bluuebunny/crossref_metadata_2025 huggingface dataset:
The dynamics of heat lows
Authors: Rácz Zsuzsanna , Smith Roger K.
Published: February 2007

A numerical model is used to investigate dynamical aspects of the structure and evolution of a heat low in an idealized flow configuration with an area of land surrounded by sea. of particular interest is the evolution of the distributions of relative vorticity and potential vorticity. While the heat low has a minimum surface pressure in the late afternoon following strong solar heating of the land, the relative vorticity is strongest in the early morning hours following a prolonged period of low‐level convergence. Thus the heat low is not approximately in quasi‐geostrophic balance. the low‐level convergence is associated with the sea‐breeze and later with the nocturnal low‐level jet. the effects of differing sea area, land area and Coriolis parameter on various aspects of the heat low are investigated.

Although a cyclonic vortex, the heat low is characterized by an anticyclonic potential‐vorticity anomaly relative to its environment throughout much of the lower troposphere on account of the greatly reduced static stability in the convectively well‐mixed boundary layer; however, the surface temperature maximum over land corresponds with a cyclonic potential‐vorticity anomaly at the surface. the reduced static stability in the mixed layer has the further consequence that the horizontal components of relative vorticity and horizontal potential‐temperature gradient make a non‐negligible contribution and of opposite sign to the potential vorticity in certain flow regions.

Two processes associated with the flow evolution in the model appear to be fundamental to understanding a range of low‐level atmospheric phenomena over the arid interior of Australia: these are the deep convective mixing over land during the daytime and the development of a nocturnal low‐level jet, which leads to convergence in the trough. Such phenomena include the diurnal behaviour of dry cold fronts and the generation of nocturnal wind surges and bores. It is reasonable to assume that similar processes operate in other arid regions of the world where deep convective mixing over land produces local maxima of diabatic heating in the lower atmosphere.
Observations of stratocumulus cloud break‐up over land
Authors: Price Jeremy D.
Published: February 2007

Detailed measurements were made during three episodes of boundary‐layer stratocumulus clouds occumng at Cardington, Bedfordshire, UK, using the UK Meteorological Oftice tethered‐balloon system. no cases were of cloud which dispersed or partly dispersed during the day, and the other was of persistent cloud. Analysis of these data reveals that one cloud deck dispersed owing to solar heating of the boundary layer warming the cloud above its dew‐point, and the other dispersed because of a combination of the above and entrainment of drier and warmer air into the top of the cloud caused by strong wind shear. This was the only incidence where entrainment was seen to affect significantly the morphology of a cloud deck. Data for the persistent case suggest that insufficient solar radiation penetrated into the boundary layer to cause significant warming and that the dynamics of the cloud were governed by radiative cooling at cloud top. the differences in the initial morphology of the cloud decks that caused them to evolve differently are discussed.
Simulation of fog with the ECMWF prognostic cloud scheme
Authors: Teixeira João
Published: February 2007

A prognostic cloud scheme has been operational at the European Centre for Medium‐Range Weather Forecasts (ECMWF) since 1995. In this paper the performance of the cloud scheme in simulating fog is assessed. A case‐study is performed with the one‐column version of the ECMWF model in order to analyse how the model reproduces the main mechanisms of fog generation and dissipation. A climatology of the model's fog is produced using data from the ECMWF Re‐Analysis and compared with climatological data. High resolution (T213L31) operational forecasts with the prognostic cloud scheme are compared with synoptic reports of visibility for Europe. In this context the relation between fog and temperature, on the one hand, and fog and wind speed, on the other, is explored in some detail.

The comparison between the simulated and the observed visibility, in the one‐column model case‐study, shows that the evolution of fog is properly simulated. the analysis of the behaviour of the different parametrized physical processes suggests that the subtle balance between the various processes, fundamental for a realistic fog simulation, is achieved. This study also shows that in radiation fog a cooling process like radiation, that only changes the temperature without changing the moisture, is rather essential in order to reach saturation. the comparison between the model's fog climatology and the climatological data shows that the model is able to reproduce most of the major fog areas, particularly over the ocean.

The analysis of the results from high‐resolution fog forecasts for Europe and the comparison with synoptic observations of visibility shows that the model simulates realistically some of the fog over Europe, particularly at 00 and 06 UTC. At 12 UTC the fog in the model is usually underestimated. In general, the number of situations where the visibility is between 2 km and 11 km is underestimated by the model. It is shown that the model is always slightly closer to saturation than the observations and this might be helping the model in producing fog. the model reproduces the observed behaviour of fog as a function of temperature and wind speed rather realistically.
Construction of correlation functions in two and three dimensions
Authors: Gaspari Gregory , Cohn Stephen E.
Published: February 2007

