalous westerly water vapour transport (WVT) branch. This anomalous WVT branch is related to a North Atlantic‐Europe‐Tibetan Plateau (NA‐E‐TP) wave‐train that originates in the North Atlantic due to the ocean–atmosphere interaction. In December, a circum‐global teleconnection (CGT) wave‐train can induce anomalous westerly WVT in the TRS region, resulting in increased precipitation there. This CGT wave‐train is triggered by warm SST anomalies in the central‐eastern tropical Pacific associated with the El Niño‐Southern Oscillation (ENSO). The interannual variability of TRS precipitation in January and February is affected by southwesterly WVT anomalies over the TRS region, which are associated with a southeastward propagating wave‐train over Eurasia caused by the North Atlantic Oscillation (NAO). In March, the interannual variability of TRS precipitation is modulated by the leading mode of the Eurasian circulation that resembles the Scandinavian (EU1) pattern, which can cause anomalous southwesterly WVT in the TRS region. In winter, the characteristics of sea surface temperature anomalies, weather systems, and atmospheric circulations associated with interannual variations of monthly precipitation in the TRS region are different for individual winter months.

Ionospheric response to solar and magnetospheric protons during January 15–22, 2005: EAGLE whole atmosphere model results

We present an analysis of the ionosphere and thermosphere response to Solar Proton Events (SPE) and magnetospheric proton precipitation in January 2005, which was carried out using the model of the entire atmosphere EAGLE. The ionization rates for the considered period were acquired from the AIMOS (Atmospheric Ionization Module Osnabrück) dataset. For numerical experiments, we applied only the proton-induced ionization rates of that period, while all the other model input parameters, including the electron precipitations, corresponded to the quiet conditions. In January 2005, two major solar proton events with different energy spectra and proton fluxes occurred on January 17 and January 20. Since two geomagnetic storms and several sub-storms took place during the considered period, not only solar protons but also less energetic magnetospheric protons contributed to the calculated ionization rates. Despite the relative transparency of the thermosphere for high-energy protons, an ionospheric response to the SPE and proton precipitation from the magnetotail was obtained in numerical experiments. In the ionospheric E layer, the maximum increase in the electron concentration is localized at high latitudes, and at heights of the ionospheric F2 layer, the positive perturbations were formed in the near-equatorial region. An analysis of the model-derived results showed that changes in the ionospheric F2 layer were caused by a change in the neutral composition of the thermosphere. We found that in the recovery phase after both solar proton events and the enhancement of magnetospheric proton precipitations associated with geomagnetic disturbances, the TEC and electron density in the F region and in topside ionosphere/plasmasphere increase at low- and mid-latitudes due to an enhancement of atomic oxygen concentration. Our results demonstrate an important role of magnetospheric protons in the formation of negative F-region ionospheric storms. According to our results, the topside ionosphere/plasmasphere and bottom-side ionosphere can react to solar and magnetospheric protons both with the same sign of disturbances or in different way. The same statement is true for TEC and foF2 disturbances. Different disturbances of foF2 and TEC at high and low latitudes can be explained by topside electron temperature disturbances.

Strengthened Linkage between November/December North Atlantic Oscillation and Subsequent January European Precipitation after the Late 1980s

AbstractThis work investigates the nonsynchronous relationship between the North Atlantic Oscillation (NAO) and winter European precipitation. The results indicate that the linkage between early-winter (November and December) NAO and the following January precipitation and atmospheric circulation over the North Atlantic and European sectors became statistically significant after the late 1980s. Before the late 1980s, January precipitation and atmospheric circulation are weakly correlated with early-winter NAO. After the late 1980s, by contrast, the positive phase of the early-winter NAO is generally followed by an anomalous meridional dipole pattern with barotropic structure over the North Atlantic, which provides conditions for more (less) precipitation south of Iceland (east of the Azores). Further analysis elucidates that this regime shift may be partly attributed to the change of early-winter NAO, which is concurrent with significant change in the intensity of the synoptic and low-frequency eddy interaction over the Atlantic–European sectors. Anomalous positive sea level pressure and geopotential height, along with zonal wind anomalies associated with a positive early-winter NAO over the North Atlantic, are more significant and extend more northeastward after the late 1980s, which may be induced by an intensified transient eddy feedback after the late 1980s, as well as the enhanced storm-track activity over the North Atlantic. Thus, early-winter NAO can induce significant ocean temperature anomalies in the North Atlantic after the late 1980s, which extend downward into the middle parts of the thermocline and persist until the following January to trigger NAO-like atmospheric circulation patterns. Analyses from the Community Earth System Model large ensemble simulations indicate the effects of internal climate variability on such a strengthened linkage.

