Intercomparison of the seasonal cycle of tropical surface stress in 17 AMIP atmospheric general circulation models

 The mean state of the tropical atmosphere is important as the nature of the coupling between the ocean and the atmosphere depends nonlinearly on the basic state of the coupled system. The simulation of the annual cycle of the tropical surface wind stress by 17 atmospheric general circulation models (AGCMs) is examined and intercompared. The models considered were part of the Atmospheric Model Intercomparison Project (AMIP) and were integrated with observed sea surface temperature (SST) for the decade 1979–1988. Several measures have been devised to intercompare the performance of the 17 models on global tropical as well as regional scales. Within the limits of observational uncertainties, the models under examination simulate realistic tropical area-averaged zonal and meridional annual mean stresses. This is a noteworthy improvement over older generation low resolution models which were noted for their simulation of surface stresses considerably weaker than the observations. The models also simulate realistic magnitudes of the spatial distribution of the annual mean surface stress field and are seen to reproduce realistically its observed spatial pattern. Similar features are observed in the simulations of the annual variance field. The models perform well over almost all the tropical regions apart from a few. Of these, the simulations over Somali are interesting. Over this region, the models are seen to underestimate the annual mean zonal and meridional stresses. There is also wide variance between the different models in simulating these quantities. Large model-to-model variations were also seen in the simulations of the annual mean meridional stress field over equatorial Indian Ocean, south central Pacific, north east Pacific and equatorial eastern Pacific oceans. It is shown that the systematic errors in simulating the surface winds are related to the systematic errors in simulating the Inter-Tropical Convergence Zone (ITCZ) in its location and intensity. Weaker than observed annual mean southwesterlies simulated by most models over Somali is due to weaker than observed southwesterlies during the Northern Hemisphere summer. This is related to the weaker than observed land precipitation simulated by most models during the Northern Hemisphere summer. The diversity in simulation of the surface wind over Somali and equatorial Indian ocean is related to the diversity of AGCMs in simulating the precipitation zones in these regions. Received: 2 August 1996 / Accepted: 7 February 1997     

Radon variations in a Hungarian village

 A steady radon exhalation is assumed in most publications. In a village of North-East Hungary, however, high radon concentrations have been measured, differing strongly in neighbouring houses and varying in time, due to the interplay of geochemical phenomena. Received: 20 November 1995 · Accepted: 18 June 1996     

Solar radio burst (type III)

This article describes the observations of a type III radio burst observed at 103 MHz simultaneously by the two radio telescopes situated at Rajkot (22.3°N, 70.7°E) and Thaltej (23°N, 72.4°E). This event occurred on September 30, 1993 at about 0430 UT and lasted for only half a minute. The event consisted of several sharp spikes in a group. The rise and fall time of these are comparable, however the peaks of individual spikes varied by a factor of four. The comparison of these observations with the data of solar radio spectrograph HiRAS indicates that this was a metric radio burst giving highest emission at about 103 MHz.     

Three-dimensional coronal structures using clark lake observations

We have undertaken a study of coronal features observed at meter-decameter wavelengths using the Clark Lake radioheliograph. Among the coronal structures we have studied are the radio manifestations of coronal streamers on the solar disk and above the solar limb. We have analyzed the radio data quantitatively, using ray-tracing models for comparison with the maps. Our study provides information about the streamers' three-dimensional shapes, scales, and density profiles, for comparison with related observations using white-light coronagraphs.     

Signal processing strategies for a bathymetric sidescan sonar

A dominant source of errors in swath bathymetry is acoustic interference. In 1989 the author published an analysis of these errors and predicted depth accuracies for a system which reduced their effect by averaging. This present paper shows how a considerable improvement in performance may be obtained by a variety of signal processing strategies that include the use of several widely spaced receivers and the elimination of the most unsatisfactory measurements before averaging. Simulations show how impressive sea bed profiles can be produced with a single ping, even at low signal-to-interference ratios     

Spine–sheath polarization structures in four active galactic nuclei jets

We present the results of multifrequency (15 + 8 + 5 GHz) polarization Very Long Baseline Array (VLBA) observations of the three BL Lacertae objects 0745+241, 1418+546 and 1652+398 together with 5-GHz VLBI Space Observatory Programme (VSOP) observations of 1418+546 and 1.6- and 5-GHz VSOP observations of the blazar 1055+018. The jets of all these sources have polarization structure transverse to the jet axis, with the polarization E vectors aligned with the jet along the jet spine and 'sheaths' of orthogonal E vectors at one or both edges of the jet. The presence of polarization aligned with the jet near the 'spine' may indicate that the jets are associated with helical B fields that propogate outward with the jet flow; the presence of orthogonal polarization near the edges of the jet may likewise be a consequence of a helical jet B field, or may be owing to an interaction with the ambient medium on parsec scales. We have tentatively detected interknot polarization in 1055+018 with E aligned with the local jet direction, consistent with the possibility that the jet of this source is associated with a helical B field.     

Stationary distribution function for non-adiabatic particle motion in a one dimensional current sheet

The problem of pitch angle scattering in field configurations similar to those found in the geomagnetic tail has been studied previously by Tsyganenko (1982). Tsyganenko used a scattering matrix to map pitch angle distributions through the current sheet. By using numerical solutions of the resulting integral equations he showed for weakly non-adiabatic particles the Stationary Distribution Function (SDF) was isotropic. Using his procedure the SDF was found to develop anisotropies with increasing non-adiabaticity. The work presented here shows analytically that for any degree of scattering the SDF must be isotropic for a general planar field reversal. Computations of particle trajectories have been used to verify some aspects of the analytic work.     

A 20‐ka climate record from Central Himalayan loess deposits

The southwest monsoon that dominated Central Himalaya has preserved loessic silt deposits preserved in patches that are proximal to periglacial areas. The occurrence of such silts suggests contemporary prevalence of cold and dry northwesterly winds. Field stratigraphy, geochemistry, mineral magnetism, infrared stimulated luminescence (IRSL) and radiocarbon dating has enabled reconstruction of an event chronology during the past 20 ka. Three events of loess accretion could be identified. The first two events of loess deposition occurred betweem 20 and 9 ka and were separated by a phase of moderate weathering. Pedogenesis at the end of this event gave rise to a well‐developed soil that was bracketed around 9 to > 4 ka. This was followed by the third phase of loess accretion that occurred around 4 to > 1 ka. Episodes of loess deposition and soil formation are interpreted in terms of changes in the strength of the Indian southwest monsoon. Copyright © 2005 John Wiley & Sons, Ltd.     

VLA observations of extreme hydrogen deficient stars

VLA observations at 2 and 6 cm have been obtained for six hydrogen-deficient stars R CrB, HD 160641, BD — 9°4395, V348 Sgr, MV Sgr and Sgrv Upper limits to the massloss rates have been estimated for some of these using the upper limits to the radio flux density. National Radio Astronomy Observatory’s Very Large Array is operated by Associated Universities Inc. under contract with National Science Foundation, USA