This article focuses on the construction, directly in physical space, of simply parametrized covariance functions for data‐assimilation applications. A self‐contained, rigorous mathematical summary of relevant topics from correlation theory is provided as a foundation for this construction. Covariance and correlation functions are defined, and common notions of homogeneity and isotropy are clarified. Classical results are stated, and proven where instructive. Included are smoothness properties relevant to multivariate statistical‐analysis algorithms where wind/wind and wind/mass correlation models are obtained by differentiating the correlation model of a mass variable. the Convolution Theorem is introduced as the primary tool used to construct classes of covariance and cross‐covariance functions on three‐dimensional Euclidean space R3. Among these are classes of compactly supported functions that restrict to covariance and cross‐covariance functions on the unit sphere S2, and that vanish identically on subsets of positive measure on S2. It is shown that these covariance and cross‐covariance functions on S2, referred to as being space‐limited, cannot be obtained using truncated spectral expansions. Compactly supported and space‐limited covariance functions determine sparse covariance matrices when evaluated on a grid, thereby easing computational burdens in atmospheric data‐analysis algorithms.

Convolution integrals leading to practical examples of compactly supported covariance and cross‐covariance functions on R3 are reduced and evaluated. More specifically, suppose that gi and gj are radially symmetric functions defined on R3 such that gi(x) = 0 for |x| > di and gj(x) = 0 for |xv > dj, O < di,dj ≦, where |. | denotes Euclidean distance in R3. the parameters di and dj are ‘cut‐off’ distances. Closed‐form expressions are determined for classes of convolution cross‐covariance functions Cij(x,y) := (gi * gj)(x‐y), i ≠ j, and convolution covariance functions Cii(x,y) := (gi * gi)(x‐y), vanishing for |x ‐ y| > di + dj and |x ‐ y| > 2di, respectively, Additional covariance functions on R3 are constructed using convolutions over the real numbers R, rather than R3. Families of compactly supported approximants to standard second‐ and third‐order autoregressive functions are constructed as illustrative examples. Compactly supported covariance functions of the form C(x,y) := Co(|x ‐ y|), x,y ∈ R3, where the functions Co(r) for r ∈ R are 5th‐order piecewise rational functions, are also constructed. These functions are used to develop space‐limited product covariance functions B(x, y) C(x, y), x, y ∈ S2, approximating given covariance functions B(x, y) supported on all of S2 × S2.
Lightning activity as an indicator of climate change
Authors: Reeve N. , Toumi R.
Published: February 2007

Data from the Optical Transient Detector lightning sensor are analysed to investigate the hypothesis that global lightning activity will increase should the average global temperature increase. It is shown that changes in global monthly land lightning activity are well correlated with changes in global monthly land wet‐bulb temperatures. the correlation is strongest in the northern hemisphere and weak in the southern hemisphere. the conclusion is that a high land‐area to sea‐area ratio is necessary for a good correlation. Contrary to expectation, the tropics show no correlation. the results predict that a change in the average land wet‐bulb temperature of the globe of just 1K would result in a change in lightning activity of about 40%.
An economical second‐order advection scheme for numerical weather prediction
Authors: Malguzzi P. , Tartagione N.
Published: February 2007

A very simple second‐order Eulerian scheme for the advection equation, based on a forward (backward) time integration on even (odd) grid points, is studied. the proposed scheme is similar, but not equivalent, to the so‐called ‘hopscotch method’, developed in the 1960s and early 1970s for the advection‐diffusion equation, and is stable up to Courant number 2.0. It is shown that, in the case of the advection equation, the proposed scheme has the same advantages yielded by the forward‐backward scheme in the case of the shallow‐water equations; in particular, it is equivalent to the application of centred time and space differencing on the Eliassen grid. the new scheme, unlike the classical leapfrog scheme, can be coupled to the forward‐backward integration of the gravity‐wave problem in primitive‐equation models.

With the aid of the proposed scheme, an explicit version of the atmospheric, Bologna Limited‐Area Model (developed in recent years at the FISBAT Institute of the National Council of Research of Italy) is devised, and a comparison with the semi‐implicit version of the same model is performed. the explicit version runs with a double time step, achieving the same accuracy with significantly less computer time and storage. It is suggested that the new time scheme is particularly suitable for numerical weather prediction on massively parallel computing machines based on SIMD (Single Instruction Multiple Data) and distributed memory architecture, which strongly penalize non‐local algorithms.
A cloud‐detection scheme for use with satellite sounding radiances in the context of data assimilation for numerical weather prediction
Authors: English S. J. , Eyre J. R. , Smith J. A.
Published: February 2007