The Development and Use of Isoscapes to Determine the Geographical Origin of Quercus spp. in the United States

The stable isotope ratios of oxygen, hydrogen, carbon and sulfur from extracted wood of 87 samples of oaks from the United States were analysed. Relationships with climate variables and the stable isotope ratios of the 69 training dataset samples were investigated to a monthly resolution using long-term monthly mean climate data from NASA and the University of East Anglia’s Climate Research Unit, in conjunction with forecast data for hydrogen and oxygen isotope ratios in precipitation. These relationships were used to construct model isoscapes for oxygen, hydrogen, carbon and sulfur for US oak with the aim of using them to forecast isotopic patterns in areas that were not sampled and predict values in samples not used to construct the models. The leading predictors for isoscape generation were oxygen isotope ratios in January precipitation for oak oxygen isotope ratios, hydrogen isotope ratios in July precipitation for oak hydrogen isotope ratios, water vapour in April for carbon isotope ratios, and reflected shortwave radiation in March in combination with sulfate concentration in May for oak sulfur isotopes. The generated isoscapes can be used to show regions an unknown sample may have originated from with a resolution dependent on the rarity of the stable isotope signature within the United States. The models were assessed using the data of 18 samples of georeferenced oak. The assessment found that 100% of oxygen, 94% of hydrogen, 78% of carbon, and 94% of sulfur isotope ratios in the 18 test dataset samples fell within two standard deviations of the isoscape models. Using the results of the isoscapes in combination found that there were 4/18 test samples which did not fall within two standard deviations of the four models, this is largely attributed to the lower predictive power of the carbon isoscape model in conjunction with high local variability in carbon isotope ratios in both the test and training data. The method by which this geographic origin method has been developed will be useful to combat illegal logging and to validate legal supply chains for the purpose of good practice due diligence.

Modern pollen and its relationship with vegetation and climate in the Mu Us Desert and surrounding area, northern China: Implications of palaeoclimatic and palaeocological reconstruction

This study establishes the spatial and quantitative relationship between modern pollen, vegetation and climate based on 84 surface pollen samples from a nature reserve and ecological restoration area in the Mu Us Desert and surrounding areas, including seven natural vegetation types. Pollen spectra, discriminant analysis and spatial distribution patterns of the main pollen types are used to explore the relationship between pollen assemblages and vegetation types. Results show that each vegetation type can be characterized by a distinct modern pollen assemblage as follows: (1) coniferous forest: Pinus Diploxylon-type and Picea; (2) deciduous forest: Pinus Diploxylon-type, Quercus, Artemisia and Poaceae; (3) deciduous shrub: Pinus Diploxylon-type, Artemisia and Poaceae; (4) grass meadow: Poaceae and Cyperaceae; (5) grass steppe: Poaceae and Artemisia; (6) desert steppe: Chenopodiaceae, Artemisia and Poaceae; and (7) desert: Chenopodiaceae, Nitraria and Artemisia. The higher percentage and lower concentration of Pinus pollen in the Mu Us Desert can be explained by effective long-distance transport from the mountain forest, while Picea has a limited pollen dispersal ability via wind-borne dispersal. Poaceae pollen is well represented and has a higher percentage in several different types of ecological conditions. Modern pollen–climate relationships are assessed using redundancy analysis and weighted averaging partial least squares regression. Significant relationships exist between pollen assemblages and mean annual temperature (R2 = 0.67; RMSEP 1.7 °C; max. bias 2.5 °C), July precipitation (R2 = 0.58; RMSEP 17.8 mm; max. bias 26.0 mm), and January precipitation (R2 = 0.82; RMSEP 0.9 mm; max. bias 1.8 mm). Furthermore, the climate variables obtained by spatial interpolation of data from meteorological stations are used to cross-check the data reconstructed by pollen-climate transfer functions. Our work provides a more comprehensive understanding of the qualitative and quantitative relationships between modern pollen and vegetation/climate and serves as a basis for the paleovegetation and paleoclimate reconstruction in arid and semiarid regions of northern China and similar areas.

