Showing papers in "Meteorology and Atmospheric Physics in 1999"

The Extreme Precipitation Event of 11 to 16 February 1996 over South Africa

Abstract: The extreme precipitation event of 11 to 16 February 1996 was one of four significant events during the 1995/96 rainfall season over southern Africa. Extensive flooding and related damage was recorded at this time, with historical records showing one of the highest flood peaks of the past century. This extreme event is analysed using a combination of mesoscale numerical modelling and Lagrangian trajectory analysis, allowing a comprehensive three-dimensional reconstruction of the associated atmospheric structure and its evolution. The adjustments in the circulation patterns as well as the timing and contribution of different moisture source regions are clearly important in influencing the duration and intensity of this extreme rainfall event over southern Africa. The moisture that contributed to precipitation during the event, as well as to the south-western part of the country, was imported mainly from the Indian Ocean tothe east and south-east, suggesting that the equatorial Indian Ocean may not be the predominant source of moisture as previously believed.

Long Range Forecasting Indian Summer Monsoon Rainfallby a Hybrid Principal Component Neural Network Model

Abstract: The existing methods based on statistical techniques for long range forecasts of Indian monsoon rainfall have shown reasonably accurate performance, for last 11 years. Because of the limitation of such statistical techniques, new techniques may have to be tried to obtain better results. In this paper, we discuss the results of an artificial neural network model by combining two different neural networks, one explaining assumed deterministic dynamics within the time series of Indian monsoon rainfall (Model I) and other using eight regional and global predictors (Model II). The model I has been developed by using the data of past 50 years (1901–50) and the data for recent period (1951–97) has been used for verification. The model II has been developed by using the 30 year (1958–87) data and the verification of this model has been carried out using the independent data of 10 year period (1988–97). In model II, instead of using eight parameters directly as inputs, we have carried out Principal Component Analysis (PCA) of the eight parameters with 30 years of data, 1958–87, and the first five principal components are included as input parameters. By combining model I and model II, a hybrid principal component neural network model (Model III) has been developed by using 30 year (1958–87) data as training period and recent 10 year period (1988–97) as verification period. Performance of the hybrid model (Model III) has been found the best among all three models developed. Rootmean square error (RMSE) of this hybrid model during the independent period (1988–97) is 4.93% as against 6.83%of the operational forecasts of the India Meteorological Department (IMD) using the 16 parameter Power Regression model. As this hybrid model is showing good results, it is now used by the IMD for experimental long-range forecasts of summer monsoon rainfall over India as a whole.

Metphomod – a Numerical Mesoscale Model for Simulation of Regional Photosmog in Complex Terrain: Model Description and Application During Pollumet 1993 (Switzerland)

Abstract: A prognostic three-dimensional Eulerian model for the simulation of summer smog was implemented which fully couples meteorological processes and gas phase chemistry. The model includes modules for atmospheric dynamics, atmospheric turbulence, transport, gas-phase chemistry, short- and long-wave radiation transfer, and surface interactions, including emission and deposition of trace gases. The model was tested in a numerical study of 29–30 July 1993 on the Swiss Plateau and compared to the data of the POLLUMET measurement campaign. Model results were found to be in good agreement with measurements. The model revealed a complex interaction of long range transport, dispersion of plumes from urban areas, and local influences of the complex Swiss topography on local wind systems, and local ozone concentration maxima during that episode.

A Study of Orographic Blocking and Barrier Wind Development Upstream of the Southern Alps, New Zealand

Abstract: Upper level and surface wind data for 1994 are used to provide an initial identification of the orographic effect on regional airflow patterns upwind of the mountain barrier. A case study of the development of upstream blocking and barrier jets is also provided.

