Semidiurnal Variations Research Articles

Statistics of Antarctic surface meteorology based on hourly data in 1957–2007 at Syowa Station

Statistical characteristics of the surface meteorology are examined at Syowa Station (69°00′S, 39°35′E), which is located on an island near the coastal region of the Antarctic continent, based on operational observations made over the 50-year period from February 1957 through January 2007, which includes missing periods equivalent to 5 years. Statistics are obtained for the surface temperature, sea level pressure (SLP), and horizontal winds in terms of frequency distribution, frequency power spectra, seasonal variation, diurnal variation, inter-annual variation, and trends, using hourly observation data, and several interesting characteristics are elucidated. The mean temperature, SLP, and wind speed over the 50-year period are −10.5 °C, 986 hPa, and 6.6 m s −1, respectively. The frequency distribution of temperature is far from the normal one, because less variation exists in summer at higher temperatures. The predominant wind direction is northeasterly (southwestward), and a weak secondary peak is observed in the southerly (northward) direction in the frequency distribution. The directional constancy of winds is 0.78. The frequency spectra over a wide range of 2 h to 20 years exhibit clearly isolated peaks corresponding to annual and diurnal frequencies and their higher harmonics. An important finding is that the spectral shape is proportional to a power of the frequency with a transition frequency for all physical parameters. The transition frequencies correspond to about 5 days for temperature and winds and 3 days for SLP, most likely due to cyclonic activity. A peak in the 11-year solar cycle is not identified in any spectrum. Another interesting feature is the dominance of semi-annual and semi-diurnal variations in SLP, while annual and diurnal variations are dominant for temperature and winds. Statistically significant trends are not detected for annual mean surface temperature and SLP over the 50-year period, while a positive trend is significant for wind speed. These trends are also examined as a function of the months. The inter-annual variation of SLP is well correlated with that of the Antarctic Oscillation index, indicating that Syowa Station can be regarded as a typical Antarctic station. Furthermore, statistical analysis was conducted for blizzards (severe snow storms), in terms of duration, and seasonal and inter-annual variation in frequency of occurrence. It is shown that the blizzards are dominant in the period from late March to late October. No systematic variation in blizzard frequency was observed during the 50-year period. Instead, the frequency depends largely on the year with a minimum of nine in 1988 and a maximum of 42 in 1982. As a typical example, a synoptic chart is used to show the cause of a strong snow storm on 27 May 1996, when the absolute maximum gust was observed.

VHF radar observations of surface currents off the northern Opal coast in the eastern English Channel

Two very high-frequency radars (VHFR) operating on the Opal coast of eastern English Channel provided a nearly continuous 35-day long dataset of surface currents over a 500 km 2 area at 0.6–1.8 km resolution. Argo drifter tracking and CTD soundings complemented the VHFR observations, which extended approximately 25 km offshore. The radar data resolve three basic modes of the surface velocity variation in the area, that are driven by tides, winds and freshwater fluxes associated with seasonal river discharge. The first mode, accounting for 90% of variability, is characterized by an along-shore flow pattern, whereas the second and third modes exhibit cross-shore, and eddy-like structures in the current velocity field. All the three modes show the dominant semi-diurnal variability and low-frequency modulation by the neap-spring tidal cycle. Although tidal forcing provides the major contribution to variability of local currents, baroclinicity plays an important role in shaping the 3D velocity field averaged over the tidal cycle and may strongly affect tracer dynamics on larger time scales. An empirical orthogonal function (EOF) decomposition and a spectral rotary analysis of the VHFR data reveal a discontinuity in the velocity field occurring approximately 10 km offshore which was caused by the reversal in the sign of rotation of the current vector. This feature of local circulation is responsible for surface current convergence on ebb, divergence on flood and strong oscillatory vertical motion. Spectral analysis of the observed currents and the results of the Agro drifter tracking indicate that the line of convergence approximately follows the 30-m isobath. The most pronounced feature of the radar-derived residual circulation is the along-coast intensification of surface currents with velocity magnitude of 0.25 m/s typical for the Regions of Freshwater Influence (ROFI). The analysis has provided a useful, exploratory examination of surface currents, suggesting that the circulation off the Opal coast is governed by ROFI dynamics on the hypertidal background.

