A Study on the Impact of Observation Assimilation on the Numerical Simulation of Tropical Cyclones JAL and THANE Using 3DVAR

In this work, the impact of assimilation of conventional and satellite remote sensing observations (Oceansat-2 winds, MODIS temperature/humidity profiles) is studied on the simulation of two tropical cyclones in the Bay of Bengal region of the Indian Ocean using a three-dimensional variational data assimilation (3DVAR) technique. The Weather Research and Forecasting (WRF)-Advanced Research WRF (ARW) mesoscale model is used to simulate the severe cyclone JAL: 5–8 November 2010 and the very severe cyclone THANE: 27–30 December 2011 with a double nested domain configuration and with a horizontal resolution of 27 × 9 km. Five numerical experiments are conducted for each cyclone. In the control run (CTL) the National Centers for Environmental Prediction global forecast system analysis and forecasts available at 50 km resolution were used for the initial and boundary conditions. In the second (VARAWS), third (VARSCAT), fourth (VARMODIS) and fifth (VARALL) experiments, the conventional surface observations, Oceansat-2 ocean surface wind vectors, temperature and humidity profiles of MODIS, and all observations were respectively used for assimilation. Results indicate meager impact with surface observations, and relatively higher impact with scatterometer wind data in the case of the JAL cyclone, and with MODIS temperature and humidity profiles in the case of THANE for the simulation of intensity and track parameters. These relative impacts are related to the area coverage of scatterometer winds and MODIS profiles in the respective storms, and are confirmed by the overall better results obtained with assimilation of all observations in both the cases. The improvements in track prediction are mainly contributed by the assimilation of scatterometer wind vector data, which reduced errors in the initial position and size of the cyclone vortices. The errors are reduced by 25, 21, 38 % in vector track position, and by 57, 36, 39 % in intensity, at 24, 48, 72 h predictions, respectively, for the two cases using assimilation of all observations. Simulated rainfall estimates indicate that while the assimilation of scatterometer wind data improves the location of the rainfall, the assimilation of MODIS profiles produces a realistic pattern and amount of rainfall, close to the observational estimates.     

Updraft properties deduced statistically from Rawin soundings

Between 1966 and 1973, National Severe Storms Laboratory mesonetwork soundings were taken near thunderstorms and squall lines. Thirty-four of these soundings sampled updrafts at various levels. Because of instrumental noise and atmospheric fine structure, many of the individual soundings were hard to interpret. However, significant results were obtained from the data as a whole and from the four fastest updraft cases. Typically, the updrafts were associated with moderate thunderstorms that formed in baroclinic regions with maritime tropical air near the surface overlaid by drier air aloft. The immediate environments were potentially, conditionally, and latently unstable but were stable with respect to parcel displacement from the lifted condensation level. The updrafts were generally located in regions of strong radar reflectivity gradient at 0° antenna tilt. The updraft soundings often showed pseudo-adiabatic lapse rates immediately above the updraft condensation level. Statistical analysis of the data confirms that updrafts are warm core at mid- and upper levels and that they are relatively cool at low levels (below 700 mb). Important effects of perturbation pressure gradient forces are emphasized. We also find that local updraft speed is positively correlated at the 95% confidence level with local potential wet-bulb temperature between 0.9 and 9.3 km MSL. The average vertical velocity is a maximum at a relatively low height (4.8 km MSL); this agrees with other experiments that involve tracking objects released below cloud base. However, flight train icing and other extraneous factors may be influencing this result. The average horizontal winds in the updraft and environment agree with recent conceptual Great Plains thunderstorm models.     