A scheme for detecting cloud‐affected radiances is described. the method is used to determine the probability of cloud‐free conditions given the observations and the prior knowledge we have about the atmosphere from a numerical weather prediction (NWP) model. This is achieved using a likelihood method. It combines the strengths of some alternative methods (e.g. comparison of infra‐red and microwave channels sounding the lower troposphere and comparison of infra‐red window channels with sea surface temperature) in a powerful and flexible method. It is powerful because it uses different types of information simultaneously. It is flexible because it makes no assumption about which instrument is being processed, or what type of prior information (NWP, climatology etc.) is used. Therefore, it can readily be extended to new situations and data types (e.g. Advanced TIROS Operational Vertical Sounder (ATOVS)). It is suitable for use on general cloud‐detection problems, using combined microwave and infra‐red data. It has been tested using TIROS Operational Vertical Sounder (TOVS) radiances. the new method has been compared with an alternative cloud‐detection method tailored specifically for TOVS and has been developed to a level of robustness adequate for operational use. the new method gave very similar results to the alternative method, especially over the ocean. the differences that did occur have been investigated by comparing with cloud information derived from the Advanced Very High Resolution Radiometer (AVHRR). Both the alternative method and the new scheme were found to have deficiencies when dealing with very low cloud. Some cloud missed by the existing scheme is identified by the new scheme. Over land, cloud detection is more difficult. the two schemes disagree more often, but validation using AVHRR is also more difficult because of increased surface heterogeneity and more variable emissivity and surface temperature errors. the new method is therefore shown to perform at least as well as an alternative method in operational use, whilst gaining the flexibility required for future systems. the implications for ATOVS are discussed.
Impact of a simplified physical package in 4D‐var analyses of fastex situations
Authors: Janisková M. , Veersé F. , Thépaut J.‐N. , Desroziers G. , Pouponneau B.
Published: February 2007

In the context of the FASTEX (Fronts and Atlantic Storm‐Track EXperiment), this paper presents a number of tests and validations of the four‐dimensional variational (4D‐Var) data assimilation system under development at Météo‐France, over the Intensive Observation Periods IOP3 and IOP17, in particular the impact of adding a simplified physical package within the incremental approach. the impact of using a 6‐hour 4D‐Var system (instead of a standard optimum interpolation or a 3D‐Var scheme) is positive for the description and the forecast of Atlantic cyclones. Moreover, despite the lack of tuning and remaining deficiencies in the parametrization schemes. the analysis and prediction of Atlantic cyclones seem better handled with a 4D‐Var system including physical processes in the assimilating model.
Analysis and model dependencies in medium‐range ensembles: Two transplant case‐studies
Authors: Harrison M. S. J. , Palmer T. N. , Richardson D. S. , Buizza R.
Published: February 2007

There are numerous occasions on which operational forecasts from different meteorological centres diverge in the medium range. These differing evolutions may be due to the different analyses used by the centres or may be a result of the different models used to produce the forecasts; or a combination of both factors may be involved. Ensemble forecasts in routine production at the European Centre for Medium‐Range Weather Forecasts (ECMWF) are designed to address the problem of the impact of small analysis differences on forecast evolution. the initial perturbations, generated using the singular‐vector technique, are designed to maximise spread within the ensemble while being consistent with possible analysis errors. the aim of the current work is to examine, in the context of ensemble forecasting, the effects of model and analysis differences on forecast evolution. Two detailed case‐studies have been performed using the ECMWF model at T63 resolution and the UK Meteorological Office (UKMO) Unified Model run at a comparable resolution. In each case four 33‐member ensembles were produced using all possible combinations of ECMWF and UKMO models and analyses. the same initial perturbations, derived using the singular‐vector technique with the ECMWF model, were used for all four ensemble sets.

Significant differences between all four ensemble sets are found in each case‐study. the relative impact of model and analysis dependencies varies with geographical region and forecast range; however the greater overall impact is, for both region and range, from the model differences. the varying evolutions of the four ensemble sets are shown to have a substantial impact on synoptic features in the predicted 500 hPa height fields, such that a forecaster would have issued different predictions depending on which ensemble data were available. the synoptic differences are consistent with the statistically significant model and analysis dependencies found using analysis of variance applied to anomaly correlations and to the distribution of ensemble members in phase space. It is demonstrated that the calculation of the initial perturbations is relatively insensitive to differences between the initial analyses of the two centres, but that the dispersion of initial conditions resulting from the 16 singular vectors used to generate the perturbations does not cover the dispersion arising from the analysis fields. In both of the case‐studies each of the four ensemble sets contains information not available in any of the other ensembles. These results raise the possibility that, to maximise the information available to the forecaster, both models and both analyses should be used in the generation of ensemble predictions.
Overview of the field phase of the fronts and Atlantic Storm‐Track EXperiment (FASTEX) project
Authors: Joly Alain , Browning Keith A. , Bessemoulin Pierre , Cammas Jean‐Pierre , Caniaux Guy , Chalon Jean‐Pierre , Clough Sidney A. , Dirks Richard , Emanuel Kerry A. , Eymard Laurence , Gall Robert , Hewson Tim D. , Hildebrand Peter H. , Jorgensen Dave , Lalaurette François , Langland Rolf H. , Lemaǐtre Yvon , Mascart Patrick , Moore James A. , Persson P. Olag. , Roux Frank , Shapiro Melvyn A. , Snyder Chris , Toth Zoltan , Wakimoto Roger M.
Published: February 2007

The field phase of the Fronts and Atlantic Storm‐Track Experiment (FASTEX) project took place between 5 January and 27 February 1997 with the deployment of a unique set of observing facilities across the North Atlantic. the major objective was to document the life cycle of a representative set of mid‐latitude cyclones. Other objectives were to test the practical feasibility of ‘adaptive’ observations with a view to improving the prediction of these same cyclones and to document the internal structure of the associated cloud systems using combined airborne Doppler radars and dropsondes. Another goal of FASTEX was to measure air‐sea exchange parameters under conditions of strong winds with high seas.

