bluuebunny/crossref_metadata_2025_split huggingface dataset:
On the interaction of tropical‐cyclone‐scale vortices. III: Continuous barotropic vortices
Authors: Holland Greg J. , Dietachmayer Gary S. 
Published: February 2007

AbstractThe interaction of cyclonic vortices in spherical geometry is investigated using a shallow‐water model on an adaptive grid. It is found that the mutual approach and merger of binary cyclones support the compound vortex‐patch findings of Part II. As interaction commences, distortion of the weak outer vorticity fields leads to a change in the advecting flow over each vortex core, which leads to mutual approach or retreat depending on the shape of the vorticity fields. Raped merger occurs when the cyclone cores approach within the critical separation distance defined in Part II.The core merger of cyclones seems to the largely independent of environment and the rotation of the earth. The initial approach can be quite sensitive, however. Non‐merging binary cyclones sweep equatorial parcels of air over long trajectories towards the pole to create exceptionally strong anticyclonic gyres. These can be of sufficient strength to capture the nearest cyclone and induce the escape mode described in Part I. Some sensitivity to latitude (and hence strength of the earth‐vorticity gradient), therefore, is found. A large anticyclone in a subtropical ridge location is found to be sheared and torn apart by the interacting cyclones, with little effect on merger.The observations in Part I that interactions of three cyclones could be broken down into separate binary interaction sequences is supported by experiments with three vortices. When two of the vortices merge, the interaction evolves to a small/large cyclone situation. When no merger occurs, each pair of cyclones proceeds through a distinctive orbit/escape cycle.
Analysis of the surface hydrology in a regional climate model
Authors: Giorgi Filippo , Hostetler Steven W. , Brodeur Christine Shields 
Published: February 2007

AbstractThis paper discusses the surface hydrology of a multi‐year simulation of present day climate over the United States (US) conducted with a regional climate model (RegCM) nested within a general circulation model (GCM). The RegCM, which is run with a 60 km gridpoint spacing is interactively coupled with a state‐of‐the‐art surface physics package that includes full surface hydrology calculations (the Biosphere‐Atmosphere Transfer Scheme or BATS). The hydrologic budgets of ten regional drainage basins in the US are analysed. Model results are compared with available observations and with results from previous modelling experiments to evaluate the feasibility of using nested RegCM/GCM models for hydrology studies. In our experiment, the model captures the basic seasonality of the basin hydrologic budgets, although the simulated precipitation amounts are too high over the western US and too low over the eastern US. As a result, runoff, snow cover and soil water content are underestimated over the eastern US basins, while evaporation and runoff are overestimated in some of the western US basins. Topographically induced characteristics of precipitation, snow cover and runoff are well simulated over the mountainous western regions. Also well captured is the inter‐basin variation of hydrologic budgets which occurs in response to different climatic settings. The springtime snowmelt and peak runoff season generally occurs in the model earlier in the year than is observed. Although our work indicates that the coupled regional modelling system can be useful in applications to hydrological studies, results from this experiment indicate that better accuracy in the simulation of regional climatic variables and more detailed representation of some hydrologic processes would be required before the coupled modelling system could be used to provide accurate assessments of hydrologic responses to climate change.
A wide cold‐frontal rainband and its relationship to frontal topography
Authors: Locatelli John D. , Martin Jonathan E. , Hobbs Peter V. 
Published: February 2007

AbstractA wide cold‐frontal rainband associated with a vigorous surface cold front was investigated using conventional meteorological data and the velocity and reflectivity data from high‐resolution Doppler radar. It was found that the movement of the rainband was closely related to the velocity of the frontal surface, and that the substructural features of the rainband corresponded closely with the topographical features of that surface. The evolution of the topography of the frontal surface is described, and some processes that might affect frontal topography, and therefore the structure and movement of rainbands, are examined.
Reply to comments by S. B. Feldstein and W. A. Robinson on ‘Spatial structure of ultra‐low frequency variability of the flow in a simple atmospheric circulation model’ (October 1992, 118, 1211–1233)
Authors: James I. N. , James P. M. 
Published: February 2007

AbstractThe note by Feldstein and Robinson (hereafter referred to as FR) raises two questions. The first is whether the EOF1 of zonal wind in the numerical integrations that we carried out is correlated with the atmospheric angular velocity or with the mean angular momentum. The second is whether the ultra‐low‐frequency fluctuations which we observed have any dynamical significance or are merely ‘climate noise’. We reply to each of these issues in turn, and are grateful to FR for raising some interesting questions about our results.
Water vapour continuum absorption in mid‐latitudes: Aircraft measurements and model comparisons
Authors: Rudman S. D. , Saunders R. W. , Kilsby C. G. , Minnett P. J. 
Published: February 2007

