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Abstract
A steady-state two-layer model has been developed for the baroclinic boundary layer. The lower layer is the constant flux surface layer (SL) in which the eddy viscosity K varies with height and stability according to the Monin-Obukhov similarity theory; the upper one is the Ekman layer in which K is fixed at the value attained at the top of the SL. The equations of motion in the Ekman layer are solved using the Green's function approach. The lower boundary condition gives two equations from which the nondi-mensionalized friction velocity u */V g0 and the cross-isobaric angle α0 can be obtained in terms of the other known parameters. These equations are compared with the resistance laws. The boundary condition also is given a geometrical interpretation. It has been shown that if Vg (z) is linear, the variation of α0 and u */V g0 with θ, the angle between Vg (0) (surface isobars) and thermal wind (isotherms), is sinusoidal. Also, there is a phase difference of 90° between the variation of α0 and u */V g0, and the amplitude of variation of α0 is found to be proportional to the non-dimensionalized magnitude of thermal wind. ATEX 1969 observations are used to test the wind profiles obtained by the model.
Abstract
A steady-state two-layer model has been developed for the baroclinic boundary layer. The lower layer is the constant flux surface layer (SL) in which the eddy viscosity K varies with height and stability according to the Monin-Obukhov similarity theory; the upper one is the Ekman layer in which K is fixed at the value attained at the top of the SL. The equations of motion in the Ekman layer are solved using the Green's function approach. The lower boundary condition gives two equations from which the nondi-mensionalized friction velocity u */V g0 and the cross-isobaric angle α0 can be obtained in terms of the other known parameters. These equations are compared with the resistance laws. The boundary condition also is given a geometrical interpretation. It has been shown that if Vg (z) is linear, the variation of α0 and u */V g0 with θ, the angle between Vg (0) (surface isobars) and thermal wind (isotherms), is sinusoidal. Also, there is a phase difference of 90° between the variation of α0 and u */V g0, and the amplitude of variation of α0 is found to be proportional to the non-dimensionalized magnitude of thermal wind. ATEX 1969 observations are used to test the wind profiles obtained by the model.
Abstract
It is shown that, in any secant polar stereographic projection, a small circle on a sphere projects into a circle. This property provides a simple relationship between KH , the horizontal component of curvature of a horizonal curve and K H ′, the curvature of its projection on a secant polar stereographic map. KH can be computed by subtracting from thc map factor times K H ′ the earth's curvature multiplied by a correction factor that depends only on the latitude of the place and inclination of the curve to the latitude circle. This factor vanishes if the curve is along a meridian but takes an extreme value if it is along a latitude. For a given orientation of the curve, the value of this factor increases gradually as the location of the curve moves from the Pole to the Equator and more rapidly after it crosses the Equator. It is less than 1 in the Northern Hemisphere but can exceed unity in the Southern Hemisphere.
Abstract
It is shown that, in any secant polar stereographic projection, a small circle on a sphere projects into a circle. This property provides a simple relationship between KH , the horizontal component of curvature of a horizonal curve and K H ′, the curvature of its projection on a secant polar stereographic map. KH can be computed by subtracting from thc map factor times K H ′ the earth's curvature multiplied by a correction factor that depends only on the latitude of the place and inclination of the curve to the latitude circle. This factor vanishes if the curve is along a meridian but takes an extreme value if it is along a latitude. For a given orientation of the curve, the value of this factor increases gradually as the location of the curve moves from the Pole to the Equator and more rapidly after it crosses the Equator. It is less than 1 in the Northern Hemisphere but can exceed unity in the Southern Hemisphere.