These objectives were successfully achieved. Intensive Observation Periods were conducted on 19 occasions. High‐resolution vertical profiles through the same cyclones at three different stages of their life cycle have been obtained on more than 10 occasions. the calculation of critical areas where further observations were needed to limit the growth of forecast error, was undertaken using different techniques, and flights were planned and executed in these areas in time to achieve this. Combined dropsonde and Doppler radar observations of cloud systems are available for 10 cases. A unique air‐sea turbulent exchange dataset has been obtained.
Forecast skill of the ECMWF model using targeted observations during FASTEX
Authors: Montani A. , Thorpe A. J. , Buizza R. , Undén P.
Published: February 2007

The impact of targeted observations on forecast accuracy is investigated for five case‐studies of cyclogenesis during the Fronts and Atlantic Storm‐Track EXperiment (FASTEX). Calculations of localized singular vectors (SVs) have been made using the European Centre for Medium‐Range Weather Forecasts (ECMWF) forecasting model to find the sensitive regions for each case‐study. Two sets of analyses have been prepared: a ‘control’ set consisting of analyses in which all the FASTEX data have been removed from the operational analysis, and a ‘perturbed’ set obtained by adding to the control the data from the dropsondes deployed in regions highlighted as sensitive by SVs. Analysis and forecast differences have been compared with the structure of the SVs as they evolve during the forecast. Model integrations, starting from the two different analyses, have been performed. the impact of the dropsonde data on the forecast skill has been assessed on the basis of both subjective and objective scores. Re‐runs of the model from perturbed analyses almost always give better objective scores than those starting from the control analyses. the impact is more evident if the verification area coincides with the regions over which the SVs have been optimized. In this case, short‐range prediction errors (up to day 2) are reduced on average by 15%, with a maximum error reduction of about 37%.
Targeting observations with the quasi‐inverse linear and adjoint NCEP global models: Performance during FASTEX
Authors: Pu Zhao‐Xia , Kalnay Eugenia
Published: February 2007

This note gives a brief summary of the targeting results obtained with both adjoint and quasi‐inverse linear NCEP global spectral models during the Fronts and Atlantic Storm‐Track EXperiment (FASTEX). Questions concerning the feasibility of the methods and related issues about targeting observations are briefly discussed.
Dynamical Characterization of the FASTEX cyclogenesis cases
Authors: Baehr Christophe , Pouponneau Béatrice , Ayrault Franck , Joly Alain
Published: February 2007

A series of diagnostics based on the automatic tracking of cyclones is applied to the 25 life cycles captured during the field phase of the Fronts and Atlantic Storm‐Track Experiment (FASTEX). the tracking enables the various cases to be set into a common frame of reference that moves with the system. Time‐filtering is used to further separate the events into a basic‐state and perturbations. Information is obtained on environmental properties such as strain or baroclinicity.

Combining these diagnostics, it is shown that instability mechanisms based on the existence of potential vorticity strips or the control of these instabilities by deformation may be involved in the initial genesis of a number of cases, but do not seem to characterize specific types nor to determine a systematic behaviour.

On the other hand, it is shown that rapid‐deepening phases leading Eastern Atlantic systems to their maximum amplitude happen suddenly and at the same time as crossing the jet stream even in the absence of a clear additional upper‐level feature. For cold‐air systems, development results from meeting a diffluent zone. This is unlike the American east‐coast systems that are associated with jet exit/entrance complex.

Another remarkable feature is the generation of a frontal wave as a result of the parent vorticity maximum being apparently stretched and split. This has occurred in 25% of cases.

A number of other properties of the FASTEX cases are shown, such as several occurrences of large phase changes in sharp contrast to the idea of phase locking.
Surface fluxes in the North Atlantic current during CATCH/FASTEX
Authors: Eymard L. , Caniaux G. , Dupuis H. , Prieur L. , Giordani H. , Troadec R. , Bessemoulin P. , Lachaud G. , Bouhours G. , Bourras D. , Guérin C. , Le Borgne P. , Brisson A. , Marsouin A.
Published: February 2007