Responses of potential suitable area of Paris verticillata to climate change and its dominant climate factors

Prediction of the potential distribution of species based on the data of its current distribution in combination with climatic variables is important for understanding species evolution and reasonable conservation. Based on 220 distribution sites in China and 12 low-correlation climatic variables, we analyzed the potential distribution of Paris verticillata at present and future (2050s and 2070s) using the MaxEnt model and ArcGIS program. Further, we analyzed the dominant driving factors for its geographic distribution. The results showed that the area under the curve indices (AUC) was 0.940, with high prediction accuracy. The potential suitable regions of P. verticillata were mainly distributed in the Greater Xing'an Mountains, the Xiao Xing'an Mountains, the Changbai Mountains, the Qinling-Daba Mountains, Hebei, Shanxi and north Shandong under current climate scenario. Those regions accounted for 18.1% of the total suitable area in the country, of which the highly suitable areas accounted for 7.0% and the lowly suitable area 11.1%. The total suitable areas of P. verticillata in the 2050s and 2070s would decline under the climate change scenarios of RCP 2.6, RCP 4.5, RCP 6.0 and RCP 8.5. The highly suitable area would decline, but the lowly suitable area would increase. With the global climate change, both the range and the geometric center of its distribution would gradually spread to higher altitude in the northeast. The cumulative contributions of four dominant factors reached as high as 89.2%, namely, precipitation of wettest month, mean annual temperature, isothermality, and precipitation of January. Their appropriate ranges were 100-275 mm, -0.1-16 ℃, 21-35 and 3-14 mm, respectively.

The impact of climate indices on precipitation variability in Baluchistan, Pakistan

Pakistan’s biggest province in terms of area, Baluchistan appears to have been affected from the climate variability since last few decades. No substantive research works have been carried out in analyzing the precipitation variability in Baluchistan and linkage to large-scale teleconnection. The goal of this paper is to determine possible linkages of precipitation with large scale atmospheric and oceanic circulation indices in the months which have shown changes in precipitation trends in Baluchistan. These climate indices may be the possible predictors for the precipitation in Baluchistan in the respective months. Mann-Kendall (MK) statistical test was used to identify the monthly significant precipitation trends in thirteen meteorological stations located in four regions of Baluchistan. The noteworthy trend out of significant trends is selected using Theil and Sen’s slope (TS). Decreasing trend is identified in January whereas increasing trend is identified in June mostly in stations located in North Eastern region of Baluchistan (Region1). The changes in the significant trend in January and June under the influence of climate indices are then determined by Partial Mann-Kendall (PMK). Empirical Orthogonal Function (EOF), Principal Component Analysis (PCA), correlation technique between Principal Components (PC) of Region1 precipitation and climatic Indices are used to filter out the relevant climatic indices. It is found out that North Atlantic Oscillation (NAO), Equatorial Indian Ocean Zonal Wind Index (EQWIN), ENSO Modoki Index (EMI) on annual scale whereas Pacific Decadal Oscillation (PDO), Atlantic Multi-decadal Oscillation (AMO) on decadal scale are influencing the January precipitation. It is also found out that El Nino Southern Oscillation-Multivariate ENSO Index (ENSO-MEI), EMI, NAO, PDO and AMO are influencing the June precipitation. These are the dominating indices explains the precipitation variability in January and June in this Region1. This research will impart awareness in the society from the impact of precipitation trend variability.