A NUMERICAL STUDY OF THE DIURNAL VARIATION OF METEOROLOGICAL PARAMETERS IN THE PLANETARY BOUNDARY LAYER
I. DIURNAL VARIATION OF WINDS
Abstract
The diurnal variation of various meteorological parameters in the Planetary Boundary Layer at different latitudes was studied adopting the basic framework of the simple one dimensional model of Estoque and modifying it in the light of the latest theories of atmospheric turbulence. Following are the results concerning the variation of wind: i) The phase angle of the diurnal wind speed wave shifts with height, the rate of shift varying with latitude. The latter is negative at latitudes north of 30°N., zero at about 30°N., and becomes positive south of 30°N. ii) Low level wind maximum occurs before midnight in midlatitudes, slightly after midnight at 30°N., at sunrise at 17.5°N., and later farther south. iii) The amplitude of the diurnal wind speed wave increases from north to south, reaches a maximum a little below 3O°N., and then decreases rapidly. The super-geostrophic winds are strongest between 40°N. and 20°N., suggesting that these latitudes are more favorable for the occurrence of low level jet than any others. The height of the low level wind maximum is below 500 m. north of 30°N., at about 550 m. between 30°N., and 12.5°N. and higher farther south. iv) The winds attain an absolute minimum value by sunrise north of 30°N., and only a relative minimum by about sunset south of 30°N. v) A semidiurnal oscillation of wind speed occurs in the layers below 400 m. north of 30°N., but is not noticed at latitudes south of say 30°N. vi) The Ekman layer appears to be shallower in latitudes south of 30°N. than in more northern latitudes.
Abstract
The diurnal variation of various meteorological parameters in the Planetary Boundary Layer at different latitudes was studied adopting the basic framework of the simple one dimensional model of Estoque and modifying it in the light of the latest theories of atmospheric turbulence. Following are the results concerning the variation of wind: i) The phase angle of the diurnal wind speed wave shifts with height, the rate of shift varying with latitude. The latter is negative at latitudes north of 30°N., zero at about 30°N., and becomes positive south of 30°N. ii) Low level wind maximum occurs before midnight in midlatitudes, slightly after midnight at 30°N., at sunrise at 17.5°N., and later farther south. iii) The amplitude of the diurnal wind speed wave increases from north to south, reaches a maximum a little below 3O°N., and then decreases rapidly. The super-geostrophic winds are strongest between 40°N. and 20°N., suggesting that these latitudes are more favorable for the occurrence of low level jet than any others. The height of the low level wind maximum is below 500 m. north of 30°N., at about 550 m. between 30°N., and 12.5°N. and higher farther south. iv) The winds attain an absolute minimum value by sunrise north of 30°N., and only a relative minimum by about sunset south of 30°N. v) A semidiurnal oscillation of wind speed occurs in the layers below 400 m. north of 30°N., but is not noticed at latitudes south of say 30°N. vi) The Ekman layer appears to be shallower in latitudes south of 30°N. than in more northern latitudes.
Abstract
To study the climatological structure of the atmospheric fields during the onset phase of the Indian summer monsoon, a composite analysis of different meteorological parameters over Indian stations is carried out. The composites are constructed relative to a uniform set of onset dates over south Kerala. Over the peninsular Indian stations, the rainfall composites show sudden and sharp increases with onset except in the case of east coast stations, where rainfall does not substantially change with the onset of the summer monsoon. The composite wind analysis demonstrates how the upper-tropospheric subtropical westerlies weaken and shift poleward and the tropical easterlies strengthen and spread north with the onset of the monsoon. The onset vortex that takes the monsoon northward along the west coast in many years is clearly discernible between 600 and 400 hPa in the composite streamline charts. The relative humidity builds up suddenly in the vertical a few days before the onset at the respective stations. The vertically integrated zonal moisture transport at individual stations over the peninsula increases sharply with respect to the south Kerala onset, with appropriate lag in time. The composite outgoing longwave radiation fields over the north Indian Ocean show rapid buildup of convective activity over the southeast Arabian Sea and east Bay of Bengal with the approach of the monsoon.
Abstract
To study the climatological structure of the atmospheric fields during the onset phase of the Indian summer monsoon, a composite analysis of different meteorological parameters over Indian stations is carried out. The composites are constructed relative to a uniform set of onset dates over south Kerala. Over the peninsular Indian stations, the rainfall composites show sudden and sharp increases with onset except in the case of east coast stations, where rainfall does not substantially change with the onset of the summer monsoon. The composite wind analysis demonstrates how the upper-tropospheric subtropical westerlies weaken and shift poleward and the tropical easterlies strengthen and spread north with the onset of the monsoon. The onset vortex that takes the monsoon northward along the west coast in many years is clearly discernible between 600 and 400 hPa in the composite streamline charts. The relative humidity builds up suddenly in the vertical a few days before the onset at the respective stations. The vertically integrated zonal moisture transport at individual stations over the peninsula increases sharply with respect to the south Kerala onset, with appropriate lag in time. The composite outgoing longwave radiation fields over the north Indian Ocean show rapid buildup of convective activity over the southeast Arabian Sea and east Bay of Bengal with the approach of the monsoon.