Evidence of a Convective Boundary Layer Developingon the Antarctic Plateau during the Summer

Abstract: The development of a convective boundary layer over the Antarctic Plateau is documented by a Doppler minisodar data-set recorded during a 10 day campaign in January 1997. The vertical velocities associated with thermals do not exceed 1 m/s, while the depth of the convective layer, usually less than 200 m, never surpasses 300 m. Measurements of momentum flux, sensible heat flux, wind speed and radiation budget show characteristics that are typical of a convective boundary layer evolution. The diurnal behaviour of absolute humidity, however, exhibits features that are not expected, e.g. anticorrelation with incoming net radiation and air temperature.

A Case of Convection Development over the Western Mediterranean Sea: A Study through Numerical Simulations

Abstract: A convective case producing heavy precipitation in the western Mediterranean region, characterized by pronounced upper level forcing and main rainfall over the sea, is studied. On the day of the event (September 28th, 1994), more than 140 mm of precipitation were recorded in coastal lands of eastern Spain, and 180 mm were estimated over the sea with radar data. Synoptically, the case appears to combine warm and moist easterly advection at low levels, typically observed in torrential rainfall events of the region, with a less common strong upper level dynamical forcing. A set of mesoscale numerical simulations using the Hirlam model is performed to investigate the mechanisms responsible for the convection development, and to assess the influence of the orography on the rainfall field. Model output diagnosis indicates that in addition to the lower level forcing, a two-jets interaction is decisive for the triggering and driving of the convection during the event. Moreover, a non-topographic simulation reveals a relatively weak influence of the orography on this event when compared with other similar heavy precipitation cases in eastern Spain. Previous studies have shown an orographic influence of more than 90% on the rainfall whereas in this case about 50% of the precipitation over the area is attributed to the orographic forcing. The study is extended with an analysis of the individual effects of the Atlas and Iberian Peninsula, by means of a factor separation technique. It is shown that the Atlas range induces a redistribution of the precipitation over the Mediterranean, whereas local enhancements can be attributed to the Iberian topography.

On the role of the cross equatorial flow on summer Monsoon rainfall over India using NCEP/NCAR reanalysis data

Abstract: The role of the cross equatorial flow from the southern Indian Ocean on the Indian Summer monsoon is examined using the National Centre for Environmental Prediction (NCEP)/National Centre for Atmospheric Research (NCAR) data for the period January 1982 to December 1994. A comparison of NCEP/NCAR data with the satellite data retrieved from the Special Sensor Microwave Imager (SSM/I) sensor onboard the Defense Meteorological Satellite Program (DMSP) exhibited a negative bias for the wind speeds greater than 4 m/s. whereas in the case of specific humidity, SSMI values exhibited a positive bias and the precipitable water derived from the satellite data exhibited a negative bias. The NCEP reanalysis is able to depict the mean annual cycle of both the cross equatorial flow and moisture flow into the Indian subcontinent during the monsoon season, but it fails to depict these differences during excess (1983, 1988, 1994) and deficit monsoon (1982, 1986, 1987) composites. Further, it is seen that inter hemispheric flow far exceeds the excess moisture available over the Arabian Sea indicating that it is the cross equatorial flow which decides the fate of the Indian summer monsoon.

Acoustic Travel Time Tomography - A Method for Remote Sensingof the Atmospheric Surface Layer

Abstract: Acoustic travel time tomography is demonstrated as a technique for remote monitoring of near surface air temperature and wind fields. An experimental procedure is presented which provides line-averaged values of the effective sound speed changing with air temperature and wind vector. In this study the travel times of sound signals at defined propagation paths between different acoustic sources and receivers were measured. The travel time data were inverted into effective sound speed values by using a special tomographic algorithm (Simultaneous Iterative Reconstruction Technique) to obtain area-averaged meteorological quantities (air temperature, wind speed). The method of acoustic travel time tomography will be applied to the atmosphere in order to directly provide spatially averaged data which are needed for evaluation of large-eddy simulation and microscale meteorological models as well as to complete conventional point measurements.