Tidal signatures in mesospheric turbulence

Abstract. We search for the presence of tidal signatures in high latitude mesospheric turbulence as parameterized by turbulent energy dissipation rate estimated using a medium frequency radar, quantifying our findings with the aid of correlation analyses. A diurnal periodicity is not particularly evident during the winter and spring months but is a striking feature of the summer mesopause. While semidiurnal variation is present to some degree all year round, it is particularly pronounced in winter. We find that the maximum in the summer 24-h variation corresponds to that of the westward phase of the diurnal tide, and that the maximum in the winter 12 h variation corresponds to that of the southward phase of the semidiurnal tide. This information is used to infer the horizontal propagation direction of gravity waves: during the summer the eastward direction is consistent with closure of the summer vortex, while in winter the inferred directions require more complex arguments.

The semidiurnal variation in GPS‐derived zenith neutral delay

Zenith neutral delay (ZND) estimates derived from ground‐based GPS receivers exhibit variations at harmonics of the solar day. The aim of this work is to characterize the semidiurnal (S2) variation and determine its probable origin. Data from 100 GPS sites are compared with surface pressure measurements to reveal close agreement between the estimated ZND S2 variation and the S2 surface pressure tide. Error analysis suggests that the S2 variation in ZND estimates is not due primarily to orbit, solid earth, or Earth orientation modeling errors. Atmospheric loading and mapping function errors are each expected to contribute less than 11% to the estimated ZND S2 amplitude. Local incongruities reflect the influence of water vapor or site‐dependent errors.

Submarine volcanic activity, ocean‐acoustic waves and internal ocean tides

Submarine volcanic event often generates acoustic waves (T‐waves) traveling over long distances through the low velocity channel (SOFAR) of the ocean. By a method of coherent stacking of T‐waves from a submarine volcanic activity in northern Mariana, we found a significant semidiurnal variation of T‐wave travel times. The amplitude of variation is an order of larger than those reported in the previous ocean sound transmission experiments. Ray‐theoretical consideration for the numerically simulated ocean tides indicates that such large T‐phase travel time variation is a consequence of large up‐and‐down movement of seawater around the axis of the SOFAR channel due to the M2 internal tide effectively converted from external tidal forcing. T‐phases, a ubiquitous feature of the ocean acoustic noise field, can be used to infer internal tidal motion and the associated ocean mixing.

Determination of rock mass deformation modulus by means of Traveling Load Tests—Part II: Traveling Load Test practice in an open pit

A case study makes clear the applicability and high potential of the Traveling Load Test in an open pit. The induced tilt-loading distance curve is measured by means of a series of stationary loading tests over a short time, so as to avoid semidiurnal variation of initial tilt by the earth tide. Through a series of data processing, the deformation modulus of rock mass is determined as a function of Poisson's ratio by the values of the peak tilt and peak distance on the induced tilt-loading distance curve and the magnitude of load subjected, regardless of the depth of tilt measurement and the thickness of surface damages. The test results represent the macroscopic deformation modulus of rock mass, covering an area much wider than the conventional load tests. Additionally, analyzing the signal to noise ratio of the traveling load test in an open pit, it is discussed that the lack of load is a problem to be settled by future studies.

Mean diurnal variations of noctilucent clouds during 7 years of lidar observations at ALOMAR

Abstract. From 1997 to 2003, noctilucent clouds (NLC) were observed by lidar above the ALOMAR observatory in Northern Norway (69° N) during a total of 1880 measurement hours. This data set contains NLC signatures for 640h, covering all local times, even during the highest solar background conditions. After data limitation imposing a threshold value of 4x10-10m-1sr-1 for the volume backscatter coefficient of the NLC particles, a measure for the cloud brightness, local time dependencies of the NLC occurrence frequency, altitude, and brightness were determined. On average, over the 7 years NLC occurred during the whole day and preferably in the early morning hours, with a maximum occurrence frequency of ~40% between 4 and 7 LT. Splitting the data into weak and strong clouds yields almost identical amplitudes of diurnal and semidiurnal variations for the occurrence of weak clouds, whereas the strong clouds are dominated by the diurnal variation. NLC occurrence, altitude, as well as brightness, show a remarkable persistence concerning diurnal and semidiurnal variations from 1997 to 2003, suggesting that NLC above ALOMAR are significantly controlled by atmospheric tides. The observed mean anti-phase behavior between cloud altitude and brightness is attributed to a phase shift between the semidiurnal components by ~6h. Investigation of data for each individual year regarding the prevailing oscillation periods of the NLC parameters showed different phase relationships, leading to a complex variability in the cloud parameters.

