Diurnal Wind Variations Research Articles

Some aspects of the monsoon circulation and monsoon rainfall

The south Asian summer monsoon from June to September accounts for the greater part of the annual rainfall over most of India and southeast Asia. The evolution of the summer and winter monsoon circulations over India is examined on the basis of the surface and upper air data of stations across India. The salient features of the seasonal reversals of temperature and pressure gradients and winds and the seasonal and synoptic fluctuations of atmospheric humidity are discussed. The space-time variations of rainfall are considered with the help of climatic pentad rainfall charts and diagrams. The rainfall of several north and central Indian stations shows a minimum around mid-August and a maximum around mid-February which seem to be connected with the extreme summer and winter positions of the ITCZ and the associated north-south shifts in the seasonal circulation patterns. Attention is drawn to the characteristic features of the monsoon rainfall that emerge from a study of daily and hourly rainfall of selected stations. Diurnal variations of temperature, pressure, wind and rainfall over the monsoon belt are briefly treated.

Meridional Circulation in the Thermosphere. II. Solstice Conditions

Abstract The mean meridional circulation and latitudinal variation of temperature in the thermosphere are calculated for solstice conditions. The heat and momentum sources that drive the thermospheric circulation are solar EUV and UV heating, high-latitude heating due to auroral processes, and a momentum source due to the correlation of diurnal variations of wind and ion drag. The results show a solar-driven, summer-to-winter circulation that is modified by the high-latitude heat source. The high-latitude heat source reinforces the summer-to-winter circulation in the summer hemisphere, but reverses the circulation in the mid-latitude winter hemisphere at F-layer heights with transition from one cell to another in the midlatitude winter hemisphere. Below about 150 km, however, the summer-to-winter circulation is maintained at all latitudes. The zonal winds at midlatitudes are generally eastward in the winter hemisphere and westward in the summer hemisphere. At F-layer heights, there is a significant temper...

Diurnal Variations of the Summertime Wind and Force Field at Three Midwestern Locations

Abstract A comprehensive summary of diurnal wind variations in the midwestern region of the United States is presented. Analyses are based on seven summers of four per day soundings at Fort Worth, Tex., Topeka, Kan., and International Falls, Minn. It is found that the diurnal oscillations are most prominent at Fort Worth, of significant amplitude at Topeka, and, although of lesser amplitude, still detectable at International Falls. An analysis is made of the forcing required to account for that part of the wind oscillation which cannot be attributed to Coriolis effects. This analysis indicates that the forcing is comparatively small at Fort Worth when the wind oscillations are largest owing to a resonance there with natural inertial oscillations. Significant forcing is present at higher latitude stations even though the manifestation of the forcing in the wind field is somewhat smaller in amplitude. The data suggest that forcing mechanisms at low and high attitudes may propagate to cause wind oscillations...

Observational evidence for time‐dependent zonal winds in the exosphere

One of the few methods that allows the determination of zonal thermospheric winds is the analysis of the variation of the inclination of satellite orbits. Until now mainly the diurnally averaged zonal wind velocity was determined by this method. From the analysis of the orbit of the balloon satellite Explorer 24 (1964‐76A) both the diurnal average and the diurnal variation of the zonal wind have been determined in the height region between 540‐610 km. Within the limits of the accuracy of the method there results a diurnally averaged zonal wind velocity that is nearly zero. This is contrary to the results of previous investigations by a similar method where an average rotational velocity of the exosphere appreciably less than the rotational velocity of the earth has been found. The diurnal component of the zonal wind is shown to have an amplitude of 90 ± 65 m/sec, and the zonal wind has its maximum from West to East at 23.96 ± 2.33 hr local time. Both the amplitude and the phase of the diurnal component of the zonal exospheric wind velocity agree approximately with the theoretical results of Blum and Harris.

Effects of heating at high latitudes on global thermospheric dynamics

Three dimensional numerical calculations of thermospheric wind, density and temperature fields generated by solar EuV heating were described in a recent paper by Straus et al. (1975). These model calculations indicated that many of the global characteristics of the thermosphere are generated by EUV heating alone. However, several observed features were found to be poorly represented by the model results. These include the amplitude of the diurnal temperature variation and properties of the diurnal variations of the meridional wind and pressure gradient fields. The present paper describes investigation of the extent to which heating of the thermosphere at high latitudes would contribute to the resolution of these difficulties. This heating lowers the day to night temperature ratio, since it provides a heat source at night. It brings calculated meridional winds and pressure gradients into better agreement with observations, since it raises isobaric surfaces at high latitudes. The amount of heating at high latitudes required to make these modifications is consistent with that due to quiet time joule heating.