AbstractRecently reported measurements by the Meteorological Research Flight C‐130 aircraft over the tropical Atlantic have shown that existing models of infrared water vapour continuum absorption underestimate its magnitude by up to 30%. This paper presents additional results from more recent measurements made close to the south‐west approaches of the British Isles and over the eastern Mediterranean Sea. These measurements are compared with two models: GENLN2, a line‐by‐line model, and the widely used band model LOWTRAN7. The measured radiances suggest a consistent underestimate in both models of the water vapour continuum absorption. The modified parametrization for the water vapour continuum absorption, inferred from the tropical results, was also used here to check its validity for lower water vapour amounts and lower temperatures. As with the tropical results, increasing the self‐broadening coefficient by a constant factor, or increasing the temperature dependence, improves the agreement between the measurements and the model predictions.
The influence of the Indian Ocean/Indonesian SST gradient on the Australian winter rainfall and circulation in an atmospheric GCM
Authors: Frederiksen Carsten S. , Balgovind Ramesh C. 
Published: February 2007

AbstractA number of atmospheric general circulation experiments have been conducted with the aim of studying the impact of an enhanced sea surface temperature (SST) gradient, between the Indonesian archipelago and the central Indian Ocean, on the Australian winter circulation and rainfall. Particular emphasis is on modelled synoptic events and the sensitivity of mean winter climate anomalies to changes in the strength of the anomalous SST gradient.It is shown that an atmospheric general circulation model, forced by an anomalous SST gradient to produce mean winter circulation and precipitation anomalies qualitatively similar to the observed, simulates quite realistic synoptic events over the Australian continent. In particular, the model simulates an increased frequency of major Australian north‐west cloud bands. These events are associated with a triad of tropical systems in the Bay of Bengal, the Cocos Islands and the Indonesian archipelago, strong cross‐equatorial flow into a region north‐west of Australia, and interactions with short‐wave mid‐latitude disturbances.The mean climate anomalies, over Australia, are shown to be largely independent of whether a fixed or evolving anomalous gradient is used, but very sensitive to the magnitude of the SST anomaly. This is seen in the greater variability of the results over Australia. It is also suggested that the presence and strength of the warm anomaly in the Indonesian region has a greater bearing on the results than the cold anomaly, in the Indian Ocean. Global tropospheric circulation and precipitation anomaly patterns, associated with an enhanced gradient, are also shown to be consistent with some observed patterns, and to be robust with respect to changes in the magnitude and distribution of the SST anomalies. This suggests that an enhanced SST gradient may also be associated with rainfall variability in a number of geographical locations.
Radiative damping of equatorial waves in the middle atmosphere
Authors: Vijayan Lekshmi , Reddy C. A. 
Published: February 2007

AbstractTheoretical estimates are made of the radiative damping of various Kelvin and Rossby‐gravity (RG) wave modes in the 20–70 km region, for a period of 24 months (1979–80), using the observed monthly mean zonal‐wind profiles at 8.5°N latitude and realistic values of scale‐dependent cooling coefficients. The net changes in wave amplitudes at various height regions are calculated by combining the attenuation due to radiative cooling and the growth due to density decrease. The calculated height variations of wave amplitudes explain quite well several features of the observed height structures of Kelvin waves. They also explain the observed rapid decay of RG waves below about 35 km when their period exceeds 3 days.The calculated momentum flux convergences of various waves are used to estimate the mean‐flow accelerations generated by the damped waves. The Kelvin waves are found to be capable of generating the accelerations needed to drive the westerly phase of the quasi‐biennial oscillation (QBO) and the semi‐annual oscillation (SAO) in the zonal wind. However, RG waves seem to be capable of driving the easterly phase of the QBO but not the SAO. The remarkable consistency of our theoretical results and the observed patterns of wave amplitudes and mean‐flow accelerations demonstrate that radiative damping is the dominant dissipative process for the equatorial waves in the 20–70 km region and our theoretical approach is capable of characterizing the equatorial wave behaviour and the associated flow accelerations with a good quantitative accuracy.
Impact of localized tropical and extratropical SST anomalies in ensembles of seasonal GCM integrations
Authors: Ferranti L. , Molteni F. , Palmer T. N. 
Published: February 2007