Sea-breeze Front Variations in Space and Time

Abstract: This paper is a contribution to experimental meteorology: A sea-breeze front was investigated by aircraft observations and thorough numerical analysis using an unprecedented number of runs crossing the same front within a timespan of \(\). The 33 runs were flown in a situation of offshore geostrophic wind of 5 m/s in 1000 hPa and with the strategy of obtaining information on the four-dimensional field (t=time, x=cross-coastal coordinate, y=coast-parallel coordinate, z=height): 9 runs in x-direction (and reverse) at different heights to yield x,z-cross-sections of the observed meteorological quantities (specific humidity q, potential temperature Θ and the components u, v and w of the wind velocity), assuming a frozen structure in time; the next 7 runs again in x-direction but all at the same level and on the same track to yield x,t-diagrams of the same quantities in order to study the temporal changes compared to those with x and z; the next 10 runs as a zig-zagging flight track crossing the front but drifting in y-direction, all at the same height, in order to obtain the y-dependency; andfinally 7 runs for another x,z-cross-sectional analysis, which can be compared to that evaluated from the runs at the beginning of the mission.

Modifications of a Valley Wind System by an Urban Area - Experimental Results

Abstract: The Austrian city of Graz at the south-eastern edge of the Alps frequently experiences wintertime stagnations during anticyclonic flow conditions, leading to high local concentrations of primary pollutants This paper investigates the dominant three-dimensional local flow structures in the Graz region during a representative January stagnation period in 1998 using data obtained from a field experiment that supplemented the routine meteorological network with an array of sodars and tethersondes and a meteorological tower Important modifications to the temperature and wind fields over Graz and its surroundings are attributed to both topographical and urban effects The main modifications to the along-valley wind system in the Mur valley that runs through Graz from north to south are caused by near-surface temperature field differences between a warmer north and a cooler south part of the city and the regular development of a nighttime down-valley low-level jet and its upward lifting when crossing the city centre

Effects of a Total Solar Eclipse on the Mesoscale Atmospheric Circulation over Europe – A Model Experiment

Abstract: ¶On August the 11th, 1999 Central Europe saw a spectacular astronomical event, a total solar eclipse. We present a model study concerning the meteorological effects of this eclipse in central Europe using the state-of-the-art limited area forecast model Deutschland-Modell DM from the German Weather Service DWD. Under typical summer radiation conditions very strong anomalies in the surface energy flux and temperature in screen height are simulated. The main temperature signal in the lower troposphere is delayed by about one hour with respect to the surface. Furthermore it is connected with a well defined dynamical signal which is reminiscent to a large scale land – sea circulation. The event could be used as a test case for mesoscale atmospheric models.

The role of acoustic sounding in a high-technology era.

Abstract: This paper presents a brief synopsis of past, current and anticipated progress and problems in the use of acoustic remote sensing for basic and applied research of the lower atmosphere. The potential and reality of the sodar for determination of meteorological parameters and turbulence characteristics is discussed. Sodars’ place alongside other ground-based remote sensors, including radar wind profilers, radioacoustic sounding systems (RASS) and lidars, is elucidated. Areas of atmospheric research where Doppler sodar has certain advantages are described such as cost, sensitivity, spatial and temporal resolution and surface layer measurements. The use of sodar in networks of integrated radar/RASS systems designed to supply uninterrupted monitoring of atmospheric parameters for improvements in forecasts of weather and air quality is demonstrated.