Winter wheat response to climate change under irrigated and dryland conditions in the US southern High Plains

A rise in global temperature has been observed since the 1980s. The objective of the study was to understand the winter wheat (Triticum aestivum L.) responses and yield performances to the changing climate. The study was performed using growing degree day (GDD) model and regression analysis method. Wheat data (1940 to 2016) were obtained from the Southern Regional Performance Nursery (SRPN) experiments at Bushland, Texas. Wheat genotypes were divided into three groups: Kharkof as a check cultivar, the highest yielding cultivar for each year, and all other cultivars. Climate data were obtained from USDA Agricultural Research Service at Bushland and the National Oceanic and Atmospheric Administration. Temperatures between 1940 and 2016 showed a curvilinear trend with a significant rise since 1980. Mean temperature during wheat growing season (September 15 to June 15) increased at a rate of 0.35°C per decade since 1980. Consequently, seasonal GDD and extreme degree days (EDD) increased at a rate of 69°C d and 19°C d per decade. From 1940 to 2016, wheat heading date has shifted earlier by 16 days for dryland wheat and 11 days for irrigated wheat. Maximum and minimum temperatures in March and April had a negative relationship with heading date, while precipitation had a positive relationship with heading date for both dryland and irrigated wheat. Early heading may have benefits for heat escape but also has the disadvantage of reducing precipitation prior to heading—a loss of 25 mm when heading shifted earlier by 16 days for dryland wheat. The five-year moving average yields increased steadily from about 1960 to 1980, plateaued until the late 1980s, and then declined significantly. However, average genetic yield gains were 505 kg ha−1 under dryland and 1,671.4 kg ha−1 under irrigated conditions for all new cultivars during 1940 to 2016. Yield trends for all three cultivar groups had similar patterns over the years. Increase in maximum temperature, frequencies of maximum temperature greater than or equal to 31°C, and minimum temperature during grain filling period had significant negative effect on grain filling days and yield (except for minimum temperature under dryland conditions). Precipitation during the vegetative growth (November, January, and March) and grain filling (May) had a significant positive effect on dryland wheat yields, but only the January precipitation had significant effect on wheat yield under irrigated conditions. The study indicated that the rising temperature and shifting precipitation timings as a result of early heading are less favorable conditions for winter wheat production in the US southern High Plains.

Seasonal changes of the diurnal variation of precipitation in the upper Río Chagres basin, Panamá.

This study elucidated the characteristics of climatological seasonal changes in the diurnal variations of precipitation at four ground stations in the upper Río Chagres basin in the Panama Canal watershed. The seasonal changes differed among the stations, although they are located within an area of only 414 km2. Precipitation peaks in the early afternoon at 1500 local standard time (LST) were observed at all the stations. At Chamon, monthly-mean hourly precipitation at every hour exceeded 0.3 mm h–1 throughout November and December. The occurrence of morning precipitation in January and March distinguished the seasonal precipitation pattern at Esperanza from the pattern at the other stations. Analyses of the seasonal changes in the diurnal variation with pattern correlations and rotational empirical orthogonal functions grouped the stations into two pairs: no morning peak at Chico and Río Piedras in the downstream basin and morning peak at Chamon and Esperanza in the upstream basin.

Habitat Selection and Genetic Structure of the Endangered Frog Species Odorrana wuchuanensis (Anura: Ranidae)

Understanding the habitat selection and population genetic structure of an endangered species can play important roles in its protection. The Wuchuan odorous frog (Odorrana wuchuanensis) is endemic to the karst regions of southwest China. This frog is currently listed as "Critically Endangered" by the IUCN, but little is known about its habitat selection and population genetics. In this study, we conducted analyses of habitat selection with occurrence/absence sites and environmental data, and assessed the genetic structure between north and south populations in Guizhou provinces in China using three mitochondrial markers. The results revealed that the probability of this frog occupying cave habitats increased with higher average humidity in July and higher lowest temperature in January, but was negatively related to precipitation in January. Analyses of F statistics combined with analyses of median-joining haplotype networks and the phylogenetic tree showed low genetic differentiation between the two populations of O. wuchuanensis. Considering the small population size and geographic isolation because of the complex karst terrains, we suggest careful management practices are needed to protect this species.

