360 research outputs found
East African hydroclimatic variability: 1950-1999
The interannual variability in precipitation over East Africa is well-understood. Many studies have identified the factors influencing the interannual variability of precipitation such as El Niño – Southern Oscillation (ENSO), the Indian Ocean Dipole Mode (IODM), and Atlantic Ocean sea surface temperature and pressure variations. The relatively arid conditions in much of the East African region are not understood fully. The objective of present study is to determine the meteorological association of aridity over East Africa with regional hydroclimatic variables as well as to find global teleconnections affecting spatial distribution of aridity over East Africa. The East Africa Aridity index is calculated as the ratio of the mean seasonal precipitation to the mean seasonal potential evapotranspiration (after Budyko, 1974), and is used as a measure of aridity over East Africa. Principal components analysis was performed on the aridity index to identify characteristic modes of the temporal variability of the aridity index across East Africa. Correlation analysis was performed to identify the meteorological association in the interannual variability of aridity over East Africa and to find the global teleconnections, such as with ENSO, IODM, North Atlantic Oscillations (NAO), Tropical Atlantic SST Dipole (TASD), and Quasi Biennial Oscillations (QBO) in it. The first principal component of the aridity index was used for the correlation analysis. Correlations of the normalized difference vegetation index (NDVI) and Palmer Drought Severity Index (PDSI) with the first principal component of aridity index were calculated. The aridity index over East Africa is driven by precipitation rather than potential evapotranspiration (PET). The PET over East Africa is driven by precipitation rather than temperature. Aridity over East Africa is well correlated with the NDVI and PDSI. The ENSO influence on interannual variability of precipitation and hence on aridity is very much evident in all the seasons, while IODM influence is evident in the June – September season, the driest season for East Africa. Influence of NAO, TASD, and QBO was observed to be very small compared to that of ENSO and IODM. The teleconnections influencing the rainfall variability of East Africa also influenced variability in aridity
The WWRP Polar Prediction Project (PPP)
Mission statement: “Promote cooperative international research enabling development of improved weather and environmental prediction services for the polar regions, on time scales from hours to seasonal”. Increased economic, transportation and research activities in polar regions are leading to more demands for sustained and improved availability of predictive weather and climate information to support decision-making. However, partly as a result of a strong emphasis of previous international efforts on lower and middle latitudes, many gaps in weather, sub-seasonal and seasonal forecasting in polar regions hamper reliable decision making in the Arctic, Antarctic and possibly the middle latitudes as well.
In order to advance polar prediction capabilities, the WWRP Polar Prediction Project (PPP) has been established as one of three THORPEX (THe Observing System Research and Predictability EXperiment) legacy activities. The aim of PPP, a ten year endeavour (2013-2022), is to promote cooperative international research enabling development of improved weather and environmental prediction services for the polar regions, on hourly to seasonal time scales. In order to achieve its goals, PPP will enhance international and interdisciplinary collaboration through the development of strong linkages with related initiatives; strengthen linkages between academia, research institutions and operational forecasting centres; promote interactions and communication between research and stakeholders; and foster education and outreach.
Flagship research activities of PPP include sea ice prediction, polar-lower latitude linkages and the Year of Polar Prediction (YOPP) - an intensive observational, coupled modelling, service-oriented research and educational effort in the period mid-2017 to mid-2019
Technical Report Series on Global Modeling and Data Assimilation. Volume 7: Proceedings of the Workshop on the GEOS-1 Five-year Assimilation
The primary objective of the three-day workshop on results from the Data Assimilation Office (DAO) five-year assimilation was to provide timely feedback from the data users concerning the strengths and weaknesses of version 1 of the Goddard Earth Observing System (GEOS-1) assimilated products. A second objective was to assess user satisfaction with the current methods of data access and retrieval. There were a total of 49 presentations, with about half (23) of the presentations from scientists from outside of Goddard. The first two days were devoted to applications of data: studies of the energy diagnostics, precipitation and diabatic heating, hydrological modeling and moisture transport, cloud forcing and validation, various aspects of intraseasonal, seasonal, and interannual variability, ocean wind stress applications, and validation of surface fluxes. The last day included talks from the National Meteorological Center (NMC), the National Center for Atmospheric Research (NCAR), the Center for Ocean-Land-Atmosphere Studies (COLA), the United States Navy, and the European Center for Medium Range Weather Forecasts (ECMWF)
Investigating Aerosol Effects on Clouds, Precipitation and Regional Climate in US and China by Means of Ground-based and Satellite Observations and a Global Climate Model
Aerosols affect climate by scattering/absorbing radiation and by acting as cloud condensation nuclei (CCN) or ice nuclei (IN). One of the least understood but most significant aspects of climate change is the aerosol effect on cloud and precipitation. A hypothesis has recently been proposed that, in addition to reducing cloud effective radius and suppressing precipitation, aerosols may also modify the thermodynamic structure of deep convective clouds and lead to enhanced precipitation when complex thermodynamic processes are involved. Taking advantage of the long-term and extensive ground-based observations at the US Department of Energy's Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site, we thoroughly tested such a hypothesis and provide direct evidence of it. Moreover, the hypothesis is also supported by analysis of satellite-based observations over tropical regions from multiple sensors in the A-Train satellites constellation. Extensive analyses of the long-term ground-based and large-scale data reveal significant increases in rain rate or frequency and cloud top heights with increasing aerosol loading for mix-phase deep convective clouds, decreases rain frequency for low liquid clouds, but little impact on cloud height for liquid clouds. Rigorous tests are conducted to investigate any potential artifacts and influences of meteorological conditions.
