Ahmet Aksakal - Academia.edu (original) (raw)

Papers by Ahmet Aksakal

The Arabian Sea troposphere during a Summer Monsoon Season is characterized by convection. While ... more The Arabian Sea troposphere during a Summer Monsoon Season is characterized by convection. While deep, well organized, banded convection is observed over the Eastern Arabian Sea, shallow convection capped by an inversion is present over the Western Arabian Sea. Observations indicate that this inversion layer rises from Western to Eastern Arabian Sea during the monsoon onset and post-onset periods. Moderately

Renewable Energy, May 1, 1998

In the present study the energy potential of wind for the Eastern Province of Saudi Arabia is inv... more In the present study the energy potential of wind for the Eastern Province of Saudi Arabia is investigated. A suitable Weibull distribution is generated based on the data obtained for a duration of one complete year at a costal location in northeastern Saudi Arabia. Comparison of this model is made with the Rayleigh distribution of wind power densities. Two horizontal-axis type of wind energy conversion systems which operate at fixed rpm are considered and a model of quadratic power output function is used. It is found that the error in using the Rayleigh approximation will be less than 10% of the full rated power density level.

International Journal of Global Energy Issues, 2001

KeyLos ® 2001 is a new precipitation-hardening steel especially developed for plastic moulds. In ... more KeyLos ® 2001 is a new precipitation-hardening steel especially developed for plastic moulds. In this study the precipitation stage of KeyLos ® 2001 steel has been investigated and compared to the results obtained with 17-4 PH steel. Precipitation-hardening has been carried out at three different temperatures and the stages of hardening and overageing have been studied in order to clarify the hardening mechanisms. It has been found that hardening and softening mechanism during the precipitation-hardening treatment occur at higher temperatures and in correspondence with more prolonged treatment times than those typical for the best known 17-4 PH steel; hardness is then expected to remain stable also for very extended mould lives. Microstructural investigations by means of Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) have also been carried out and the microstructural parameters responsible for the hardening and overageing have been pointed out.

Atmospheric Research, Jun 1, 1994

The organization of convection during the pre-onset, onset, and post-onset phases of monsoon has ... more The organization of convection during the pre-onset, onset, and post-onset phases of monsoon has been studied using a linear stability model of convection which parameterized the latent heat released in preexisting cumulus convection, and the special thermodynamic and wind soundings collected during the Summer Monsoon Experiment (SMO-NEX) of 1979. The motivation for this study came from several satellite and research aircraft observations during SMONEX, which revealed that convection was organized into mesoscale bands over the Eastern (EAS), Central (CAS), and Western (WAS) Arabian seas. These bands varied in size from 20 to 100 km in length. Some (called longitudinal) were oriented parallel to the vertical shear of the base state winds; some (called transverse) were perpendicular. The linear stability model performed a normal mode analysis of convection and examined the factors governing the orientation of bands; longitudinal bands resulted when precipitation efficiency was high, and transverse, when the efficiency was low. The phase propagation of bands was reduced when more moisture was available. The linear stability model utilized a One Dimensional Steady State cloud (1DSS) model to estimate the latent heat released in preexisting cumulus convection. The 1DSS model developed Cumulus (Cu) clouds based on the observed temperature and humidity soundings in various parts of the Arabian Sea during different phases of the monsoon. Although this simple 1DSS model cannot reproduce faithfully all the characteristics of cumulus convection in the monsoon, some of the results, such as estimates of cloud tops, agreed well with those observed. Based on the 1DSS results and the kinematically computed mesoscale vertical velocities, three composite vertical cross sections (one for each): pre-onset, onset, and post-onset phases of the monsoon were constructed. Highlights of the cross sections were (1) during the pre-onset stage, WAS developed moderately strong Cu convection while EAS was inversion dominated; (2) during the onset stage, both CAS and EAS developed vigorous Cu convection while WAS did not show strong convection; and (3) during tPresently at the Severe Storms Branch, NASA, GSFC,

International Journal of Energy Research, Aug 1, 1999

The energy potential of wind for the eastern region of Saudi Arabia is investigated based on meas... more The energy potential of wind for the eastern region of Saudi Arabia is investigated based on measurements of a complete year data at a coastal location in eastern Saudi Arabia. A suitable Weibull distribution is generated and a comparison of this model is made with the Rayleigh distribution of wind power densities. Two horizontal-axis type of wind energy conversion systems which operate at "xed rpm are considered for the determination of the extractable wind power, and a model of quadratic power output function is used between the cut-in speed and rated speed. It is shown that small-scale wind energy systems are suitable in the eastern part of Saudi Arabia for power generation and irrigation purposes.

