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## 2017 Vol. 34, No. 5

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2017, 34(5): 571-575. doi: 10.1007/s00376-017-6329-6
Abstract:
2017, 34(5): 576-586. doi: 10.1007/s00376-017-6268-2
Abstract:
Anthropogenic climate change is expected to strengthen the vertical wind shears at aircraft cruising altitudes within the atmospheric jet streams. Such a strengthening would increase the prevalence of the shear instabilities that generate clear-air turbulence. Climate modelling studies have indicated that the amount of moderate-or-greater clear-air turbulence on transatlantic flight routes in winter will increase significantly in future as the climate changes. However, the individual responses of light, moderate, and severe clear-air turbulence have not previously been studied, despite their importance for aircraft operations. Here, we use climate model simulations to analyse the transatlantic wintertime clear-air turbulence response to climate change in five aviation-relevant turbulence strength categories. We find that the probability distributions for an ensemble of 21 clear-air turbulence diagnostics generally gain probability in their right-hand tails when the atmospheric carbon dioxide concentration is doubled. By converting the diagnostics into eddy dissipation rates, we find that the ensemble-average airspace volume containing light clear-air turbulence increases by 59% (with an intra-ensemble range of 43%-68%), light-to-moderate by 75% (39%-96%), moderate by 94% (37%-118%), moderate-to-severe by 127% (30%-170%), and severe by 149% (36%-188%). These results suggest that the prevalence of transatlantic wintertime clear-air turbulence will increase significantly in all aviation-relevant strength categories as the climate changes.
2017, 34(5): 587-598. doi: 10.1007/s00376-016-6088-9
Abstract:
Recent snow height measurements (2008-15) from nine automatic weather stations (AWSs) on the Ross Ice Shelf are used to examine the synoptic and seasonal variability in snow accumulation, and also to evaluate the performance of the Antarctic Mesoscale Prediction System (AMPS) for precipitation. The number of snow accumulation events varies from one station to another between 2008 and 2015, thus demonstrating geographic dependence. The interannual variability in snow accumulation is too high to determine its seasonality based on the current AWS observations with limited time coverage. Comparison between the AMPS and AWS snow height measurements show that approximately 28% of the AWS events are reproduced by AMPS. Furthermore, there are significant correlations between AMPS and AWS coincident event sizes at five stations (p<0.05). This finding suggests that AMPS has a certain ability to represent actual precipitation events.
2017, 34(5): 599-612. doi: 10.1007/s00376-016-6212-x
Abstract:
Based on basic equations in isobaric coordinates and the quasi-geostrophic balance, an eddy-flux form budget equation of the divergent wind has been derived. This newly derived budget equation has evident physical significance. It can show the intensity of a weather system, the variation of its flow pattern, and the feedback effects from smaller-scale systems (eddy flows). The usefulness of this new budget equation is examined by calculating budgets for the strong divergent-wind centers associated with the South Asian high, and the strong divergence centers over the Tibetan Plateau, during summer (June-August) 2010. The results indicate that the South Asian high significantly interacts with eddy flows. Compared with effects from the mean flow (background circulation), the eddy flows' feedback influences are of greater importance in determining the flow pattern of the South Asian high. Although the positive divergence centers over the Tibetan Plateau intensify through different mechanisms, certain similarities are also obvious. First, the effects from mean flow are dominant in the rapid intensification process of the positive divergence center. Second, an intense offsetting mechanism exists between the effects associated with the eddy flows' horizontal component and the effects related to the eddy flows' convection activities, which weakens the total effects of the eddy flows significantly. Finally, compared with the effects associated with the convection activities of the mean flow, the accumulated effects of the eddy flows' convection activities may be more favorable for the enhancement of the positive-divergence centers.
2017, 34(5): 613-622. doi: 10.1007/s00376-016-6139-2
Abstract:
A new parameterization of canopy asymmetry factor on phase function, which is dependent on the leaf normal distribution and leaf reflection/transmission, is derived. This new parameterization is much more accurate than the existing scheme. In addition, the new solutions for both the diffuse and direct radiation can be obtained using the Eddington approximation. It is found that the direct radiation can be described as a function of the diffuse radiation. This new approach offers a substantial improvement in accuracy, as compared with the hemispheric constant method, for both isotropic and anisotropic cases. Given the analytical nature of the solution and its high accuracy, we recommend the new parameterization for application in land surface radiation modeling.
2017, 34(5): 623-634. doi: 10.1007/s00376-016-6111-1
Abstract:
Remote sensing of atmospheric water vapor using global positioning system (GPS) data has become an effective tool in meteorology, weather forecasting and climate research. This paper presents the estimation of precipitable water (PW) from GPS observations and meteorological data in Algeria, over three stations located at Algiers, Bechar and Tamanrasset. The objective of this study is to analyze the sensitivity of the GPS PW estimates for the three sites to the weighted mean temperature (T m), obtained separately from two types of T m-T s regression [one general, and one developed specifically for Algeria (T s stands for surface temperature)], and calculated directly from ERA-Interim data. The results show that the differences in T m are of the order of 18 K, producing differences of 2.01 mm in the final evaluation of PW. A good agreement is found between GPS-PW and PW calculated from radiosondes, with a small mean difference with Vaisala radiosondes. A comparison between GPS and ERA-Interim shows a large difference (4 mm) in the highlands region. This difference is possibly due to the topography. These first results are encouraging, in particular for meteorological applications in this region, with good hope to extend our dataset analysis to a more complete, nationwide coverage over Algeria.