Contrasting climate-growth relationship between Larix gmelinii and Pinus sylvestris var. mongolica along a latitudinal gradient in Daxing’an Mountains, China

The Daxing’an Mountains is one of the areas with the most serious climate warming in northern China. Dahurian larch (Larix gmelinii) and Mongolian Scots pine (Pinus sylvestris var. mongolica) are two major coniferous species in boreal forests of the region. Their growth-climate relationship is crucial for understanding the effects of climate change on boreal forest ecosystems. To examine and compare the changes of climate-growth relationship between larch and pine, a total of 418 tree-ring cores of the two species were collected at six sites in the Daxing’an Mountains, and the tree-ring chronologies were developed. The results showed that water availability (Palmer Drought Severity Index, PDSI) played a key role in the stable growth of larch and pine. The temperature and precipitation in January, June-August are important factors affecting the radial growth of the two coniferous species along the latitude gradient. The correlation coefficients of growth and the seasonal temperature and precipitation of larch and pine showed a completely opposite trend with the increase of latitude. In summer and autumn, the correlation coefficients between larch growth and seasonal mean temperature decreased first and then increased with the increase of latitude, while that of pine, on the contrary, increased first and then decreased. In winter, spring and autumn, the correlation coefficients between larch growth and seasonal total precipitation decreased first and then increased with the increase of latitude, while that of pine was opposite. However, the correlation coefficients between larch and pine growth and PDSI showed the same trend with the increase of latitude, decreasing at first and then increasing. Before and after rapid warming (around 1980), the correlation coefficients between larch and pine growth and PDSI showed a completely opposite change. Our findings emphasize that the growth-climate relationships of Dahurian larch and Mongolian Scotts pine shows an opposite trend with latitude, which means that the two species may exhibit a completely opposite response with climate change along the latitude gradient.

Effects of the North Atlantic Oscillation (NAO) and meteorological variables on the annual Alcarria honey production in Spain

La Alcarria is one of the most significant regions for honey production in Spain. Times series data of the annual Alcarria honey production from 2000 to 2013 revealed that the total production was highly variable ranging from ca. 60,000 kg in 2000 to almost three times as much in 2011, but without a remarkable trend through time. We hypothesized that these local fluctuations in honey production may be explained by broad-scale climatic patterns. Based on the multiple linear models, we found a significant positive relationship between honey production as a function of the overall means of the North Atlantic Oscillation (NAO) index and precipitation in January, which accounts for 80% of the variability in the Alcarria honey production. We propose that the effect of a negative phase of the NAO and precipitation in January could predict the Alcarria honey production a few months in advance.

Impact of Surface Potential Vorticity Density Forcing over the Tibetan Plateau on the South China Extreme Precipitation in January 2008. Part I: Data Analysis

The external source/sink of potential vorticity (PV) is the original driving force for the atmospheric circulation. The relationship between surface PV generation and surface PV density forcing is discussed in detail in this paper. Moreover, a case study of the extreme winter freezing rain/snow storm over South China in January 2008 is performed, and the surface PV density forcing over the eastern flank of the Tibetan Plateau (TP) has been found to significantly affect the precipitation over South China in this case. The TP generated PV propagated eastward in the middle troposphere. The associated zonal advection of positive absolute vorticity resulted in the increasing of cyclo-nic relative vorticity in the downstream region of the TP. Ascending air and convergence in the lower troposphere developed, which gave rise to the development of the southerly wind. This favored the increasing of negative meridio-nal absolute vorticity advection in the lower troposphere, which provided a large-scale circulation background conducive to ascending motion such that the absolute vorticity advection increased with height. Consequently, the ascending air further strengthened the southerly wind and the vertical gradient of absolute vorticity advection between the lower and middle troposphere in turn. Under such a situation, the enhanced ascending, together with the moist air transported by the southerly wind, formed the extreme winter precipitation in January 2008 over South China.

