Geoid determination using adapted reference field, seismic Moho depths and variable density contrast

 The traditional remove-restore technique for geoid computation suffers from two main drawbacks. The first is the assumption of an isostatic hypothesis to compute the compensation masses. The second is the double consideration of the effect of the topographic–isostatic masses within the data window through removing the reference field and the terrain reduction process. To overcome the first disadvantage, the seismic Moho depths, representing, more or less, the actual compensating masses, have been used with variable density anomalies computed by employing the topographic–isostatic mass balance principle. In order to avoid the double consideration of the effect of the topographic–isostatic masses within the data window, the effect of these masses for the used fixed data window, in terms of potential coefficients, has been subtracted from the reference field, yielding an adapted reference field. This adapted reference field has been used for the remove–restore technique. The necessary harmonic analysis of the topographic–isostatic potential using seismic Moho depths with variable density anomalies is given. A wide comparison among geoids computed by the adapted reference field with both the Airy–Heiskanen isostatic model and seismic Moho depths with variable density anomaly and a geoid computed by the traditional remove–restore technique is made. The results show that using seismic Moho depths with variable density anomaly along with the adapted reference field gives the best relative geoid accuracy compared to the GPS/levelling geoid. Received: 3 October 2001 / Accepted: 20 September 2002 Correspondence to: H.A. Abd-Elmotaal     

Dleineation and characterization of geomorphological features in a part of lower maharahstra metamorphic plateau using IRS-ID LISS-III Data

In the present study, an attempt has been made to delineate and characterize the different geomorphic units of Tundiya river catchment in a part of Lower Maharahstra Metamorphic Plateau, north-eastern part of Nagpur district, Maharashtra. The drainage, contour and delineated geological units have been overlaid on IRS-ID LISS III satellite imagery (bands 2,3 and 4) in EASI/PACE analysis system to delineate and characterize different geomorphological units and analysis of their processes based on the field observations. The study area is basically of metamorphic in origin with different geological formations and is influenced by the various fluvio-morphological processes. Based on the satellite data analysis, the distinct geomorphological units viz., table top summits, structural hills, subdued plateau, linear ridges, shallow, moderate and deeply buried foot slopes, shallow valley fills and deep valley fills have been delineated and characterized. The information generated from satellite data in the form of vector layers has been used in GIS to generate geological and geomorphological maps of the study area. The present study demonstrates that IRS-ID LISS-III data in conjunction with geology, drainage and contour parameters to enable detailed evaluation of different geomorphological units and analysis of their processes based on the field observations. The delineated geomorphological units can be utilized for evaluation and management of natural resources and geo-environment on sustainable basis at river catchment level.     

Stationary spatially modulated fourier transform spectro-radiometer

Journal of the Indian Society of Remote Sensing - A stationary, compact, spatially modulated Fourier Transform spectro-radiometer based on triangular, common path Sagnac interferometer has been...     

A closer look at the climatological discontinuities present in the NCEP/NCAR reanalysis temperature due to the introduction of satellite data

Principal component analysis was applied to NCEP/NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research) reanalyses data for monthly temperature at given pressure levels between 1948–2000. The series composed with the time coefficients of the main components were tested for possible discontinuities. The study proved useful in gaining a better understanding of the impact of satellite observations in the reanalyses. The period 1975–1979 proved to be the most affected by inhomogeneities, in particular in August–September 1976 and December 1978–January 1979. The latter time corresponds with the introduction of satellite infrared and microwave retrievals, which gave global coverage to the observing network. Inhomogeneities due to satellite data especially affect patterns in the tropics for levels between 700 and 50 hPa and over the Southern Ocean in the layer 500 to 250 hPa, i.e. the affected regions are larger than previously determined with other methods. Greatest shifts were observed in the tropics at 100 and 150 hPa, where the discontinuity is equal to 1.6–2.0 standard deviations.     

Atmospheric Methyl Iodide at Cape Grim, Tasmania, from AGAGE Observations

Atmospheric mixing ratios of methyl iodide (CH₃I) and other methyl halides have been measured at Cape Grim, Tasmania (41°S, 145°E), since early 1998 as part of the Advanced Global Atmospheric Gases Experiment (AGAGE). This paper analyses about 1700 ambient air CH₃I measurements from the 14-month period (March 1998–April 1999). Mixing ratios peaked during the summer, despite faster photolytic loss, suggesting local oceanic emissions were about 2.2–3.6 times stronger in summer than in winter. Back trajectories show that CH₃I levels are strongly dependent on air mass origin, with highest mixing ratios in air from the Tasman Sea/Bass Strait region and lowest levels in air originating from the Southern Ocean at higher latitudes. CH₃I mixing ratios were not well correlated with other methyl halides in unpolluted marine air. The large variations with season and air mass origin suggest that high frequency, continuous data from key locations will make a significant contribution to the understanding of sources and sinks of this important short-lived atmospheric species.     