Scenario prediction of changes in water balance components of the Olenek and Indigirka rivers in the context of possible climate change in the region of the Republic of Sakha (Yakutia)

Forecasts of the dynamics of meteorological characteristics in the basins of the Olenek and Indigirka rivers (the Republic of Sakha) in the XXI century have been obtained for four IPCC global climate scenarios of SRES family, corresponding to specified scenarios of economic, technological, political, and demographic development of human civilization. The forecasts have been used to calculate scenarios of possible changes in water balance components for the basins under consideration up to the year of 2063. The calculation procedure involves a physically-based model of heat and mass exchange between the land surface and the atmosphere SWAP and climate scenario generator MAGICC/SCENGEN.     

Seismic crustal study of the Oslo Rift

A seismic refraction investigation across the southern part of the Oslo Rift has been made, based on quarry blasts at three localities. The study shows a three-layered crust with the followingP-wave velocities: . the upper mantleP-wave celocity, is 8.07 km/s. The velocity-depth relationship for the uppermost crust, obtained by solving the Wiechert-Herglotz integral equation numerically, shows a continuously decreasing velocity gradient in the region of the Oslo Rift which approaches zero at a depth of 9 km, the corresponding increase in theP-wave velocity being from 5.55 km/s to 6.34 km/s. The interface separating the subsurface layer ( =6.60 km/s) from the uppermost layer , interpreted as the Conrad discontinuity, is essentially horizontal in the investigated part of the Oslo Rift at a depth of approximately 15 km. A deep crustal layer with aP-wave velocity of 7.10 km/s appears to be related to the rift, though the top of this layer extends somewhat eastwards beneath the Precambrian rocks from the southern part of the rift at a depth of approximately 20 km. The Moho discontinuity is elevated beneath the Oslo Region compared with the surrounding area. A broad regional gravity high of about 45 mgal is observed along the entire rift zone. It is suggested that this anomaly is caused by the elevation of the sub-Conrad and Moho discontinuities during the rifting processes.     

Model studies on electro-telluric interpretation problems of some geological inhomogeneities

Summary The rotating nature of telluric field is simulated in an electrolytic model tank. Variations of this rotating field due to the presence of a few two and three dimensional geological models, simulated in the tank, are studied. The telluric ellipses, recorded on a C.R.O. Screen, are found to be useful for qualitative and quantitative interpretation of the non-linearly polarised telluric field data. Relative usefulnesses of the various parameters, e.g., (i)M=ratio of the semimajor and semiminor axis, (ii)K = square root of normalized surface areas, (iii) = linear eccentricity ((a ²/b ²-1) and (iv)e = conventional eccentricity ((1-b ²/a ²)), of a telluric ellipse are tested. Linear eccentricity appears to be the most sensitive parameter. Methods for determining depth and horizontal extent of a structure from a telluric map are suggested. Near surface inhomogeneities, as revealed from the experiment are unlikely to affect the telluric map due to a basement structure.     

Assessment of the statistical significance of global changes in the annual river runoff in XXI century due to possible anthropogenic warming of climate

A number of uncertainties of forecasts of changes in the annual runoff depths at global scale, obtained using information on results of integration of 21 IPCC climate models is studied. Following possible errors of these forecasts are calculated: errors of models; differences between main (IPCC) scenarios of emission of greenhouse gases in the atmosphere and resultant changes of global temperatures; mistakes in estimates of average long-term observed values of the runoff depths for the “control” period. Global maps of a “significance index” of forecasted changes in the runoff depths (estimations of changes in the annual runoff depths divided by mean square root values of errors of these estimations) for 2025, 2050, 2075 and 2100 are presented. It is shown that the most significant global changes of the runoff depths (growth in the north of Eastern Siberia, of the Russian Far East, of North America, falling in the “Greater Mediterranean Region”) are predicted for the second quarter of 21st century. Further changes of the runoff amplify only in the Amazon basin (reduction, by 2075). Almost everywhere else (including almost all European territory of Russia, Western Siberia, south of Eastern Siberia and of the Far East) the significance of changes in the runoff depths during 21st century is negligible.     