Meteorology of the upper atmosphere

Upper atmosphere meteorologists are concerned with the structure and dynamics of the neutral atmosphere from roughly the tropopause through the thermosphere, especially on a global scale. Some of the exciting developments in upper atmospheric meteorology over the last 4 years include advances in understanding the role of various trace gases in the stratosphere that influence the photochemistry of ozone; a description of seasonal, latitudinal, and diurnal variations of winds and composition in the thermosphere; and significant advances in interpreting various upper atmosphere circulation systems in terms of coupling to the troposphere by various types of tropical and extratropical wave processes. Notable on the methodology side has been the application of increasingly advanced in situ and remote sensing techniques utilizing satellite platforms, especially the use of mass spectrometers for measuring composition in the thermosphere and infrared radiometers for inferring global stratospheric temperature. Numerical models of dynamic processes have increased in sophistication with, on the one hand, extension of tropospheric general circulation models to stratospheric levels and, on the other hand, development of a large number of different simplified dynamic models for treating various aspects of thermospheric dynamics. This paper reviews progress over the last 4 years in upper atmospheric meteorology.

Diurnal variation of the neutral thermospheric winds determined from incoherent scatter radar data

Incoherent scatter radar measurements of the diurnal variation of the electron and ion temperatures, electron density, and vertical plasma drift are used to determine the diurnal variation of the neutral thermospheric winds. The ionospheric properties needed to determine the neutral winds are (1) the horizontal neutral wind component along the magnetic declination, SD (determined from plasma drift measurements and F region theory) and (2) the longitudinal derivative of the neutral exospheric temperature, ∂T∞/∂γ (derived from ion and electron temperatures and density measurements above 300 km). The atmospheric response to various forcing functions is determined from a three-dimensional dynamic model of the neutral thermosphere, the ion drag being specified by the electron density measurements. Harmonic components of the pressure force in terms of the latitudinal and longitudinal variations of exospheric temperature are used to determine the SD response coefficients from the model. A least squares fit of the measured and calculated diurnal variation of SD and ∂T∞/∂γ gives the Fourier coefficients necessary to construct the appropriate latitudinal and longitudinal pressure gradients. The pressure gradients are then used to calculate the diurnal winds for two geomagnetically quiet days over Millstone Hill during equinox. Amplitudes of 200 m s−1 during the night and 50 m s−1 during the day are obtained. The diurnal winds determined by using three different empirical models of the thermospheric structure are also calculated and compared with the winds determined from the least squares fit.

Diurnal variations of winds over an upwelling region off Oregon

The diurnal variation of surface winds off the coast of Oregon is described and compared with a recent analysis of winds off the coast of Peru. The Oregon wind speeds have a distinct 24-h periodicity, while the Peru wind speeds were reported to have an irregular 12-h variation. The long-and trans-shore components of both winds exhibit 24-h periodicities; the ratio of the long-shore to trans-shore diurnal amplitudes off Oregon is 2.8, twice the ratio found off Peru. Although meteorological conditions off Oregon were quasi-stationary during the period investigated, there were considerable day-to-day variations in diurnal amplitudes and phases. Diurnal amplitudes were found to be correlated with the daily mean long-shore winds.

The Diurnal Wind Variation in the Lowest 1500 ft in Central Oklahoma. June 1966–May 1967

One year of wind data from a television tower in northern Oklahoma City has been analyzed an a diurnal basis. The annual mean speeds below the third level at 296 ft are lowest at night and highest during the day, and conversely the speeds up to the seventh level at 1458 ft are lowest during the day and highest at night. Generally, speed changes occur in short time periods shortly after sunrise and near sunset. Resultant wind directions veer most with height between 1700–1000 CST and least during midday. At all levels winds veer with time between 2100 and approximately 1100 CST and then back with time during the remaining hours.

エクマン層の風速と温度場の日変化

エクマン層の風速と温度場の日変化

Diurnal wind variations in Antarctica

AbstractFor the three Antarctic ice shelf stations considered (Norway Station, Maudheim, and Halley Bay) it is shown that from spring to autumn a systematic diurnal variation of the mean vector wind at the surface exists. Different factors that may be responsible for this variation are discussed. The results seem to agree well with the assumption that in areas far from the ice front (the seaward edge of the ice shelf) the main cause is the larger net gain of momentum in the lowest layers during the middle of the day, while closer to the ice front there is an additional effect of the varying contrast between the air temperature over the sea and over the ice shelf.

Diurnal wind variations in Antarctica

Diurnal wind variations in Antarctica

Diurnal variation of temperature and wind in the equatorial stratosphere

Diurnal variation of temperature and wind in the equatorial stratosphere

Diurnal variation of temperature and wind in the equatorial stratosphere

AbstractRocket soundings were made at Gan (0°41′S, 73°09′E) after sunset and before sunrise to determine the change of temperature and wind during the night. The soundings showed that the change of temperature varied with height, being positive around 54 km, negative around 44 km, and mainly positive between 30 and 40 km, in general agreement with Lindzen's tidal theory. The zonal wind also showed a tidal variation. An analysis of day and night rocket soundings at Ascension Island (07°59′S, 14°25′W) indicates that the winds show a tidal variation, but the heights of the isobaric surface in the stratosphere are always a maximum around noon, suggesting an error in the temperature measurements due to solar radiation.