AbstractA series of 120‐day ensemble integrations of a general circulation model, designed to assess the impact of geographically localized sea‐surface‐temperature (SST) anomalies in both the tropics and extratropics, are described. These experiments contribute firstly to an appraisal of the relative roles of tropical and extratropical SST anomalies on interannual variability of the large‐scale circulation in the northern extratropics, and secondly to an assessment of the role of quasi‐stationary diabatic‐heating anomalies on model systematic error, including blocking activity.Overall it is found that SST anomalies associated with El Niño and La Niña have a larger and more reproducible impact on the extratropics than the chosen extratropical SST anomalies. These extratropical anomalies were localized to the north‐west Pacific, and north‐west Atlantic, with realistic amplitude. However, unlike earlier studies, a response to the extratropical North Pacific SST anomalies has been obtained over the North Pacific which is correlated with the sign of the imposed SST anomaly. The response to extratropical SST anomalies in the north‐west Atlantic are similar to the results obtained from an earlier study. The downstream responses to the extratropical Pacific and Atlantic SST anomalies are qualitatively similar to one another. Overall it is concluded that the northern large‐scale flow is influenced by such extratropical SST anomalies.The response to idealized tropical SST anomalies was also studied. In particular, a localized anomaly over Indonesia had a very substantial impact on the Hadley circulation, on zonal flow, and on blocking frequency over the North Pacific and Europe. This response was such as to reduce model systematic error: locally in the vicinity of the SST anomaly, remotely around the tropics, and remotely in the extratropics. A similar, though weaker, impact on Euro‐Atlantic blocking was obtained with an idealized Caribbean SST anomaly.Further statistical and dynamical analyses suggested that the extratropical response to the Indonesian SST anomaly occurs through the creation of two distinct planetary‐scale regimes, in one of which the formation of blocks is much favoured by increased ridges on the north‐eastern side of the oceans.
Importance of microphysical processes in the dynamics of a CSI mesoscale frontal cloud band
Authors: Marecal Virginie , Lemaitre Yvon 
Published: February 2007

AbstractMicrophysical processes in a cloud band observed during the MFDP/FRONTS 87 experiment are studied. The importance of precipitation evaporation in the downdraught zone of the cloud‐band circulation, due to conditional symmetric instability (CSI), is clearly evidenced thanks to a two‐dimensional (2D) microphysical retrieval model applied to an observed 2D wind field. This study shows the crucial role of snow evaporation in maintaining the descent of the CSI circulation in a nearly saturated state. The evaporation creates a cooling which reduces the resistance of the atmosphere to subsiding motions, and thus reinforces the ascending ones. Comparisons with results on the effect of snow evaporation on mesoscale motions obtained by other authors with a one‐dimensional model clearly show the effect of the horizontal wind component.
Internal variability in a perpetual January integration of a troposphere‐stratosphere‐mesosphere GCM
Authors: Pawson Steven , Meyer Arnold , Leder Silke 
Published: February 2007

AbstractA 1200‐day perpetual January integration of the Berlin troposphere‐stratosphere‐mesosphere general circulation model (GCM) has been performed with constant boundary conditions. The long‐term mean climatology represents the response to the different forcing terms in the zonal‐mean momentum budget: the winter stratospheric mean climatology is determined by the balance between the mean meridional circulation (m.m.c.), acting via the Coriolis force, and the Eliassen–Palm flux divergence. The winter stratospheric mean state oscillates between periods of strong wind (undisturbed conditions) and weaker winds (following minor warmings) on periods of several hundred days; this is the response to transient eddy forcing from the troposphere which has an almost‐red power spectrum, with maximum power on time‐scales of tens of days. As previously found in observations, there is a near cancellation between the wave‐ and the m.m.c.‐induced forcing of the mean flow, but the acceleration is well correlated with the wave forcing. The dominant spatial modes of variability of the GCM are isolated using principal‐component analysis. The dominant orthogonal modes of wind and temperature extend from the winter into the summer hemisphere. They represent structures close to thermal‐wind balance, with a dipolar structure in the lower stratospheric temperature field associated with variations in the strength of the polar‐night jet and weaker wind anomalies in the summer subtropics (22°S, 5hPa). There is some evidence of long‐period fluctuations in the vertical velocity in the lower tropical stratosphere associated with the high‐latitude velocity variations; these are consistent with the equatorial temperature anomalies. Time‐delayed point correlations with the zonal velocity at 62°N, 1 hPa reveal that the weak anticorrelation of velocity in the northern subtropics (28°N, 1 hPa), which is weak at zero‐lag, increases with time to about 58% at 20‐days lag. Singular‐spectrum analysis of the zonal velocity at the three reference points, which isolates orthogonal modes of temporal variability, reveals that the correlations are increased when only the low‐frequency velocity variations are considered.
Dynamically consistent, quasi‐hydrostatic equations for global models with a complete representation of the Coriolis force
Authors: White A. A. , Bromley R. A. 
Published: February 2007

