Changing climate affects vegetation growth in the arid region of the northwestern China

The northwestern China is a typical dry-land region of Inner Asia, where significant climate change has been observed over the past several decades. How the regional vegetation, particularly the grassland-oasis-desert complex, responds to such climatic change is poorly understood. To address this question, we investigated spatio-temporal changes in vegetation growth and their responses to a changing climate by biome and bioregion, using satellite-sensed Normalized Difference Vegetation Index (NDVI) data from 1982 to 2003, along with corresponding climate data. Over the past 22 years, about 30% of the total vegetated area showed an annual increase of 0.7% in growing season NDVI. This trend occurred in all biomes and all bioregions except Sawuer, a subregion of the study area with no significant climate change. Further analyses indicated that NDVI change was highly correlated with the current precipitation and evapotranspiration in growing season but was not associated with temperature. We also found that NDVI was positively correlated with the preceding winter precipitation. These findings suggest that precipitation may be the key cause of vegetation growth in this area, even for mountain forests and grasslands, whose growth are often regarded to be limited by low temperate in winter and early spring.     

Go with the flow: geospatial analytics to quantify hydrologic landscape connectivity for passively dispersed microorganisms

Understanding the diverse ways that landscape connectivity influences the distribution of microbial species is central to managing the spread and persistence of numerous biological invasions. Here, we use geospatial analytics to examine the degree to which the hydrologic connectivity of landscapes influences the transport of passively dispersed microbes, using the invasive plant pathogen Phytophthora ramorum as a case study. Pathogen occurrence was analyzed at 280 stream baiting stations across a range of watersheds – exposed to variable inoculum pressure – in California over a 7-year period (2004–2010). Using logistic regression, we modeled the probability of pathogen occurrence at a baiting station based on nine environmental variables. We developed a novel geospatial approach to quantify the hydrologic connectivity of host vegetation and inoculum pressure derived from least cost distance analyses in each watershed. We also examined the influence of local environmental conditions within the immediate neighborhood of a baiting station. Over the course of the sampling period, the pathogen was detected at 67 baiting stations associated with coastal watersheds with mild climate conditions, steep slopes, and higher levels of inoculum pressure. At the watershed scale, hydrologic landscape connectivity was a key predictor of pathogen occurrence in streams after accounting for variation in climate and exposure to inoculum. This study illustrates a geospatial approach to modeling the degree to which hydrologic systems play a role in shaping landscape structures conducive for the transport of passively dispersed microbes in heterogeneous watersheds.     

凝结反馈对云层垂直发展影响的一个线性理论

根据小尺度垂直运动的线性分析,作者证明凝结反馈对云层的垂直发展有着很强的制约作用。这种制约作用在温度和湿度较大的情况下更为明显。作者引出了在凝结反馈作用下非定常运动中的频率公式和定常运动下的波速公式。这样,南方雨季中每次雨停时常常见到的云层迅速分层的现象得到了可能的理论解释。     

The role of convection in surface property and velocity fluctuations

Time series of temperature, humidity and horizontal wind speed are presented from data collected at two tower sites during the 1968 Barbados Experiment. Marked and periodic fluctuations on the convective scale are evident in the traces. It is postulated that these fluctuations are due to moderate cumulus convection. Estimates show that departures in temperature and humidity at the surface must be due to downward mixing from near or above the convective cloud base. Such cloud layer-subcloud layer mixing must change the thermodynamic character of the subcloud layer. This in turn suggests that cloud convection may play an important part in governing the transport of energy across the air-sea interface.     

Analysis of aircraft measurements of momentum flux in the subcloud layer over the tropical atlantic ocean during gate

The momentum flux data obtained by the gust probe aboard the NOAA DC-6 aircraft during GATE are analyzed. Vertical profiles are obtained for Phases I and III and correlated with vertical wind velocity profiles using the geostrophic departure method. Reasonable agreement is obtained using the horizontal equations of motion with negligible advective acceleration. The vertical profiles of momentum flux and wind speed variance compare well with the numerical model results of Deardorff (1972) and Wyngaard et al. (1974). Vertical distributions of power spectra for vertical eddy motion and cospectra corresponding to the momentum flux components are obtained along with the height variation of the dominant length scales of vertical eddy motion and the dissipation rate of turbulence kinetic energy. When normalized by mixed-layer similarity, these results agree well with previous determinations in the boundary layer over tropical oceans and over land.     

Multiple Residence and Cyclical Migration: A Life Course Perspective*

As a result of restrictive time-space bounds in viewing migration, surprisingly little is known about the tempos and rhythms of geographical mobility in America. We discuss limitations of the conventional definition of migration and develop a life course framework of multiple residence and cyclical migration. Results of an Arizona-based case study reveal that multiple residence is common and more diverse than the annual influx of elderly snowbirds. Coming to grips with multiple residence and recurrent mobility in the United States represents a fundamental challenge in population and migration studies.