Detection of inversions and mixing height by REMTECH PA2 Sodar in comparison with collocated radiosonde measurements

Abstract: A REMTECH PA2 Doppler Sodar is operated regularly at the Czech Hydrometeorological Institute (CHMI) observatory in Prague, collocated with a routine rawinsonde sounding system. The Air Pollution Control Division of CHMI utilises the sodar data in air pollution studies and as an information support for the smog warning system operated in Prague. Besides of the basic software for echo strength and wind profile evaluation, optional routines for deriving parameters such as inversion and mixing height, stability class etc. were delivered by the sodar manufacturer. Based on a sufficiently large data set (more than one year) of synchronous sodar and rawinsonde measurements, an analysis and comparison of inversion and mixing heights provided by both sounding systems have been accomplished in order to evaluate the correctness and accuracy of sodar estimates of these parameters. In contrast to the wind speed and wind direction data, for which a satisfactory agreement with other kind of measurements has been reported by many studies, the results for inversion and mixing height detection were totally disappointing. A direct applicability of inversion height and mixing height data provided by the REMTECH’s automatic routines in air pollution studies or smog warning systems is quite problematical with the present “state of the art”.

Inland Propagation of Sea Breeze under Opposing Offshore Wind

Abstract: According to past experience, the nearly stagnant conditions caused by the presumed equilibrium between the Saronikos Gulf sea breeze and an opposing synoptic flow is identified as the principal mechanism leading to high pollution episodes in Athens during the summer. However, previous experimental work has not examined in detail the interaction of the sea breeze flow with the opposing background flow. In this context, recent experimental work covering the basic key-locations of the Athens Basin focused on the inland propagation of the southerly sea breeze from the coast to the northern part of the basin mainly under moderate northerly background wind. During this campaign, a network of four meteorological stations established along the Athens Basin and a high range acoustic sounder at the centre of Athens operated over a two months time period in the summer of 1993. In addition, tethered balloon flights in the centre of Athens and on a sea vessel about 15 km offshore were employed during an experimental day with moderate opposing background wind. The results from this experimental campaign include the documentation of the sea breeze delay and its intensity as a function of a sea breeze index and features of the vertical structure of the sea breeze over land as well as over sea.

First Results of Measurements with a Newly-Designed Phased-Array Sodar with RASS

Abstract: At a special measuring site for boundary-layer studies as well as land-surface processes the Meteorological Observatory Lindenberg of the German Weather Service (DWD) has recently put into operation a newly-desi gned phased-array SODAR/RASS, which has been developed by METEK on behalf of the DWD. This system provides the vertical profiles of the three-di mensional wind vector in the boundary layer on an operational basis and is furthermore suitable for getting information on the profile of virtual temperature up to about 400 m in height based on the addition of RASS components. The following paper describes both the technique of this SODAR/RASS and the various modes of operation as well as the different options in managing the system. First evaluations on the data availability concerning the maximum height coverage will give an impression on the system’s capabilities. Finally, the accuracy of the derived profiles of winds and temperature will be investigated by means of comparisons of the SODAR/RASS data with measurements of a six-sonde tethered-balloon system as well as meteorological data of a 99 m tower in the vicinity of the system.

Use of Coded Waveforms for SODAR Systems

Abstract: Pulse-compression, phase-encoding and chirp techniques are frequently used in EM systems to improve system performance. Simple averaging-over-range and averaging-over frequency schemes have been used in some SODAR systems, but generally code techniques are problematic because of the high fractional Doppler shift of 0–0.04.

A Numerical Study of Precipitation Characteristics over Taiwan Island during the Winter Season

Abstract: Two-thirds of the land mass of Taiwan island is mountainous, which affects the airflow and precipitation systems over the island. In this study, we discuss the characteristics of precipitation systems when the prevailing wind direction is from the north-east during winter. Observations indicate that rainfall amounts were higher in northeastern Taiwan (the upstream side of the mountains) and that a rainfall shadow occurred in southwestern Taiwan.

Mesoscale Convective System over the Yellow Sea - A Numerical Case Study

Abstract: A mesoscale convective system (MCS) case that developed over the Yellow Sea (12–13 July 1993) is studied by using a 23-level, 30 km-mesh Penn State/NCAR mesoscale model MM5. This MCS was generated in northern China, south of the Changma front, in a convectively unstable environment, under the influence of a short-wave trough accompanied by a marked cold vortex aloft.