On the seasonal asymmetry of the diurnal and semidiurnal geomagnetic variations

The diurnal and semidiurnal variations of the geomagnetic field are investigated at 18 observatories using long series of hourly values (up to 97 years at Sitka). The seasonal variations of amplitude of the 12‐hour and 24‐hour lines are obtained for the H and Z components using a 28‐day sliding window. The Fourier analysis is performed using either all days within the window or only the five quietest days. At midlatitudes a strong lack of symmetry about the summer solstice is observed for both lines and both components. This effect is enhanced when selecting quiet days. When averaged over the entire series, the sign of this seasonal asymmetry is the same at 9 out of 10 midlatitude observatories for both lines and for a given component; it is opposite for the H and Z components. Such a coherent seasonal asymmetry is not found at low and high latitudes. At high latitudes a strong annual variation is found inside the polar caps, while a strong semiannual variation is found in the auroral zones. When selecting quiet days, these two effects are weaker, and the seasonal asymmetry becomes comparable to that at midlatitudes. At all latitudes the year‐to‐year variations of the seasonal asymmetry are uncorrelated with solar activity. It is suggested that lower thermospheric winds may have a similar seasonal asymmetry between spring and autumn equinoxes. Such an asymmetry is present in the published literature but has been overlooked. Possible causes for this wind asymmetry are reviewed.

Sporadic E morphology from GPS‐CHAMP radio occultation

The scintillations of phase and amplitude in terms of signal‐to‐noise ratio (SNR) of the GPS radio occultation signal are caused by thin ionization layers. These thin irregular electron density layers in the E region ionosphere are often called sporadic E (Es). For a monthly retrieval of Es morphology we use the variances of the phase and SNR fluctuations of worldwide ∼6000 GPS/CHAMP occultations in the E region. The Es climatology is studied globally with the SNR and phase variances in terms of monthly zonal means, seasonal maps, and diurnal and long‐term variations. The zonal mean variances reveal strong, extended Es activities at summertime midlatitudes but weak, confined activities in wintertime high latitudes, peaking at ∼105 km. Global maps at 105‐km altitude show clear dependence of Es activities on the geomagnetic dip angle, where the summertime midlatitude Es occurs mostly at dip angles of 30°–60° and the wintertime high‐latitude enhancement occurs mostly at dip angles >80°. The midlatitude Es variances exhibit a strong semidiurnal variation with peak hours near 0800–1000 and 2000 local solar time, respectively. The peak hours are delayed slightly with decreasing height, suggesting influences from the semidiurnal tide. To provide more insights on the observed SNR and phase variances, we model radio wave propagation for the CHAMP observing geometry under several perturbed cases in the E region ionosphere. The model simulations indicate that the SNR variance has the maximum response to Es perturbations at vertical wavelengths of ∼1.2 km, whereas the phase response maximizes at ∼2 km (for the 1‐s variance analysis). The characteristic scale depends little on the truncation time used in the SNR variance analysis, but it increases with the truncation time for the phase variances. Initial studies show that reasonable global Es morphology can be produced on a monthly and seasonal basis with the CHAMP one‐antenna occultations. Better results from other existing and upcoming GPS occultation missions are anticipated in future studies, and they will significantly improve our understanding of this important phenomenon.

High accuracy satellite drag model (HASDM)

The dominant error source in force models used to predict low-perigee satellite trajectories is atmospheric drag. Errors in operational thermospheric density models cause significant errors in predicted satellite positions, since these models do not account for dynamic changes in atmospheric drag for orbit predictions. The Air Force Space Battlelab’s High Accuracy Satellite Drag Model (HASDM) estimates and predicts (out three days) a dynamically varying global density field. HASDM includes the Dynamic Calibration Atmosphere (DCA) algorithm that solves for the phases and amplitudes of the diurnal and semidiurnal variations of thermospheric density near real-time from the observed drag effects on a set of Low Earth Orbit (LEO) calibration satellites. The density correction is expressed as a function of latitude, local solar time and altitude. In HASDM, a time series prediction filter relates the extreme ultraviolet (EUV) energy index E 10.7 and the geomagnetic storm index a p , to the DCA density correction parameters. The E 10.7 index is generated by the SOLAR2000 model, the first full spectrum model of solar irradiance. The estimated and predicted density fields will be used operationally to significantly improve the accuracy of predicted trajectories for all low-perigee satellites.

Semi-diurnal variation in cosmic ray intensity under different geomagnetic conditions

Semi-diurnal variation in cosmic ray intensity under different geomagnetic conditions

Solar activity and atmospheric tide effect on the polar conductivity and the vertical electric field in the stratosphere at low latitude

Balloonborne measurements of the polar conductivity and the vertical electric field were carried out over a period of 1984–1994 from Hyderabad, Central India. The conductivity values show positive correlation with solar activity between 20 and 35 km. Between 5 and 20 km the conductivity values show an anti-correlation with solar activity. The vertical electric field does not show solar cycle effect. However, the electric field and the conductivity show a semi-diurnal variation at balloon float altitude.