Impact of Land Surface Processes on the South Asian Monsoon Simulations Using WRF Modeling System

The Weather Research and Forecasting model has been used to examine the role of land surface processes on Indian summer monsoon simulations. Isolated experiments have been carried out with physical parameterization schemes (land surface and planetary boundary layer) and data assimilation to examine their relative roles in the representation of regional hydroclimate in model simulations. The impact of vegetation green fraction on the model simulations has been extensively studied by replacing the default United States Geological Survey (USGS) vegetation cover data with that of Indian Space Research Organisation (ISRO) data. Results indicate that differences in the treatment of surface processes in the model lead to large differences in precipitation simulation over the Indian domain. Several hydroclimate parameters from the simulations using ISRO and USGS vegetation green fractions were examined. It is seen that the role of vegetation green fraction in these experiments has been to increase latent heat flux to the atmosphere. Two sets of data assimilation experiments were also carried out for an entire year using the same set of observed data but with different land surface parameterization schemes. It is found that evenwhen using the same observed data, the differences in land surface schemes reduce the impact and contribution of observed data being assimilated into the model. The hydroclimate over the region becomes a function of the land surface scheme. This study highlights the importance of vegetation green fraction and land surface schemes in the context of the regional hydroclimate over South Asia.     

Crustal Architecture of the Inverted Central Lapland Rift Along the HUKKA 2007 Profile

We have studied the lateral velocity variations along a partly buried inverted paleo–rift in Central Lapland, Northern Europe with a 2D wide-angle reflection and refraction experiment, HUKKA 2007. The experiment was designed to use seven chemical explosions from commercial and military sites as sources of seismic energy. The shots were recorded by 102 stations with an average spacing of 3.45 km. Two-dimensional crustal models of variations in P-wave velocity and Vp/Vs-ratio were calculated using the ray tracing forward modeling technique. The HUKKA 2007 experiment comprises a 455 km long profile that runs NNW–SSE parallel to the Kittilä Shear Zone, a major deformation zone hosting gold deposits in the area. The profile crosses Paleoproterozoic and reactivated Archean terranes of Central Lapland. The velocity model shows a significant difference in crustal velocity structure between the northern (distances 0–120 km) and southern parts of the profile. The difference in P-wave velocities and Vp/Vs ratio can be followed through the whole crust down to the Moho boundary indicating major tectonic boundaries. Upper crustal velocities seem to vary with the terranes/compositional differences mapped at the surface. The lower layer of the upper crust displays velocities of 6.0–6.1 km/s. Both Paleoproterozoic and Archean terranes are associated with high velocity bodies (6.30–6.35 km/s) at 100 and 200–350 km distances. The Central Lapland greenstone belt and Central Lapland Granitoid complex are associated with a 4 km-thick zone of unusually low velocities (<6.0 km/s) at distances between 120 and 220 km. We interpret the HUKKA 2007 profile to image an old, partly buried, inverted continental rift zone that has been closed and modified by younger tectonic events. It has structural features typical of rifts: inward dipping rift shoulders, undulating thickness of the middle crust, high velocity lower crust and a rather uniform crustal thickness of 48 km.     

Ranges of AGW propagation in the Earth’s atmosphere

The propagation of atmospheric gravity waves (AGWs) is studied in the context of geometrical optics in the nonisothermal, viscous, and thermal-conductive atmosphere of Earth in the presence of wind shifts. Parametric diagrams are plotted, determining the regions of allowed frequencies and horizontal phase velocities of AGWs depending on the altitude. It is shown that a part of the spectrum of AGWs propagates in stationary air in an altitude range from the Earth’s surface through the ionospheric F1 layer. AGW from nearearth sources attenuate below 250 km, while waves generated at altitudes of about 300 km and higher do not reach the Earth’s surface because of the inner reflection from the thermosphere base. The pattern changes under strong thermospheric winds. AGW dissipation decreases with an adverse wind shift and, hence, a part of the wave spectrum penetrated from the lower atmosphere to the altitudes of F2 layer.     