Abstract
The altitude profiles of water vapor density, ρ, in the troposphere at low latitudes have been studied using radiosonde observations over nine stations in India. An attempt has been made to evolve a satisfactory model for the altitude variation of ρ in terms of the surface value. It has been found that a simple exponential relation with constant scale height parameter is not always adequate for this purpose. The scale height parameter shows significant variation with altitude. A double exponential profile having two different scale height parameters is found to be more suitable for most of the cases.
Abstract
The altitude profiles of water vapor density, ρ, in the troposphere at low latitudes have been studied using radiosonde observations over nine stations in India. An attempt has been made to evolve a satisfactory model for the altitude variation of ρ in terms of the surface value. It has been found that a simple exponential relation with constant scale height parameter is not always adequate for this purpose. The scale height parameter shows significant variation with altitude. A double exponential profile having two different scale height parameters is found to be more suitable for most of the cases.
Abstract
In this study the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) version 3.5.2 was used to simulate the Indian summer monsoon during the two contrasting years of 1987 and 1988, a dry year and a wet year, respectively. Three different convection parameterization schemes of Betts–Miller–Janjic, Kain–Fritsch, and Grell were used to study the sensitivity of monsoon to cumulus effects. The model was integrated for a period of 6 months, starting from three different initial conditions of 0000 UTC on 1, 2, and 3 May of each year using the NCEP–NCAR reanalysis data as input. The 6-hourly reanalysis data were used to provide the lateral boundary conditions, and the observed weekly Reynolds sea surface temperature, linearly interpolated to 6 h, was used as the lower boundary forcing. The results show that all three cumulus schemes were able to simulate the interannual and intraseasonal variabilities in the monsoon with reasonable accuracy. However, the spatial distribution of the rainfall and its quantity were different in all the schemes. The Grell scheme underestimated the rainfall in both the years. The Kain–Fritsch scheme simulated the observed rainfall well during July and August, the peak monsoon months, of the year 1988 but overestimated the rainfall in June and September of 1988 and throughout the monsoon season of 1987. The Betts–Miller–Janjic scheme simulated less rainfall in the drought year of 1987 and overestimated the rainfall in June and July of 1988. The circulation patterns simulated by the Betts–Miller–Janjic and Kain–Fritsch schemes are comparable to the observed patterns.
Abstract
In this study the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) version 3.5.2 was used to simulate the Indian summer monsoon during the two contrasting years of 1987 and 1988, a dry year and a wet year, respectively. Three different convection parameterization schemes of Betts–Miller–Janjic, Kain–Fritsch, and Grell were used to study the sensitivity of monsoon to cumulus effects. The model was integrated for a period of 6 months, starting from three different initial conditions of 0000 UTC on 1, 2, and 3 May of each year using the NCEP–NCAR reanalysis data as input. The 6-hourly reanalysis data were used to provide the lateral boundary conditions, and the observed weekly Reynolds sea surface temperature, linearly interpolated to 6 h, was used as the lower boundary forcing. The results show that all three cumulus schemes were able to simulate the interannual and intraseasonal variabilities in the monsoon with reasonable accuracy. However, the spatial distribution of the rainfall and its quantity were different in all the schemes. The Grell scheme underestimated the rainfall in both the years. The Kain–Fritsch scheme simulated the observed rainfall well during July and August, the peak monsoon months, of the year 1988 but overestimated the rainfall in June and September of 1988 and throughout the monsoon season of 1987. The Betts–Miller–Janjic scheme simulated less rainfall in the drought year of 1987 and overestimated the rainfall in June and July of 1988. The circulation patterns simulated by the Betts–Miller–Janjic and Kain–Fritsch schemes are comparable to the observed patterns.