CATCH (Couplage avec l'Atmosphère en Conditions Hivernales) was the oceanic component of FASTEX (Fronts and Atlantic Storm‐Track Experiment). It took place in January and February 1997, in the Newfoundland Basin near 47°N, 40°W, a region characterized by the presence of the warm North Atlantic Current and cold surrounding waters. CATCH was devoted to the study of the parametrization of surface turbulent fluxes in strong winds and changing directions, the surface‐flux variability related to the passage of atmospheric fronts and the influence on fluxes of the strong sea surface temperature gradients associated with the North Atlantic Current. This paper presents first results of ship data analysis. A large range of wind and stratification conditions were experienced: 5% of measured winds were higher than 20 m s‐1; 30% of unstable stratification (air‐sea temperature differences lower than −5 °C) and 30% of very dry conditions (air‐sea moisture differences lower than −2.5 g kg−1) were sampled. Surface turbulent heat and momentum fluxes were obtained using the inertial‐dissipative method from which a bulk algorithm was derived. A significant increase of latent‐heat and momentum‐transfer coefficients with increasing wind is obtained. This parametrization is compared to others published using the CATCH dataset. For high winds and unstable stratifications, differences between schemes reach 200 W m−2 for latent‐heat flux values of 600 W m−2. Radiative and turbulent ship‐measured fluxes are compared with modelled fluxes from the European Centre for Medium‐Range Forecasts (ECMWF) along the ship's trajectory: each component of the net heat budget is higher in the ECMWF model, consequently the heat loss of the ocean is 35% higher in the model. Finally, the effect of sea surface temperature fronts on surface turbulent fluxes is analysed by evaluating the contribution of the various terms in the flux variations, showing a significant impact of the surface temperature change in all unperturbed cases.
The evolution of the tropical western Pacific atmosphere‐ocean system following the arrival of a dry intrusion
Authors: Parsons David B. , Redelsperger Jean‐Luc , Yoneyama Kunio
Published: February 2007

Recent studies using TOGA COARE data have found that extremely dry air from middle‐latitude waves frequently intrudes into the equatorial troposphere over the western Pacific. Using sounding data taken during the COARE, the magnitude of the advection of water vapour for one event is calculated, and it is estimated that the lime for the atmosphere to recover to moist conditions was ∼ 10‐20 days. From the magnitude of the drying and from the frequency of these events, it is proposed that dry intrusions must be a major contributor to the tropospheric moisture budget over the region during the COARE, making it difficult for the atmosphere to reach a radiative‐convective equilibrium, intrusions, instead, can help to recharge the tropical atmosphere by decreasing convective activity and, thus, driving the atmosphere toward unusually large values of convective available potential energy. A variety of atmospheric and oceanic measurements are also used to study the recovery process in detail. A conceptual model is proposed based on this work and previous investigations. As in past studies, the recovery of the atmosphere to moist conditions is accomplished through detrainment from convective clouds that began to form soon after the arrival of the dry air mass and slowly deepen in height as the recovery progresses. Previous investigators concluded that the entrainment of dry air into convective ceils is generally the factor that tends to suppress convective activity and limits the height of any convection that does develop under these adverse conditions. The idea that entrainment limits convective activity is consistent with the commonly held perception that the western Pacific is a region where there is little inhibition to deep convection and, when inhibition does occur, it can be removed by surface fluxes within hours. In contrast, it is found that convective inhibition can be large enough to suppress convection following dry intrusions, and that the diurnal variation in rainfall is due partly to modulations in convective inhibition. The modulations in convective inhibition are, in turn, caused by diurnal variations in the vertical profiles of radiation, in surface fluxes, and perhaps in large‐scale subsidence, leading to a minimum in convective inhibition during the late afternoon. In contrast, studies of this type of convection have generally emphasized diurnal variations in the surface fluxes, and often ignored convective inhibition and diurnal variations in atmospheric radiative heating.
Variational cloud‐clearing with TOVS data
Authors: Joiner J. , Rokke L.
Published: February 2007

A number of studies have shown that the use of passive microwave and infrared satellite observations in data assimilation systems can increase forecast skill. Considerable effort has been expended over the past two decades, particularly with the TIROS Operational Vertical Sounder (TOVS), to achieve this result. The positive impact on forecast skill is a result of more rigorous treatment of quality control, improvements in systematic error correction schemes, and advances in data assimilation systems. Yet, there Still remains potential for improving the use of satellite data, particularly cloud‐contaminated observations, in data assimilation. Here, we use a one‐dimensional variational framework (1DVAR) as a first step towards improving the treatment of cloudy data by cloud‐clearing. Cloud‐clearing is a procedure that removes cloud radiative effects through comparison of partly cloudy adjacent pixels. The JDVAR approach simultaneously extracts cloud‐clearing parameters and information about the atmospheric and surface state from microwave and infrared observations. The variational framework ensures that the state estimate is consistent with all available measurements. The 1DVAR cloud‐clearing approach can also be extended to three or four dimensions (3DVAR, 4DVAR). Our TOVS cloud‐clearing implementation allows for complex cloud structures, including multiple cloud layers with wavelength‐dependent radiative properties. We present preliminary results of our 1DVAR cloud‐clearing implementation with TOVS. The results suggest that there is useful information in the cloud‐cleared data.
Propagation mechanisms for the Madden‐Julian Oscillation
Authors: Matthews Adrian J.
Published: February 2007

The Madden‐Julian Oscillation (MJO) is examined using 20‐years of outgoing long‐wave radiation and National Centers for Environmental Prediction/National Center for Atmospheric Research re‐analysis data. Two mechanisms for the eastward propagation and regeneration of the convective anomalies are suggested.