High-frequency patterns of the atmospheric circulation over the southern hemisphere and South America

Abstract: Daily 500-hPa geopotential height and 250-hPa meridional wind reanalyzed data obtained from the National Centers for Environmental Prediction are used to document austral winter (May to September) and summer (November to March) high-frequency variability in the Southern Hemisphere (SH) midlatitudes for the 1990–1994 period. Empirical orthogonal function (EOF) technique is used to determine the high-frequency patterns for these variables in selected areas. The high-frequency anomalous 500-hPa geopotential height patterns for two areas in the SH midlatitudes (the zonally global domain and the western hemisphere) and the high-frequency anomalous 250-hPa meridional wind patterns in the western hemisphere between 15° N and 70° S are discussed. The high-frequency winter and summer patterns for both variables feature a wavetrain structure in the SH midlatitudes which is related to synoptic-scale systems, such as cyclones and anticyclones associated with frontal zones. The dominant high-frequency patterns in the SH midlatitudes manifest in the eastern hemisphere while the secondary ones appear in the southeastern Pacific. Analysis of the western hemisphere data reveal that the wavetrain in the South American sector extends northeastward over the continent, thus affecting the regional weather conditions. An important result presented here concerns the preference of the intense synoptic systems in the eastern hemisphere and in the southeastern Pacific to occur in a sequential instead of an intermittent fashion. This result might have a potential for being used in weather monitoring.

Temporal scales of convective coherent structures derived from sodar data

Abstract: The temporal variability of thermal turbulence and vertical velocity derived from sodar measurements during periods of atmospheric free convection is studied using both spectral and wavelet analysis. A promising approach to analyse atmospheric processes, an advanced high-resolution spectrum estimation technique is described. Variance spectra of meteorological and turbulent parameters are shown to have their specific comb shape at a low-frequency range. Spectra and wavelet transforms of the data obtained at different sites both indicate the existence of some representative predominant temporal scales in time variations of the convective boundary layer structure. The most evident temporal scales revealed are centered around 7–9 and 18–22 minutes.

The Argonne boundary layer experiments facility : using minisodars to complement a wind profiler network.

Abstract: The Argonne Boundary Layer Experiments (ABLE) facility, located in south central Kansas, east of Wichita, is devoted primarily to investigations of and within the planetary boundary layer (PBL), including the dynamics of the mixed layer during both day and night; effects of varying land use and land form; the interactive role of precipitation, runoff, and soil moisture; storm development; and energy budgets on scales of 10 to 100 km. Located entirely within the Walnut River watershed, ABLE provides intense measurements within the northeast quadrant (Fig. 1) of the Southern Great Plains (SGP) Cloud and Radiation Testbed (CART) of the Atmospheric Radiation Measurement (ARM) Program (Stokes and Schwarz, 1994). By combining the continuous measurements of ABLE with ancillary continuous measurements of, for example, the ARM and the Global Energy Water cycle Experiment (GEWEX) (Kinster and Shukla, 1990) programs, ABLE provides a platform within which shorter, more intensive studies, such as those conducted by the Cooperative Atmosphere-Surface Exchange Studies (CASES) Program, can realize the full benefit of a wide variety of atmospheric measurements on many scales; this allows the study of hypothesized features of PBL development and dynamics, including frontal dynamics, nocturnal boundary development and breakdown, urban heat island effects, precipitation enhancement, and modification of the surface energy budget partitioning.