In-situ Formation of Light-Absorbing Organic Matter in Cloud Water

Current climate models seem to underestimate the flux of solar energy absorbed by the global troposphere. All of these models are constrained with the assumption that cloud droplets consist of pure water. Here we demonstrate in a simple laboratory experiment that aromatic hydroxy-acids which are found in continental fine aerosol can react with hydroxyl radicals under typical conditions prevalent in cloud water influenced by biomass burning. The reactions yield colored organic species which do absorb solar radiation. We also suggest that the products of such reactions may be humic-like substances whose presence in continental aerosol has been confirmed but their source mechanisms are still much sought after. We also attempt to give a first order estimate of the enhancement of water absorption at a visible wavelength under atmospheric conditions.     

Meso-gamma scale forecasts using the nonhydrostatic model LM

Summary ?The nonhydrostatic model LM was developed for small scale operational predictions. Advances in computer development will give the possibility of operational models of a rather fine scale, which will cover the meso-gamma scale. The LM is currently applied at a scale of 7 km and an increase of the operational resolution to 2.5 km is planned for the next few years. Predictions of such high resolution require to abandon the hydrostatic assumption, which is used with most current operational weather prediction models. The LM was designed to cover all resolutions from 50 m to 50 km with an efficiency making it suitable for operational use. It is a fully elastic model, using second order centred finite differences. The time integration is done using the Klemp–Wilhelmson method, treating the slow modes by a larger time step than the fast modes. The vertical propagation of the fast waves is done implicitly. After describing the design of the LM, this paper gives examples of model predictions at the meso-γ scale. Some results of the current operational application at the resolution 7 km are presented. Deficiencies in the localisation of model generated precipitation are investigated using an idealised bell shaped mountain and applying different resolutions. In this way the convergence to the correct solution can be investigated. From these results it is concluded, that orographic filtering is necessary and the effect of such filtering on precipitation forecasts is investigated. Finally, the prediction of a squall line over northern Germany is shown in order to demonstrate the potential of the model in forecasting the meso-γ scale. Received May 15, 2001; revised September 21, 2001     

Description and evaluation of the bergen climate model: ARPEGE coupled with MICOM

A new coupled atmosphere–ocean–sea ice model has been developed, named the Bergen Climate Model (BCM). It consists of the atmospheric model ARPEGE/IFS, together with a global version of the ocean model MICOM including a dynamic–thermodynamic sea ice model. The coupling between the two models uses the OASIS software package. The new model concept is described, and results from a 300-year control integration is evaluated against observational data. In BCM, both the atmosphere and the ocean components use grids which can be irregular and have non-matching coastlines. Much effort has been put into the development of optimal interpolation schemes between the models, in particular the non-trivial problem of flux conservation in the coastal areas. A flux adjustment technique has been applied to the heat and fresh-water fluxes. There is, however, a weak drift in global mean sea-surface temperature (SST) and sea-surface salinity (SSS) of respectively 0.1 °C and 0.02 psu per century. The model gives a realistic simulation of the radiation balance at the top-of-the-atmosphere, and the net surface fluxes of longwave, shortwave, and turbulent heat fluxes are within observed values. Both global and total zonal means of cloud cover and precipitation are fairly close to observations, and errors are mainly related to the strength and positioning of the Hadley cell. The mean sea-level pressure (SLP) is well simulated, and both the mean state and the interannual standard deviation show realistic features. The SST field is several degrees too cold in the equatorial upwelling area in the Pacific, and about 1 °C too warm along the eastern margins of the oceans, and in the polar regions. The deviation from Levitus salinity is typically 0.1 psu – 0.4 psu, with a tendency for positive anomalies in the Northern Hemisphere, and negative in the Southern Hemisphere. The sea-ice distribution is realistic, but with too thin ice in the Arctic Ocean and too small ice coverage in the Southern Ocean. These model deficiencies have a strong influence on the surface air temperatures in these regions. Horizontal oceanic mass transports are in the lower range of those observed. The strength of the meridional overturning in the Atlantic is 18 Sv. An analysis of the large-scale variability in the model climate reveals realistic El Niño – Southern Oscillation (ENSO) and North Atlantic–Arctic Oscillation (NAO/AO) characteristics in the SLP and surface temperatures, including spatial patterns, frequencies, and strength. While the NAO/AO spectrum is white in SLP and red in temperature, the ENSO spectrum shows an energy maximum near 3 years.