The first is a local mechanism operating over the warm‐pool region. At the phase of the MJO with a dipole structure to the convection anomalies, there is enhanced tropical convection over the eastern Indian Ocean and reduced convection over the western Pacific. Over the equatorial western Indian Ocean, the equatorial Rossby wave response to the west of the enhanced convection includes a region of anomalous surface divergence associated with the anomalous surface westerlies and pressure ridge. This lends to suppress ascent in the boundary layer and shuts off the deep convection, eventually leading to a convective anomaly of the opposite sign. Over the Indonesian sector, the equatorial Kelvin wave response to the east of the enhanced convection includes a region of anomalous surface convergence into the anomalous equatorial surface easterlies and pressure trough, which will tend to favour convection in this region. The Indonesian sector is also influenced by an equatorial Rossby wave response (of opposite sign) to the west of die reduced convection over the western Pacific, which also has a region of anomalous surface convergence associated with its anomalous equatorial surface easterlies and pressure trough. Hence, convective anomalies of either sign tend to erode themselves from the west and initiate a convective anomaly of opposite sign via their equatorial Rossby wave response, and expand to the east via their equatorial Kelvin wave response.

The second mechanism is global, involving an anomaly completing a circuit of the equator. Enhanced convection over the tropical western Pacific excites a negative mean‐sea‐level pressure (m.s.l.p.) anomaly which radiates rapidly eastward as a dry equatorial Kelvin wave at approximately 35 m s−1 over the eastern Pacific. It is blocked by the orographic barrier of the Andes and Central America for several days before propagating through the gap at Panama. After rapidly propagating as a dry equatorial Kelvin wave over the Atlantic, the m.s.l.p. anomaly is delayed further by the East African Highlands before it reaches the Indian Ocean and coincides with the development of enhanced convection at the start of the next MJO cycle.
Ice particle habits in stratiform clouds
Authors: Korolev A. , Isaac G. A. , Hallett J.
Published: February 2007

Ice crystals in clouds in the atmosphere have shapes that relate to their density, terminal fall velocity, growth rate and radiative properties. In calculations for climate‐change predictions, forecasting of precipitation, and remote‐sensing retrievals, idealized crystal shapes such as columns, needles, plates and dendrites are often assumed. The objective of this work is to study the frequency of occurrence of different habits of ice particles in natural clouds from aircraft observations. Images of cloud particles were measured by a PMS Optical Array Probe‐2DC at 25 μm resolution installed on the National Research Council Convair‐580. The processing of particle images was conducted with a newly developed algorithm for pattern recognition. Data were collected during four field projects in the Canadian and US Arctic over the North Atlantic near Newfoundland, and over the Great Lakes. Approximately 5 × 106 images of cloud particles having a size larger than 125 μm were analysed. The cloud particles were classified into four categories; spheres, irregulars, needles/columns and dendrites. The habit classification of particles was done for three different size ranges: > 125 μm, >250 μm. and >500 μm. The frequency of occurrence of different habits was found for each 5 degC temperature interval in the range −45°C < T < 0°C. It was concluded that the majority of ice particles in natural clouds were of irregular shape. The frequency of occurrence of irregular ice decreases with increasing particle size. On average, the concentration of particles larger than 125 μm was approximately constant down to −35°C, whereas the concentration of particles larger than 500 μm decreased at temperatures below −15°C. Since the data were collected in different climatic zones within many cloud lypes. and covered a significant cloud path length (3.6 × 104 km), the conclusions are applicable to most stratiform clouds containing ice.
Upper‐level barotropic growth as a precursor to cyclogenesis during FASTEX
Authors: Kucharski Fred , Thorpe Alan J.
Published: February 2007

During the Fronts and Atlantic Storm‐Track Experiment (FASTEX) two large‐scale cyclones (in intensive observation periods 11 and 17) developed in the North Atlantic. These can be classified, in their initial phase, as Petterssen Type B cyclones. Initiation of the cyclones follows transient upper‐level barotropic growth, caused by a pre‐existing upper‐level trough approaching a jet associated with a strong baroclinic zone. The cyclones later developed further due to baroclinic energy conversion. A favourable orientation of the trough relative to the baroclinic zone ('horizontal tilt against the shear) is crucial for this transient barotropic growth. This scenario is therefore proposed as a general dynamical mechanism leading to Type B cyclogenesis.
Accounting for overlap of fractional cloud in infrared radiation
Authors: Li J.
Published: February 2007