Effect of remote clouds on surface UV irradiance

Clouds affect local surface UV irradiance, even if the horizontal distance from the radiation observation site amounts to several kilometers. In order to investigate this effect, which we call remote clouds effect, a 3-dimensional radiative transfer model is applied. Assuming the atmosphere is subdivided into a quadratic based sector and its surrounding, we quantify the influence of changing cloud coverage within this surrounding from 0% to 100% on surface UV irradiance at the sector center. To work out this remote clouds influence as a function of sector base size, we made some calculations for different sizes between 10 km × 10 km and 100 km × 100 km. It appears that in the case of small sectors (base size 20 km × 20 km) the remote clouds effect is highly variable: Depending on cloud structure, solar zenith angle and wavelength, the surface UV irradiance may be enhanced up to 15% as well as reduced by more than 50%. In contrast, for larger sectors it is always the case that enhancements become smaller by 5% if sector base size exceeds 60 km × 60 km. However, these values are upper estimates of the remote cloud effects and they are found only for special cloud structures. Since these structures might occur but cannot be regarded as typical, different satellite observed cloud formations (horizontal resolution about 1 km × 1 km) have also been investigated. For these more common cloud distributions we find remote cloud effects to be distinctly smaller than the corresponding upper estimates, e.g., for a sector with base size of 25 km × 25 km the surface UV irradiance error due to ignoring the actual remote clouds and replacing their influence with periodic horizontal boundary conditions is less than 3%, whereas the upper estimate of remote clouds effect would suggest an error close to 10%.     

Geomagnetic activity and the North Atlantic Oscillation

The North Atlantic Oscillation (NAO) is the prominent pattern of winter climate variability that has a strong effect on weather in the North Atlantic region and the adjacent continents. At present, uncertainty prevails as to the mechanisms controlling the variability of the NAO. It is also difficult to explain why the positive phase of the NAO has prevailed over the past 37 years (1972–2008). We found high positive correlation coefficients between geomagnetic activity (used as a measure of solar wind intensity) and the NAO indices that equal 0.76 for 1962–1994 and 0.63 for 1961–2011. Positive correlations of the distribution of surface air temperature with the NAO and similarly with geomagnetic activity occur in the Northern Hemisphere. These results encourage our search for possible causes controlling the NAO. We have found that at times of high geomagnetic activity the NAO index is positive and magnetic reconnection may enable the solar wind to initiate downward winds in the magnetosphere. Wind anomalies originate at the edge of the stratospheric polar vortex and propagate downward through the troposphere taking part in the intensification of the vortex and of the westerlies. Stronger northerly winds over Greenland carry cold air southward and, together with the enhanced westerlies, advect the warm air from the Atlantic along the deep Icelandic low into Eurasia increasing temperatures there. On the other hand, at times of low geomagnetic activity, the NAO index is negative and the stratospheric polar vortex is weak. Warm air from the subtropics is carried into the Arctic and a rapid amplification of planetary waves propagating upward may cause displacement or even splitting of the weak vortex and sudden stratospheric warming. During this negative NAO phase the weakened westerlies allow more cold air to build up over North America and Eurasia.     

Global transport and localized layering of metallic ions in the upper atmosphere

A numerical model has been developed which is capable of simulating all phases of the life cycle of metallic ions, and results are described and interpreted herein for the typical case of Fe⁺ ions. This cycle begins with the initial deposition of metallics through meteor ablation and sputtering, followed by conversion of neutral Fe atoms to ions through photoionization and charge exchange with ambient ions. Global transport arising from daytime electric fields and poleward/downward diffusion along geomagnetic field lines, localized transport and layer formation through descending convergent nulls in the thermospheric wind field, and finally annihilation by chemical neutralization and compound formation are treated. The model thus sheds new light on the interdependencies of the physical and chemical processes affecting atmospheric metallics. Model output analysis confirms the dominant role of both global and local transport to the ions life cycle, showing that upward forcing from the equatorial electric field is critical to global movement, and that diurnal and semidiurnal tidal winds are responsible for the formation of dense ion layers in the 90–250 km height region. It is demonstrated that the assumed combination of sources, chemical sinks, and transport mechanisms actually produces F-region densities and E-region layer densities similar to those observed. The model also shows that zonal and meridional winds and electric fields each play distinct roles in local transport, whereas the ion distribution is relatively insensitive to reasonable variations in meteoric deposition and chemical reaction rates.     