Impact of Surface Potential Vorticity Density Forcing over the Tibetan Plateau on the South China Extreme Precipitation in January 2008. Part ll: Numerical Simulation

The surface air convergence on the eastern flank of the Tibetan Plateau (TP) can increase the in situ surface potential vorticity density (PVD). Since the elevated TP intersects with the isentropic surfaces in the lower troposphere, the increased PVD on the eastern flank of TP thus forms a PVD forcing to the intersected isentropic surface in the boundary layer. The influence of surface PVD forcing over the TP on the extreme freezing rain/snow over South China in January 2008 is investigated by using numerical experiments based on the Finite-volume Atmospheric Model of the IAP/LASG (FAMIL). Compared with observations, the simulation results show that, by using a nudging method for assimilating observation data in the initial flow, this model can reasonably reproduce the distribution of precipitation, atmospheric circulation, and PVD propagation over and downstream of the TP during the extreme winter precipitation period. In order to investigate the impact of the increased surface PVD over the TP on the extreme precipitation in South China, a sensitivity experiment with surface PVD reduced over the TP region was performed. Compared with the control experiment, it is found that the precipitation in the TP downstream area, especially in Southeast China, is reduced. The rainband from Guangxi Region to Shandong Province has almost disappeared. In the lower troposphere, the increase of surface PVD over the TP region has generated an anomalous cyclonic circulation over southern China, which plays an important role in increasing southerly wind and the water vapor transport in this area; it also increases the northward negative absolute vorticity advection. In the upper troposphere, the surface PVD generated in eastern TP propagates on isentropic surface along westerly wind and results in positive absolute vorticity advection in the downstream areas. Consequently, due to the development of both ascending motion and water vapor transport in the downstream place of the TP, extremely heavy precipitation occurs over southern China. Thereby, a new mechanism concerning the influence of the increased surface PVD over the eastern TP slopes on the extreme weather event occurring over southern China is revealed.

Altitudinal disparity in growth of Dahurian larch (Larix gmelinii Rupr.) in response to recent climate change in northeast China

Forests are sensitive to climate change at high altitude and high latitude. Dahurian larch (Larix gmelinii Rupr.) has experienced an unprecedented forest retreat northward during the last century. Whether the response of growth to climate has dissimilar patterns at different altitudes, and what the “altitudinal trends” of forest development will be in the future, remains unclear. We dendroclimatically investigated the impacts of climate change on the growth of larch forests along an altitudinal gradient. In total, 721 trees from 25 forest stands, representing an altitudinal range from 400 to 950 m a.s.l. in the Great Xing'an Mountains, northeast China, were sampled and used to develop tree-ring width chronologies. The results suggest that warming caused a decline in larch growth at low altitude, while tree growth increased at high altitude. The growth–climate relationships indicate that October–February temperatures were positively correlated with larch growth at low- and high-altitude sites, but negatively correlated at medium-altitude sites (ca. 600–700 m a.s.l.). April–May (early spring) temperatures and October–January precipitation had positive effects on growth in general (ca. 75% of all sites). The effects of summer temperature/precipitation on larch growth at high-altitude sites were opposite to that at low-altitude sites. This change of response from significantly positive/negative correlation to significantly negative/positive correlation occurred gradually along the altitudinal gradient. The relationships varied significantly with altitude both in the case of temperature (R2 = 0.425, P < 0.001) and precipitation (R2 = 0.613, P < 0.001). The shift in response of larch forest to changes in summer temperature and precipitation occurred in the areas with a mean annual temperature of ca. −4 °C and ca. −5 °C, respectively; larch growth at temperatures lower or higher than these thresholds was limited by temperature and precipitation, respectively.