The cloud‐matrix method for describing the mutual cloud‐coverage relationship between any two levels is systematically discussed. A general method is devised for calculating the effective cloud emissivity for maximum‐random overlap clouds. For several cloud configurations with extreme variation in fractional cloud amounts, the errors are generally very small (<5%). The radiative‐transfer process that corresponds to the random‐overlap cloud scheme is discussed. Compared with the purely random clouds scheme, the maximum‐random overlap scheme always produces a smaller cooling rate in the lower layers of a cloud block and a smaller downward flux. The difference in cooling rate can be about 3 K d−1 and the difference in the downward flux near the surface can be as large as 20 W m−2. The calculations show that the scheme of effective cloud emissivity commonly used in general‐circulation models could cause underestimation of cloud cooling rate. The clear‐sky and the cloudy‐sky radiation field can be obtained through a single calculation process but with different water‐vapour profiles. The results show that for the all‐sky case the separate treatment of the water‐vapour profile for clear and cloudy portions makes only a very small difference in the cooling rate and upward flux at the top of the atmosphere in comparison with the results of an averaged water‐vapour profile.
Atmosphere‐ocean thermal coupling in the North Atlantic: A positive feedback
Authors: Watanabe M. , Kimoto M.
Published: February 2007

In this article, it is examined whether a positive atmosphere‐ocean feedback is found over the North Atlantic through a series of general circulation model (GCM) experiments.

A 60‐year integration of an atmospheric GCM (AGCM) coupled with a 50 m deep motionless ocean was conducted. The coupled run was compared with two uncoupled runs, one with prescribed mid‐latitude climato‐logical sea surface temperatures (SSTs) and the other with daily SSTs derived from the coupled experiment. The uncoupled atmosphere forces the slab ocean, to obtain SST responses in these runs. Consistent with previous studies, the coupled atmosphere‐ocean fields show interannual variances larger than those in the uncoupled runs, due to reduction in the local thermal damping.

Patterns of maximum atmosphere‐ocean covariability show the North Atlantic Oscillation (NAO) and tripole SST anomalies both in the coupled and uncoupled fields, indicating a dominant role of the atmosphere in generating the SST anomalies. On the other hand, analyses of the temporal variability in the three runs suggest an active role of SST anomalies in determining the polarity of the air‐sea coupled variability longer than the interannual time‐scale. A combined analysis of forcing SST, upper‐air height, and the response SST anomalies in the uncoupled run identified that a patch of positive SST anomalies in the mid‐latitude band around 40°N effectively excite the positive phase of the NAO, which in turn reinforces the tripole SST anomalies. This relationship was further confirmed by a nine‐member ensemble AGCM experiment forced by the relevant SST anomalies. Since the forcing and response SST anomaly patterns bear some resemblance, these results manifest positive feedback at work in the coupled atmosphere‐ocean patterns. The processes responsible for this positive feedback were elaborated by a series of linear model experiments. The local thermal adjustment of the atmosphere to the SST anomalies results in increased precipitation over the Gulf Stream region. Associated diabatic heating applied to a linear baroclinic model yields a positive height response to the east, which highly influences the southern part of the NAO. This stationary response in turn induces a northward deflection in the storm track activity, leading to an eddy vorticity feedback that tends to force the positive phase of the NAO.
Exploring natural and anthropogenic variation of climate
Authors: Manabe S. , Knutson T. R. , Stouffer R. J. , Delworth T. L.
Published: February 2007

This lecture discusses the low‐frequency variability of surface temperature using a coupled ocean‐atmosphere‐land‐surface model developed at the Geophysical Fluid Dynamics Laboratory/NOAA. Despite the highly idealized parametrization of various physical processes, the coupled model simulates reasonably well the variability of local and global mean surface temperature. The first half of the lecture explores the basic physical mechanisms responsible for the variability. The second half examines the trends of local surface temperature during the last half century in the context of decadal variability simulated by the coupled model.
Sensitivity of tropical storms simulated by a general circulation model to changes in cumulus parametrization
Authors: Vitart F. , Anderson J. L. , Sirutis J. , Tuleya R. E.
Published: February 2007

A number of recent studies have examined the statistics of tropical storms simulated by general circulation models (GCMs) forced by observed sea surface temperatures. Many GCMs have demonstrated an ability to simulate some aspects of the observed interannual variability of tropical storms, in particular, variability in storm frequency. This has led to nascent attempts to use GCMs as part of programs to produce operational seasonal forecasts of tropical‐storm numbers.

In this study, the sensitivity of the statistics of GCM‐simulated tropical storms to changes in the model's physical parametrizations is examined. After preliminary results indicated that these statistics were most sensitive to details of the convective parametrization, GCM simulations with identical dynamical cores but different convective parametrizations were created. The parametrizations examined included moist convective adjustment, two variants of the Arakawa‐Schubert scheme, and several variants of the relaxed Arakawa‐Schubert (RAS) scheme; the impact of including a shallow‐convection parametrization was also examined.