Lidar studies of interannual, seasonal, and diurnal variations of polar mesospheric clouds at the South Pole

Polar mesospheric clouds (PMC) were observed by an Fe Boltzmann temperature lidar at the South Pole in the 1999–2000 and 2000–2001 austral summer seasons. We report the study of interannual, seasonal, and diurnal variations of PMC using more than 430 h of PMC data. The most significant differences between the two seasons are that in the 2000–2001 season, the PMC mean total backscatter coefficient is 82% larger and the mean centroid altitude is 0.83 km lower than PMC in the 1999–2000 season. Clear seasonal trends in PMC altitudes were observed at the South Pole where maximum altitudes occurred around 10–20 days after summer solstice. Seasonal variations of PMC backscatter coefficient and occurrence probability show maxima around 25–40 days after summer solstice. Strong diurnal and semidiurnal variations in PMC backscatter coefficient and centroid altitude were observed at the South Pole with both in‐phase and out‐of‐phase correlations during different years. A significant hemispheric difference in PMC altitudes was found, that the mean PMC altitude of 85.03 km at the South Pole is about 2–3 km higher than PMC in the northern hemisphere. Through comparisons with the NCAR Thermosphere‐Ionosphere‐Mesosphere‐Electrodynamics General Circulation Model (TIME‐GCM), the hemispheric difference in PMC altitude is attributed to the hemispheric differences in the altitudes of supersaturation region and in the upwelling vertical wind, which are mainly caused by different solar forcing in two hemispheres that the solar flux in January is 6% greater than the solar flux in July due to the Earth's orbital eccentricity. Gravity wave forcing also contributes to these differences.

A localized structure in OH airglow images near the mesopause region

An intense localized airglow‐depletion structure was observed in OH airglow images (emission altitude ∼86 km) at Shigaraki (34.9°, 136.1°) on 19 December 1998. The structure was a single‐band airglow depletion with a small horizontal width (∼15 km) but a large amplitude (∼34% from the average intensity). Airglow rotational temperature indicated a large temperature lapse rate at this time as a result of strong semidiurnal variation. We suggest that nonlinear wave‐wave interaction is the most likely cause of this localized structure.

Semidiurnal variations of stratospheric conductivity at balloon float altitude

Ten balloon flights were conducted from Hyderabad, India to measure polar conductivity. Measurements were carried out during balloon ascent as well as at float altitude. At float altitude, i.e. 32–36 km, the measurements were carried out for a period ranging from 5 hrs to 10 hrs. At sunrise the conductivity always decreases. At around 1000 hrs local time the conductivity is maximum and again decreases at around 1400 hrs. This variation shows that at float altitude, 32–36 km the conductivity shows semi-diurnal variations. We suggest that this is due to semidiurnal tide.

The Annual Behavior of the Semidiurnal and Diurnal Pressure Variations in East Antarctica

Abstract A 15-yr (1980–95) 3-hourly record of surface pressure from the Dome Concordia automatic weather station (74.30°S, 123.00°E; 3280 m MSL), located in East Antarctica, was analyzed to study the annual behavior of the semidiurnal and diurnal variation associated with atmospheric tides excited by heating due to insolation absorption by ozone and water vapor. The mean daily behavior of the pressure variation shows maxima around 0900 and 2100 LT. This variation is more intense during the austral winter. The time series of a Total Ozone Mapping Spectrometer (TOMS) were analyzed to study the correlation between the local pressure tidal oscillations and the annual behavior of total ozone over the globe. A clear correlation between the intensity of the pressure semidiurnal variation and global total ozone in both the annual and long-term trends was found. The mean annual behavior of the semidiurnal tides and global total ozone is very similar, with two maxima and one deep minimum. Maxima in semidiurnal tide...