AbstractThe spherical polar components of the Coriolis force consist of terms in sin ϕ and terms in cos ϕ, where ϕ is latitude (referred to the frame‐rotation vector as polar axis). The cos ϕ Coriolis terms are not retained in the usual hydrostatic primitive equations of numerical weather prediction and climate simulation, their neglect being consistent with the shallow‐atmosphere approximation and the simultaneous exclusion of various small metric terms. Scale analysis for diabatically driven, synoptic‐scale motion in the tropics, and for planetary‐scale motion, suggests that the cos ϕ Coriolis terms may attain magnitudes of order 10% of those of key terms in the hydrostatic primitive equations. It is argued that the cos ϕ Coriolis terms should be included in global simulation models.A global, quasi‐hydrostatic model having a complete representation of the Coriolis force is proposed. Conservation of axial angular momentum and potential vorticity, as well as energy, is achieved by a formulation in which all metric terms are retained and the shallow‐atmosphere approximation is relaxed. Distance from the centre of the earth is replaced by a pseudo‐radius which is a function of pressure only. This model is put forward as a more accurate alternative to the traditional hydrostatic primitive equations; it preserves the desired conservation laws and may be integrated by broadly similar grid‐point methods.
Cisk: A theory for the response of tropical convective complexes to variations in sea surface temperature
Authors: McBride John L. , Fraedrich Klaus 
Published: February 2007

AbstractA set of balanced and unbalanced slab‐symmetric linear 2‐layer models is analysed. the stability properties depend on a parametrized moisture budget dominated by the value of the underlying sea surface temperature. Convective heating occurs within an inner region characterizing the size of the convective disturbance. the heating is equal to the sum of two terms, representing the mid‐level and lower‐level upward mass‐fluxes.A fast and a slow mode of instability can be distinguished. the slow mode shows finite maximum growth for infinitesimally small disturbances and is the instability traditionally associated with Conditional Instability of the Second Kind (CISK). the fast mode reveals infinitely large positive growth‐rates for finite‐size disturbances.The transition of the model solution from slow to fast mode occurs at a threshold value of sea surface temperature. For the parameters chosen in this study, the threshold occurs at approximately 25.5°C.
The Asian summer monsoon and ENSO
Authors: Ju Jianhua , Slingo Julia 
Published: February 2007

AbstractThe relationship between the evolution of the Asian summer monsoon and equatorial sea‐surface‐temperature anomalies has been studied using results from an integration with the UK Universities' Global Atmospheric Modelling Programme (UGAMP) General Circulation Model (UGCM). the integration was performed as part of the Atmospheric Model Intercomparison Project and thus used the observed sea surface temperatures (SSTs) for the decade January 1979 to December 1988. the mean evolution of the Asian summer monsoon has been successfully simulated in terms of many aspects of the rapid transition of the large‐scale circulation during the boreal spring and summer. However, the results for individual years showed considerable interannual variability, both in the strength of the monsoon and in the time of onset. A relationship has been identified between the evolution of the monsoon flow and the phase of the El Niño Southern Oscillation (ENSO). In agreement with observed results, years with warm SST anomalies in the equatorial central and east Pacific Ocean (El Niño) have a weaker monsoon circulation and a delayed onset. an opposite behaviour is noted for those years with cold Pacific SST anomalies (La Niña). Diagnostics from analyses from the National Meteorological Center and the European Centre for Medium‐range Weather Forecasts, and from data on the outgoing long‐wave radiation observed by the Advanced Very High Resolution Radiometer, have been used to verify the model results. A description of the mechanism by which the phase of ENSO remotely influences the dynamics of the Asian summer monsoon has been developed involving changes in the heating patterns over Indonesia and the west Pacific in the preceding spring.
Meso‐β‐scale circulations in realistic fronts. I: Steady basic state
Authors: Fischer Claude , Lalaurette François 
Published: February 2007

AbstractThis paper investigates the occurrence of conditional symmetric instability during MFDP/Fronts87IOP2. A saturated negative equivalent‐potential‐vorticity area is detected by the mesoscale analysis in the upper troposphere, and this is confirmed by dropsounding data. Two‐dimensional hydrostatic simulations are performed with the Météo‐France PERIDOT model which give rise to the onset of symmetric instability. A complete life cycle is obtained as well as the growth rate of the perturbation (about 2.5 × 10−5 s−1 ≈ (11 hours)−1). Particular emphasis is put on the meanings and results of the kinetic‐energy conversions. Most of the perturbation energy comes from the basic state through vertical momentum fluxes, which is consistent with previous theoretical or numerical works. the decay of the perturbation is also shown to be due to its own dynamics, the symmetric rolls advecting positive equivalent potential vorticity into the unstable region. Deformations of equivalent potential temperature (θe) surfaces and m‐surfaces give an insight into the parcel trajectories: the ascending parcels follow θe‐surfaces. leading to a buckling of the m‐surfaces, and the descending parcels follow θ‐surfaces. the small value of the growth rate is stressed as well as of the maximum intensity (w ≈ 5 cm s−1). Thus, interactions with the frontogenetical forcing are expected owing to the small growth rate obtained. This is the subject of Part II of this paper.