Southeastern pacific blocking episodes and their effects on the South American weather

Abstract: Southeastern Pacific blocking episodes are studied using 17 years of reanalyzed daily data from the National Centers for Environmental Prediction (NCEP). The anomalous sea level pressure (SLP) within the area bounded by the longitudes of 130° W and 100° W and the latitudes of 50° S and 70° S is used as the base variable to determine periods with 7 or more sequential days with positive anomalies in this domain. Using these periods, composites are calculated for the SLP and its anomalies, 500-hPa geopotential height anomalies and the 250-hPa and 925-hPa wind vectors in the western southern hemisphere (SH). Composites for austral winter and summer exhibit atmospheric circulation features quite similar to those associated with the blocking episodes in the southeastern Pacific. The corresponding composite patterns of the precipitable water (Pw) and 925-hPa temperature anomalies for the South American sector are also discussed. For both seasons blocking episodes in the southeastern Pacific change the distributions of these thermodynamic variables over South America, in particular in its southern and southeastern regions by reducing (increasing) the Pw and low-level temperature in the southern South America (the central part of the continent). Therefore, monitoring the southeastern Pacific circulation patterns may lead to improved weather forecast for the South American sector.

Storm tracking and monitoring using objective synoptic diagnosis and cluster identification from infrared Meteosat imagery : A case study

Abstract: The present paper investigates the potential of combining image processing techniques based on cluster analysis of infrared (IR) Meteosat images with dynamic meteorological theory on synoptic systems. From this last point of view the highest probability of deep convective development is favoured where the overlapping of four mechanisms acting at synoptic scale is produced: upward quasi-geostrophic forcing, convergence of water vapour at low levels, convective instability in the lower troposphere and great convective available potential energy. Cloud tracking is performed over sequences of Meteosat IR images by using a shape parameterisation approach after appropriate filtering for non-significant clouds and automated identification of convective systems. The integrated methodology is applied to the case study of the heavy rainfall event which produced floods in the South of France and the North of Italy on September 27–28th, 1992. The analysis focuses on the monitoring and explanation of the zones most affected by heavy rainfall with the aim of investigating possible improvements of the predictive potential of cloud tracking and allowing identification of the areas which most lend themselves to flash floods for use in operational flood forecasting applications.

On the Atmospheric Response to Urbanization and Open-Pit Mining under Various Geostrophic Wind Conditions

Abstract: Simulations alternatively assuming a real landscape with and without open-pit mines and grown settlements were performed with a non-hydrostatic meteorological model of the meso-β-scale to elucidate whether the atmospheric response to such land-use changes is sensitive to the direction and magnitude of geostrophic wind. The results of simulations with the same geostrophic wind conditions substantiate that the daily domain-averages of the variables of state hardly differ for the different landscape realizations, except for cloud- and precipitating particles. However, land-use changes may significantly affect the local conditions over and downwind of the altered surfaces. The significant differences in the cloud- and precipitating particles, however, are not bound to the surroundings of land-use changes. The vertical component of wind vector, which is modified by the different heating of converted land-use, strongly affects cloud- and precipitation formation by the interaction cloud microphysics-dynamics. The magnitude of atmospheric response changes under the various directions and speeds of geostrophic wind for most of the field quantities and fluxes.

Evolution of a southeast Australian coastally trapped disturbance

Abstract: The southeast Australian coastally trapped disturbance (CTD) of 9–11 November 1982 that was previously studied by Holland and Leslie is re-visited. Additional observational data not considered by these authors and a numerical simulation using the Colorado State University Regional Atmospheric Modeling system (RAMS) are used for this purpose. Following initiation of the event on the south coast, mesoscale ridging propagated along the east coast to just north of Brisbane. Associated with the arrival of the event were a marked increase in surface pressure, drop in temperature and a shift and strengthening of the wind. While the simulation does not appear to capture the details of the boundary layer as well as one would like, it does represent the main features of the event, including the speed of propagation along the coast, reasonably well. Similar to the observed, the model event shows gravity current-like characteristics. The significance of topographic variability (e.g., large gap in the coastal mountains at the Hunter Valley) is considered. It is suggested that the topography and ambient stratification in southeastern Australia are less favourable for CTD occurrence than those in southern Africa and western North America where these systems have been extensively studied. Consequently, when CTD do occur in southeastern Australia, the less pronounced topography and weaker stratification may enable local effects to mask the CTD signal to some extent, thereby posing challenges in observation and forecasting.