The simulated tropical‐storm frequency, intensity, structure, and interannual variability were all found to exhibit significant sensitivities to changes in convective parametrization. A particularly large sensitivity was found when the RAS and Arakawa‐Schubert parametrizations were modified to place restrictions on the production of deep convection.

Climatologies of the GCM tropical atmosphere and composites of tropical storms were examined to address the question of whether the tropical‐storm statistics were directly impacted on by changes in convection associated with tropical storms, or if they were indirectly affected by parametrization‐induced changes in the tropical mean atmosphere. A number of results point to the latter being the primary cause. A regional hurricane model, initialized with mean states from the GCM simulation climatologies, is used to further investigate this point. Particularly compelling is the fact that versions of the RAS scheme that produce significantly less realistic simulations of tropical storms nevertheless produce a much more realistic interannual variability of storms, apparently due to an improved tropical mean climate.

A careful analysis of the background convective available potential energy (CAPE) is used to suggest that this quantity is particularly relevant to the occurrence of tropical storms in the low‐resolution GCMs, although this may not be the case with observations. If the tropical CAPE is too low, tropical storms in the low‐resolution GCMs cannot form with realistic frequency.
Boundary‐layer convection and diurnal variation of vertical‐velocity characteristics in the free troposphere
Authors: Muschinski Andreas , Chilson Phillip B. , Hooper David A. , Palmer Robert D. , Schmidt Gerhard , Steinhagen Hans
Published: February 2007

During the field experiment SOMARE‐99 (Sounding System Multifrequency Atmospheric Radar Experiment 1999), the troposphere above the Harz Mountains in Northern Germany was observed with the SOUSY VHF radar, with four microbarographs, and with 26 radiosondes launched every 3 h from nearby the radar site. This paper examines the 72 h period between 0000 UTC 25 May and 0000 UTC 28 May. Time series, root‐mean‐square values, and power spectra of radar‐observed vertical‐wind, w, fluctuations in the free troposphere and of pressure, p, fluctuations at the surface are presented and discussed. A pronounced diurnal cycle was observed: standard deviations (from 1 h periods) were about 10 cm s−1 (w) and 2 Pa (p) at night and up to 60 cm s−1 and 6 Pa during daytime. This is seen as evidence for the relevance of boundary‐layer convection for short‐period gravity waves in the free troposphere. Frequency spectra of w show a power‐law drop‐off beyond the Brunt‐Väisälä frequency with logarithmic slopes close to ‐5/3 at moderate and strong wind speeds and down to ‐3 or ‐4 at weak winds (smaller than 5 m s−1), supporting earlier work on the effects of Doppler shift on Eulerian gravity‐wave spectra. The w spectra, which have been calculated from 8 h long w time series (sampling period 1 min), have a noise floor of 10−3 m2s−2Hz−1 or less. This corresponds to an uncorrelated noise with a standard deviation of 3 mm s−1 for the 1 min samples. The concept of gravity‐wave intermittency is introduced, in analogy to a well‐established concept in turbulence physics. It is demonstrated how this concept can be used to distinguish turbulence from gravity waves empirically.
Hierarchical modelling of tropical convective systems using explicit and parametrized approaches
Authors: Liu Changhai , Moncrieff Mitchell W. , Grabowski Wojciech W.
Published: February 2007

Cloud systems observed during 1–7 September of GATE are examined in a hierarchical approach, namely: a two‐dimensional cloud‐resolving simulation using a 2 km grid length; two‐ and three‐dimensional simulations using the Kain‐Fritsch convective parametrization and 10, 15 and 25 km grid lengths; and coarse‐grid simulations without any convective parametrization. All simulations are forced by the same objectively analysed time‐varying large‐scale advection of temperature and moisture. The domain‐mean winds are relaxed to the observed wind profiles.

Both the cloud‐resolving modelling and the lower‐resolution modelling with parametrized convection realize the three observed cloud system categories (squall line, non‐squall cluster and scattered convection) and transitions among them. In particular, the well‐organized fast‐moving squall‐type cloud system observed on 4 September is realized in a three‐dimensional experiment with parametrized convection. In contrast, the lower‐resolution modelling without any convective parametrization fails to produce the squall‐type convective system during the weakly forced period but successfully represents the non‐squall cluster during strong forcing. This lack of success is mostly attributed to convective triggering and the absence of resolved downdraught‐enhanced surface fluxes. These issues are not as critical during strong large‐scale forcing.

The observed evolutions of temperature, water vapour mixing ratio, precipitation and surface moisture flux are realized in all simulations. A common deficiency is the overprediction of upper‐level relative humidity. The simulation with parametrized convection features a comparatively large low‐level water vapour mixing ratio, a surface and upper‐level cold temperature bias and a mid‐tropospheric warm bias. This is mainly attributed to deficiencies in how the Kain‐Fritsch scheme represents convective mass flux, detrainment and entrainment by cumulus congestus.