2017, 34(5): 635-649. doi: 10.1007/s00376-016-6194-8
Abstract:
Surface energy balance and the partitioning of sensible heat flux (SHF) and latent heat flux (LHF) play key roles in land-atmosphere feedback. However, the lack of long-term observations of surface energy fluxes, not to mention spatially extensive ones, limits our understanding of how the surface energy distribution has responded to a warming climate over recent decades (1979-2009) at the national scale in China. Using four state-of-the-art reanalysis products with long-term surface energy outputs, we identified robust changes in surface energy partitioning, defined by the Bowen ratio (BR = SHF/LHF), over different climate regimes in China. Over the past three decades, the net radiation showed an increasing trend over almost the whole of China. The increase in available radiative energy flux, however, was balanced by differential partitioning of surface turbulent fluxes, determined by local hydrological conditions. In semi-arid areas, such as Northeast China, the radiative energy was transferred largely into SHF. A severe deficiency in near-surface and soil moistures led to a significant decreasing trend in LHF. The combined effect of increased SHF and decreased LHF resulted in significant upward trends in the BR and surface warming over Northeast China. In contrast, in the wet monsoon regions, such as southern China, increased downward net radiation favored a rise in LHF rather than in SHF, leading to a significant decreasing trend in the BR. Meanwhile, the increased LHF partly cancelled out the surface warming. The warming trend in southern China was smaller than that in Northeast China. In addition to impacts on heat-related events, the changes in the BR also reflected recent cases of extreme drought in China. Our results indicate that information regarding the BR may be valuable for drought monitoring, especially in regions prone to such conditions.
2017, 34(5): 650-662. doi: 10.1007/s00376-016-6152-5
Abstract:
Time series of MODIS land surface temperature (T s) and normalized difference vegetation index (NDVI) products, combined with digital elevation model (DEM) and meteorological data from 2001 to 2012, were used to map the spatial distribution of monthly mean air temperature over the Northern Tibetan Plateau (NTP). A time series analysis and a regression analysis of monthly mean land surface temperature (T s) and air temperature (T a) were conducted using ordinary linear regression (OLR) and geographical weighted regression (GWR). The analyses showed that GWR, which considers MODIS T s, NDVI and elevation as independent variables, yielded much better results [R Adj2>0.79; root-mean-square error (RMSE) = 0.51°C-1.12°C] associated with estimating T a compared to those from OLR (R Adj2=0.40-0.78; RMSE = 1.60°C-4.38°C). In addition, some characteristics of the spatial distribution of monthly T a and the difference between the surface and air temperature (T d) are as follows. According to the analysis of the 0°C and 10°C isothermals, T a values over the NTP at elevations of 4000-5000 m were greater than 10°C in the summer (from May to October), and T a values at an elevation of 3200 m dropped below 0°C in the winter (from November to April). T a exhibited an increasing trend from northwest to southeast. Except in the southeastern area of the NTP, T d values in other areas were all larger than 0°C in the winter.
2017, 34(5): 663-678. doi: 10.1007/s00376-016-6175-y
Abstract:
A modified Bowen ratio (BRm), the sign of which is determined by the direction of the surface sensible heat flux, was used to represent the major divisions in climate across the globe, and the usefulness of this approach was evaluated. Five reanalysis datasets and the results of an offline land surface model were investigated. We divided the global continents into five major BRm zones using the climatological means of the sensible and latent heat fluxes during the period 1980-2010: extremely cold, extremely wet, semi-wet, semi-arid and extremely arid. These zones had BRm ranges of ($-\infty$, 0), (0, 0.5), (0.5, 2), (2, 10) and (10, $+\infty$), respectively. The climatological mean distribution of the Bowen ratio zones corresponded well with the Köppen-like climate classification, and it reflected well the seasonal variation for each subdivision of climate classification. The features of climate change over the mean climatological BRm zones were also investigated. In addition to giving a map-like classification of climate, the BRm also reflects temporal variations in different climatic zones based on land surface processes. An investigation of the coverage of the BRm zones showed that the extremely wet and extremely arid regions expanded, whereas a reduction in area was seen for the semi-wet and semi-arid regions in boreal spring during the period 1980-2010. This indicates that the arid regions may have become drier and the wet regions wetter over this period of time.
2017, 34(5): 679-684. doi: 10.1007/s00376-016-6184-x
Abstract:
The impact of La Niña on the winter Arctic stratosphere has thus far been an ambiguous topic of research. Contradictory results have been reported depending on the La Niña events considered. This study shows that this is mainly due to the decadal variation of La Niña's impact on the winter Arctic stratosphere since the late 1970s. Specifically, during the period 1951-78, the tropospheric La Niña teleconnection exhibits a typical negative Pacific-North America pattern, which strongly inhibits the propagation of the planetary waves from the extratropical troposphere to the stratosphere, and leads to a significantly strengthened stratospheric polar vortex. In contrast, during 1979-2015, the La Niña teleconnection shifts eastwards, with an anomalous high concentrated in the northeastern Pacific. The destructive interference of the La Niña teleconnection with climatological stationary waves seen in the earlier period reduces greatly, which prevents the drastic reduction of planetary wave activities in the extratropical stratosphere. Correspondingly, the stratospheric response shows a less disturbed stratospheric polar vortex in winter.