A Theoretical Study of the Diurnal Wind Variations in the Planetary Boundary Layer

An analytical solution of the wind velocity profile is obtained for the horizontally homogeneous planetary boundary layer with constant geostrophic wind velocity and with eddy diffusivity being constant in height and having periodic variations in time. The wind velocity oscillations occur for any disturbances in the flow. The time for a disturbance at the height z and eddy diffusivity A to decay appreciably is A−1z. The analytical solution is used to interpret the wind variations observed at Wichita and Oklahoma City as mainly due to the variations in the pressure gradient.

Neutral winds deduced from incoherent scatter observations and their theoretical interpretation

Ionization drifts observed with the St-Santin-Nançay incoherent scatter facility are used to determine the meridional component of the horizontal neutral winds. In a previous paper this has been done at the maximum electron density level. Here the results are presented for various F-region altitudes after evaluating the effect of the ambipolar plasma diffusion from the incoherent scatter measurements of ion density and electron and ion temperatures. The ‘experimental’ neutral winds obtained in this way are compared to ‘theoretical’ winds deduced from a model of themospheric circulation obtained by solving the partial derivative momentum transfer equation for neutrals. Three cases for summer, winter and equinox periods are considered for these comparisons. It is shown that motions induced by electric fields must be introduced during summer and equinox, but not during winter, in order to obtain a good agreement between ‘theoretical’ and ‘experimental’ diurnal variations of the neutral winds. These conclusions are drawn for the whole F-region altitude range.

Diurnal Wind Variations Over the Drakensberg Escarpment at Sani Pass

Abstract Observation of surface winds on the Drakensberg escarpment at Sani Pass reveals a marked diurnal variation of wind direction under clear weather conditions. By night north-westerly gradient winds commonly prevail while by day easterly winds are due to the overflow or a plainmountain circulation onto the 3000 m Lesotho massif at Sani Pass.

DIURNAL TEMPERATURE VARIATIONS: SURFACE TO 25 KILOMETERS1

Abstract Vertical profiles of 12-hr temperature difference are examined for evidence of structure related to the topographically forced diurnal wind variations. Wavelike features are found with amplitude of the order of 1°C and wavelengths of the order of 5–15 km. These values agree with estimates inferred from the tidal wind field on the basis of continuity considerations. The diurnal temperature range in the planetary boundary layer decreases exponentially with height, with the decay rate varying markedly from one geographical region to another.

DIURNAL VARIATION OF WIND, PRESSURE, AND TEMPERATURE IN THE TROPOSPHERE AND STRATOSPHERE OVER ENIWETOK

Abstract The diurnal and semidiurnal variations of wind, pressure, and temperature at 23 levels between 1000 and 10 mb over Eniwetok are obtained by combining two series of 6-hr soundings taken at different times in April–July 1956 and 1958. The prevalence of the semidiurnal pressure wave in the lower layers gradually yields to the first harmonic in the upper troposphere and stratosphere, as a consequence of the predominantly diurnal variation of temperature. The semidiurnal variation of temperature is found to increase slightly with height. Increase of the second harmonic amplitude of the u and v wind components with height is roughly proportional to the decrease of air density, with the phase remaining nearly constant, except for the lower layers. The phase relationship between the semidiurnal variations of pressure, and eastward and northward directed wind components is discussed in terms of a model based on a linearized form of the equations of motion, frictionless flow, and the assumption that the os...

ON THE KINETIC ENERGY SPECTRUM NEAR THE GROUND

Abstract For six stations in the northeastern United States, the spectrum of horizontal wind speed was analyzed using 10 yr of 1-min averaged, hourly surface reports. The fast Fourier transform technique was employed to estimate the spectrum between 1 cycle/2 hr and 1 cycle/2 yr. The kinetic energy spectra show two major spikes at periods of 24 hr and 1 yr. However, most of the energy is contained in the traveling cyclones and anticyclones with periods between 2 and 7 days. The apparent discrepancy between Van der Hoven's results and our results concerning the existence of an important diurnal cycle in the kinetic energy can be explained by Blackadar's theory of the diurnal wind variation with height. Van der Hoven's spectrum represents conditions near the top of the surface layer, while our data were taken well within the surface layer. A line-by-line investigation of the diurnal peak reveals a very sharp line at 2400 hr with two side lobes 3.9 min away from the main line. These side lobes are probably c...