Die Beziehung zwischen dem Luftdruck in 8 km Höhe und der Dichte in 11 Km Höhe

Zusammenfassung Oberhalb der Nullschicht addieren sich Druck und Temperatur hinsichtlich ihrer Wirkung auf die Dichte. Es existiert in etwa 11 km Höhe eine Horizontalfläche maximaler Dichteabnahme vom Hoch zum Tief. Der Luftdruck in der Schicht maximalen Horizontalgradienten des Drucks in etwa 8 km ist ein Maß für die Intensität eines Druckgebildes. Die Stärke der Korrelation zwischen dem Druck in 8 km und der Dichte in 11 km ist somit ein maß für die Stärke der Nullschichtvorgänge.

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Summary The large scale vertical motions in high and low pressure systems are, in the mean, reversed in the null layer, connected with the upper-tropospheric wind maximum layer (about 10 km altitude). Above the null layer pressure and temperature influence the air density in the same sense; below it their influence upon the air density is opposite. In an altitude of 11 km (middle latitudes) there exists a horizontal surface with a maximum of density-decrease from high to low pressure areas. Typical for the intensity of a pressure system is the pressure in the layer of maximum horizontal pressure gradient in an altitude of 8 km. The strength of the correlation between pressure in 8 km and density in 11 km is proportional to the strength of the processes in the null layer (Fig. 5a-d). The non-gradientic mass flow to high pressure areas in the null layer is great in high latitudes (high Coriolis parameter) and in winter (high meridional temperature gradient). In a low in formation more potential energy is available in winter, which can be transformed in kinetic energy of the winds in the null layer.

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Diese Arbeit wurde unterstützt vom US-Department of the Army, European Research Office, unter Kontrakt Nr. DA-91-591-EUC 2847.     

Liparitic volcanism of some regional volcanic structures in Kazakhstan,Middle Asia and Caucasus

Liparitic volcanism is a typical feature of the orogenic phase giving rise to the Kazakhstan, Middle Asia and Caucasus folded systems. The main characteristics of the liparitic volcanism common to these three regions are the following:

1. Geo-structural zonation of the volcanic structures.
2. Dismembered Moho surface within the volcanic structures.
3. Synchronous, yet independent evolution of liparitic and andesitic volcanisms.
4. Ignimbritic character of the liparitic volcanism.
5. Lateral petrochemical zonation with some features common to the liparitic and andesitic rock series.

Geo-structural and petrochemical zonations are likely governed in the regions studied by a deep-seated plutonic body.     

Wind-induced subduction at the South Atlantic subtropical front

The South Atlantic Subtropical Front, associated with the eastward-flowing South Atlantic Current, separates the colder, nutrient-rich waters of the subpolar gyre from the warmer, nutrient-poor waters of the subtropical gyre. Perturbations to the quasi-geostrophic, eastward flow generate meanders and filaments which induce cross-frontal exchange of water properties. Down-front winds transport denser waters from the South over warm waters from the North, inducing convective instability and subduction. Such processes occur over spatial scales of the order of 1 km and thus require high horizontal spatial resolution. In this modeling study, a high-resolution (4 km) regional grid is embedded in a basin-wide configuration (12 km) of the South Atlantic Ocean in order to test the importance of submesoscale processes in water mass subduction along the subtropical front. Stronger and more numerous eddies obtained in the high-resolution run yield more intense zonal jets along the frontal zone. Such stronger jets are more susceptible to instabilities, frontogenesis, and the generation of submesoscale meanders and filaments with \(\mathcal {O}(1)\) Rossby number. As a consequence, vertical velocities larger than 100 md ¹ are obtained in the high-resolution run, one order of magnitude larger than in the low-resolution run. Wind-driven subduction occurs along the frontal region, associated with negative Ertel potential vorticity in the surface layer. Such processes are not observed in the low-resolution run. A passive tracer experiment shows that waters with density characteristics similar to subtropical mode waters are preferentially subducted along the frontal region. The wind-driven buoyancy flux is shown to be much larger than thermal or haline fluxes during the wintertime, which highlights the importance of the frictional component in extracting PV from the surface ocean and inducing subduction, a process that has been overlooked in subtropical mode water formation in the region.     