Variational Data Assimilation for a Lorenz Model Usinga Non-Standard Genetic Algorithm

Abstract: Data assimilation in meteorology and oceanography for strongly nonlinear dynamical systems is challenging. The dynamical system studied here is the classical three-variable Lorenz model. In this context data assimilation with weak-constraint variational methods performs better than other methods like strong-constraint variational methods or Kalman filters. The difficulty in tracking the chaotic Lorenz orbit by assimilation of noisy observations results from the inherent instability in the system. In variational methods a cost function has to be minimized. It is known, that in the Lorenz case the structure of the cost function becomes more and more complex with increasing length of the assimilation time interval and with reduction of the observational data quality. This paper proposes a non-standard implementation of a genetic algorithm for searching the global minimum in case of a weak-constraint formulation. The good performance of this non-local search is shown, but the algorithm is computationally demanding due to a very large number of control parameters within the weak-constraint formulation and, thus, the algorithm is applicable for simple systems only.

On the influence of QBO over North Indian Ocean storm and depression tracks

Abstract: In this paper the modulation of storm and depression tracks over North Indian Ocean by the Quasi-Biennial Oscillation (QBO) of zonal winds in the equatorial stratosphere is discussed for the period 1953–1991. It was observed that during post monsoon season the storms and depressions of Bay of Bengal were mostly confined to south of 17° N and move in west/North-westward direction during easterly phase of QBO. However during the westerly phase no such similar type influence of QBO on the system tracks was observed. Also such type of QBO-System tracks association was not observed during pre-monsoon season.

Teleconnections: Northern hemisphere lower stratospheric geopotential heights and Indian monsoon rainfall

Abstract: In this study a search for teleconnection is made between the Northern Hemisphere lower stratospheric geopotential heights and Indian Monsoon Rainfall (IMR) through the correlation analysis approach. Monthly grid-point 50 hPa geopotential height data and the June to September IMR are used for the period 1958–1990.

Improving precipitation forecasts over the Global tropical belt

Abstract: This study explores the nowcasting and short-range forecasting (up to 3 days) skills of rainfall over the tropics using a high resolution global model. Since the model-predicted rainfall is very sensitive to model parameters, four key model parameters were first selected. They are the Asselin filter coefficient, the fourth order horizontal diffusion coefficient, the surface moisture flux coefficient, and the vertical diffusion coefficient. The optimal values were defined as those which contributed to the best one day rainfall forecasts in the present study. In order to demonstrate and improve the precipitation forecast skill, several numerical experiments were designed using the 14-level Florida State University Global Spectral Model (FSUGSM) at a resolution of T106. Comparisons were also made of the short-range forecasts obtained from a control experiment subjected to normal mode initialization (NMI) versus experiments based on physical initialization (PI). The latter experiments were integrated using the original FSUGSM and a modified version. This modified FSUGSM was developed here by applying a reverse cumulus parameterization alorithm to the regular forecast model, which restructures the vertical humidity distribution and constrains the large-scale model’s moisture error growth during the model integration. An improved short-range rainfall prediction skill was achieved from the modified FSUGSM in this study. The results showed a better agreement between model-based and observed rainfall intensity and pattern.

Modulation of Intraseasonal (25–70 Day) Processes by theSuperimposed ENSO Cycle Across the Pacific Basin

Abstract: Although variability is a fundamental aspect of the climate system, the interaction of different time scales of variability remains difficult to assess and verbalize. Two well-documented examples of tropical variability on different time scales are the El Nino/Southern Oscillation (ENSO) and the Madden-Julian Oscillation (MJO). Using the fifteen-year ECMWF Re-analysis (ERA) data archive and outgoing longwave radiation (OLR) data, some of the ENSO modulations of the intraseasonal MJO are examined.