Abstract
Interannual variations in spectral aerosol optical depths (AOD) were examined using the data obtained from a chain of ground-based multiwavelength solar radiometers from various locations of the Indian peninsula during the dry winter season (January–March) of 1996–2001. All of the stations revealed significant interannual variations, even though the spatial pattern of the variations differed over the years. These interannual variations were found to be significantly influenced by the extent of the southward excursion of the intertropical convergence zone (ITCZ). The years in which the southward excursion of the ITCZ was less (i.e., the years when the wintertime ITCZ was closer to the equator) showed higher AODs than the years in which the ITCZ moved far southward. The spatial variation was found to be influenced by large-scale vertical descent of an air mass over peninsular India, the Arabian Sea, the Indian Ocean, and the Bay of Bengal.
Abstract
Interannual variations in spectral aerosol optical depths (AOD) were examined using the data obtained from a chain of ground-based multiwavelength solar radiometers from various locations of the Indian peninsula during the dry winter season (January–March) of 1996–2001. All of the stations revealed significant interannual variations, even though the spatial pattern of the variations differed over the years. These interannual variations were found to be significantly influenced by the extent of the southward excursion of the intertropical convergence zone (ITCZ). The years in which the southward excursion of the ITCZ was less (i.e., the years when the wintertime ITCZ was closer to the equator) showed higher AODs than the years in which the ITCZ moved far southward. The spatial variation was found to be influenced by large-scale vertical descent of an air mass over peninsular India, the Arabian Sea, the Indian Ocean, and the Bay of Bengal.
Abstract
The effects of sea breeze on optical depth, size distribution, and columnar loading of aerosols at the tropical coastal station of Trivandrum are studied. It has been observed that sea-breeze front activity results in a significant and short-lived enhancement in aerosol optical depth and columnar loading in contrast to the effects seen on normal sea-breeze days. Examination of the changes in columnar aerosol size distribution associated with sea-breeze activity revealed an enhancement of small-particle (size less than 0.28 µ m) concentration. The aerosol size distributions deduced from optical depth measurements generally show a pronounced bimodal structure associated with the frontal activity.
Abstract
The effects of sea breeze on optical depth, size distribution, and columnar loading of aerosols at the tropical coastal station of Trivandrum are studied. It has been observed that sea-breeze front activity results in a significant and short-lived enhancement in aerosol optical depth and columnar loading in contrast to the effects seen on normal sea-breeze days. Examination of the changes in columnar aerosol size distribution associated with sea-breeze activity revealed an enhancement of small-particle (size less than 0.28 µ m) concentration. The aerosol size distributions deduced from optical depth measurements generally show a pronounced bimodal structure associated with the frontal activity.
Abstract
Using aerosol optical depth as a function of wavelength obtained from ground-based multiwavelength radiometer observations, columnar size-distribution functions of aerosols have been derived. It has been found that the nature of the derived size-distribution function is strongly dependent on season. The derived size-distribution functions are discussed in term of seasonally dependent natural aerosol sources and sinks.
Abstract
Using aerosol optical depth as a function of wavelength obtained from ground-based multiwavelength radiometer observations, columnar size-distribution functions of aerosols have been derived. It has been found that the nature of the derived size-distribution function is strongly dependent on season. The derived size-distribution functions are discussed in term of seasonally dependent natural aerosol sources and sinks.
Abstract
Altitude distribution of aerosols in the mixing region in a tropical coastal environment is studied using a bistatic continuous-wave lidar. It is found that aerosols remain fairly well mixed—their number density showing little variation with altitude up to an altitude of approximately 300 m from the surface, and above this their number density, in general, decreases with an increase in altitude. The aerosol number density shows a significant dependence on the near-surface wind speed. This dependence, could be represented fairly well by an exponential function of wind speed. The wind contribution to aerosol content is found to be at its maximum during the southwest monsoon period.
Abstract
Altitude distribution of aerosols in the mixing region in a tropical coastal environment is studied using a bistatic continuous-wave lidar. It is found that aerosols remain fairly well mixed—their number density showing little variation with altitude up to an altitude of approximately 300 m from the surface, and above this their number density, in general, decreases with an increase in altitude. The aerosol number density shows a significant dependence on the near-surface wind speed. This dependence, could be represented fairly well by an exponential function of wind speed. The wind contribution to aerosol content is found to be at its maximum during the southwest monsoon period.