The gravity field and crustal thickness of Venus

The gravity and topography of Venus obtained from observations of the Magellan mission, as well as the gravity and topography from our numerical mantle convection model, are discussed in this paper. We used the hypothesis that the geoid of degrees 2–40 is produced by sublithospheric mantle density anomalies that are associated with dynamical process within the mantle. We obtained the model dynamical admittance(the geoid topography ratio based on a convection model) by a numerical simulation of the Venusian mantle convection, and used it to correct the dynamical effect in the calculation of crustal thickness. After deducting the dynamical effect, the thickness of the Venusian crust is presented. The results show that the gravity and topography are strongly correlated with the Venusian mantle convection and the Venusian crust has a significant influence on the topography. The Venusian crustal thickness varies from 28 to 70 km. Ishtar Terra, and Ovda Regio and Thetis Regio in western Aphrodite Terra have the highest crustal thickness(larger than 50 km). The high topography of these areas is thought to be supported by crustal compensation and our results are consistent with the hypothesis that these areas are remnants of ancient continents. The crustal thickness in the Beta, Themis, Dione, Eistla, Bell, and Lada regiones is thin and shows less correlation with the topography, especially in the Atla and Imdr regiones in the eastern part of Aphrodite Terra. This is consistent with the hypothesis that these highlands are mainly supported by mantle plumes. Compared with the crustal thickness calculated with the dynamical effect, our results are more consistent with the crust evolution and internal dynamical process of Venus.     

Refraction velocities of trap basalts

Refraction surveys conducted by the Oil & Natural Gas Commission in the Cambay Basin, Gujarat, Rajahmundry area, Andhra Pradesh and near Rajamahal Traps, West Bengal, have indicated longitudinal velocities of the order of 4.2 to 5.8 km/sec for the Traps. Measurements of velocities by Sonic log in wells drilled for oil in the Cambay basin yielded values, ranging from 4.8 to 5.1 km/sec. While Sonic log velocities compare well with field refraction measurements, published values for trap basalt determined in the laboratories in India by various workers indicate a consistently high value of 6.6–7.2 km/sec. The distinctly lower value obtained by refraction methods can be attributed to weathering effects, thickness of trap flows, unsuspected inter-trappeans, dispersion in layered rocks etc. The mean values for the Traps of the Cambay, Rajamundry and Rajmahal areas come out to 4.78, 4.10 and 4.81 km/sec respectively.     

Long-period planetary waves in the mesosphere and lower thermosphere above Davis, Antarctica

The accumulation of MF radar wind and hydroxyl temperature measurements at Davis from 1997 to 2005 has enabled the compilation of a climatology of long-period (period >1 day) wave activity. A time domain filtering technique that makes allowance for the differing sampling characteristics of the measurements is described and wave amplitudes are presented for 1.7–4, 4–8 and 8–16 day period bands. Product averages of the time series yield horizontal heat and momentum fluxes for the height of the hydroxyl layer (approximately 86 km). The climatology is then discussed in terms of current knowledge of planetary wave characteristics and forcing. Heat and momentum fluxes during the year of the southern hemisphere stratospheric warming (2002) are also presented.