Renewable Energy, Aug 1, 1999

This paper presents the actual global solar radiation on a horizontal surface along with the prev... more This paper presents the actual global solar radiation on a horizontal surface along with the prevailing meteorological conditions encountered during the measurement period from 0 JanuaryÐ20 December\ for one complete year\ in the Arabian Gulf Coast near the city of Dhahran[ High resolution\ real time solar radiation and meteorological data were collected\ and processed[ Hourly\ daily\ and monthly statistics of solar radiation was made from the one! minute averaged recorded values[ The highest measured daily\ and monthly mean solar radi! ation were found to be 240 and 217 W:m −1 \ respectively[ The highest one!minute averaged solar radiation values up to 0072 W:m −1 were observed in the summer season\ from MayÐ September[ The highest hourly solar radiation value was recorded as 0942 W:m −1 in the middle of June[ Beside the global solar radiation measurements\ the main observed meteorological parameters were temperature\ pressure\ wind speed\ precipitation\ and relative humidity[ On the other hand\ the estimation of daily and monthly mean global solar radiation was performed based upon two empirical formulas which relate the solar radiation to the sunshine duration\ relative humidity\ maximum temperature\ the latitude of the monitoring location\ sunset hour and declination angles[ The agreement between the measured and estimated solar radiation values was found to be satisfactory[ Nevertheless\ the empirical formula under!estimates the solar radiation values during summer\ and over!estimates during winter[ Þ 0888 Elsevier Science Ltd[ All rights reserved[

Monthly Weather Review, Mar 1, 1997

Satellite imagery and rain gauge data are combined to create mesoscale detail in the initial stat... more Satellite imagery and rain gauge data are combined to create mesoscale detail in the initial states of relative humidity (RH) and surface moisture availability (M) for a mesoscale model simulation. The most profound impact of inserting the mesoscale initial fields was the development of a strong vertical circulation transverse to an intensifying cold front that triggered an intense frontal rainband similar to a severe squall line that was observed to develop explosively. This paper explores the causative factors leading to the formation of this intense circulation and the sensitivity of the model to the mesoscale initial fields. A substantial gradient in the initialized RH and M fields occurred across the cold front in the region where the observed frontal squall line formed. In contrast to the control run, the model simulations that incorporated the mesoscale initial analysis displayed considerable daytime warming just ahead of the front. This warming was due principally to a reduction in the RH (and, hence, low-level cloud cover) east of the front, although an increase in the cross-frontal M gradient did contribute about 25% of the warming. Increased sensible heat fluxes at the expense of decreased latent heat fluxes led to a much deeper and well-mixed prefrontal boundary layer, a more erect frontal surface, and an updraft jet just ahead of the front. A density current-like flow developed in the cold air immediately behind the front only in the presence of this cross-frontal gradient in sensible heating. Much improved forecasts of the location and timing of the frontal squall line and other precipitation systems resulted from the mesoscale initial analysis. The initial RH and M fields possessed sufficient resolution and consistency with the model dynamics to have a positive influence on the forecasts for a period of at least 12 h. This study provides evidence that differential cloud cover and evapotranspiration fields can have important impacts on frontal behavior when strong synoptic dynamics are present. Future research should attempt to improve the modeling of evapotranspiration processes, develop more objective satellite-based humidity analysis techniques, and obtain in situ mesoscale data for verification of the retrieved atmospheric and soil moisture fields.

Applied Energy, 1999

The solar energy potential of northeast Saudi Arabia is investigated based on measurements of a c... more The solar energy potential of northeast Saudi Arabia is investigated based on measurements of a complete year's data at a coastal location near Dhahran. High resolution, real time solar radiation data were collected and processed. Hourly, daily and monthly statistics of solar radiation were made from the 1 min averaged recorded values. Clearness index is discussed on the basis of hourly, daily and monthly averages. The variation of the diffuse fraction of total radiation is also determined and the results discussed.

Renewable Energy, 1999

International Journal of Global Energy Issues, 2001

The main objective of this paper is to explain not only the importance of the baseline data in ai... more The main objective of this paper is to explain not only the importance of the baseline data in air quality studies but also the general procedures and steps involved in the baseline data collection. Special considerations on the instruments and measurements for a perfect baseline data collection are also emphasized in this paper. As an example, an adequate baseline data collection procedure which took place in the North Eastern part of Saudi Arabia is briefly presented. The high resolution baseline data of (i) the air pollutants including sulphur dioxide, carbon monoxide, nitrogen oxides, ozone, (ii) the surface meteorological parameters of temperature, pressure, solar radiation, wind speed, and (iii) mixing height, which was collected by an acoustic sounder, are discussed.

Energy Sources, 2000

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