Large-scale circulation patterns and monsoon systems can be changed by scattering and absorption of solar radiation by aerosols. By means of model simulations with the National Center for Atmospheric Research Community Climate Model (NCAR/CCM3), we found that the increase of aerosol loading in China contributes to circulation changes, leading to more frequent occurrence of fog events in winter as observed from meteorological records. The increase in atmospheric aerosols over China heats the atmosphere and generates a cyclonic circulation anomaly over eastern-central China. This circulation anomaly leads to a reduction in the influx of dry and cold air over that area during winter. Weakening of the East Asian winter monsoon system may also contribute to these changes. All these changes favor the formation and maintenance of fog over this region.
The MODerate resolution Imaging Spectroradiometer (MODIS) aerosol products used in the above studies are validated using ground-based measurements from the Chinese Sun Hazemeter Network (CSHNET). Overall, substantial improvement was found in the current version of aerosol products relative to the previous one. At individual sites, the improvement varies with surface and atmospheric conditions
Barotropic response of the global atmosphere to large-scale tropical convective forcing
Spring 1999.Also issued as author's thesis (M.S.) -- Colorado State University, 1999.Includes bibliographical references.A nonlinear shallow water model on the sphere is used to study the effect of largescale tropical convective forcing on the response of the global circulation. Both the spatial and temporal characteristics of the forcing are found to significantly affect the production of tropical and extratropical circulation anomalies. The horizontal shape and location of the convective forcing determines the strength, stability, and symmetry of the upper tropospheric response, the proportion of energy transferred into westward- and eastward-dispersing Rossby modes, and the exact direction of energy propagation. Together, these effects produce teleconnection patterns similar to those observed in the atmosphere. The strongest eastward teleconnection patterns are produced when the convective forcing is meridionally elongated and/or centered off of the Equator. The timescale of the convective forcing determines the distribution of energy into Rossby, Kelvin, and gravity-inertia waves through a filtering on the wave spectrum. An analytical solution to the divergent barotropic vorticity equation is derived to highlight this timescale filter. When the forcing timescale is long, only the longest Rossby waves, with westward group velocities, can be excited at large amplitudes. As the timescale decreases, both the shorter Rossby waves, which have eastward group velocities, and the long gravity waves can be excited. The complete response of a stratified atmosphere is then shown to depend not only on the horizontal shape and timescale of the forcing, but also on its vertical structure. Lower tropospheric convective forcing excites a much more vigorous response in the circulation than does upper tropospheric forcing. Strong cyclonic vortices, which can be likened to tropical cyclones, are generated by the unstable breakdown of the flow in the lower troposphere. The initiation locations and the direction of propagation of these vortices are determined by the horizontal shape and orientation of the convective forcing, through differences in Rossby wave energy dispersion. This reasoning provides a simple explanation for observations that tropical cyclones which form in the Western North Pacific during a "reverse monsoon trough" episode tend to track to the north or northeast (Lander 1996), as opposed to the more climatologically favored northwestward tracks of most Western North Pacific tropical storms.Sponsored by the National Oceanic and Atmospheric Administration through TOGA/COARE NA37RJ0202
A study to define meteorological uses and performance requirements for the Synchronous Earth Observatory Satellite
The potential meteorological uses of the Synchronous Earth Observatory Satellite (SEOS) were studied for detecting and predicting hazards to life, property, or the quality of the environment. Mesoscale meteorological phenonmena, and the observations requirements for SEOS are discussed along with the sensor parameters
- …