A preliminary semiempirical thermosphere model of Mars: DTM‐Mars

A semiempirical thermosphere model of Mars has been constructed based on the DTM algorithm that was developed for the terrestrial thermosphere. DTM‐Mars reproduces the observed densities with a 1‐σ model uncertainty of approximately 35% outside dust season. During dust storms the uncertainty increases by approximately a factor two. The data used in this study have been derived from the Mars Global Surveyor accelerometer observations as well as from analysis of precise orbit determination results. The temperature modeling is based on the Mars‐GRAM 2000 [Justus and James, 2000] and the MTGCM [Bougher et al., 2000] models, while the CO2, O, and He constituent modeling has been achieved through the adjustment of model coefficients to density data. DTM‐Mars is preliminary because the geographical and solar activity coverage of the assimilated density data currently available is not representative of all atmospheric conditions. For example, the effect of dust storms on the thermospheric density is only represented by a temperature increase on a planetary scale, because the data did not allow the determination of a semidiurnal variation caused by local effects in the lower atmosphere. The model accuracy will be improved in the future by assimilating density data taken under different atmospheric conditions, for example, from the Mars Odyssey mission, once these become available.

Geothermal constraints on the hydrological regime of the TAG active hydrothermal mound, inferred from long-term monitoring

During August 1994 to March 1995, a period that included ODP Leg 158 drilling, bottom-water and sub-bottom temperatures were continuously logged by a long-term temperature monitoring system ‘Daibutsu’ at the base of the central black-smoker complex (CBC) and within the low heat flow zone at the TAG hydrothermal mound on the Mid-Atlantic Ridge. The temperature of hydrothermal fluid at CBC was also measured with a small high-temperature probe ‘Hobo’. Bottom-water temperature variations measured with Daibutsu at both sites have predominant semi-diurnal periods, causing the sub-bottom temperatures to fluctuate at these periods with reduced amplitudes and phase delays at sub-bottom depths. Seawater entrainment into the mound has been previously suggested at the low heat flow zone. We quantitatively evaluate the seawater entrainment rate at both sites from a one-dimensional numerical model, combined with a heat conduction model for the semi-diurnal variations. The entrainment rate of seawater at the base of CBC is estimated as 1.3±0.5×10−5 m/s, at least from August 17 to 30, 1994. On the other hand, the seawater entrainment rate at the low heat flow zone was undetected by long-term temperature monitoring at shallow sub-bottom depth. Nevertheless an increase in heat flow observed at the low heat flow zone during ODP drilling can be interpreted as a decrease in the entrainment rate of seawater. Before ODP Leg 158, Daibutsu measured three sub-bottom temperature anomalies at the base of CBC not derived from bottom-water temperature variations and Hobo also detected a CBC fluid temperature anomaly, indicating some natural changes in fluid flow within the mound. Daibutsu and Hobo also measured temperature anomalies during and after drilling at the ODP TAG-1 area. The Hobo temperature anomalies are inferred to have occurred when the cold fluid entrained through the drill holes at TAG-1 site reached or cooled the main fluid path to CBC. The entrained seawater through the drill holes appears to have contributed to dissolution and precipitation of anhydrite within the mound and perhaps affected the local permeability structure inside the mound. The temperature anomalies measured with Daibutsu at the base of CBC may have been induced by the change in the fluid flow pattern as a result of such permeability changes within the mound.

Air pollution transport in the Himalayas of Nepal and Tibet during the 1995–1996 dry season

To investigate if air pollution generated in the Himalayan foothills reached higher elevations, a sequence of surface meteorological and condensation nucleus (CN) measurements were made between October 1995 and May 1996. Measurements were made in the Kathmandu valley of the foothills, in the Dudh Kosi valley of the eastern Himalayas and at the base of Mt. Everest in Nepal and Tibet. The Kathmandu valley measurements revealed a semi-diurnal variation of CN and moisture in the mountain-valley wind system and a reduction of both in strong afternoon convection we call the “Kathmandu chimney”. The air was polluted at all times in the valley. The Dudh Kosi measurements revealed a diurnal variation in CN and moisture; both were drawn into the valley in the strong afternoon valley wind and cleaner and drier air flowed into the valley in the nighttime mountain wind. The air was unpolluted in the mornings and frequently polluted in the afternoon and evenings; some of the evening pollution was from local sources. The CN concentrations in Tibet were smaller than in Nepal at the same elevation due dilution in the deep afternoon convective boundary layer in Tibet; there also may be a “Tibetan chimney”. Evidence is presented for trans-Himalayan pollution transport: a 7-h pollution episode originating in Nepal was advected into Tibet and a 18-h episode originating in Tibet was advected into Nepal.

Periodic variations of geocoronal Balmer‐alpha brightness due to solar‐driven exospheric abundance variations

Measurements of the geocoronal Balmer‐alpha (Hα) brightness have been made at the Arecibo Observatory during 11 separate periods since 1983 using both a Fabry‐Perot interferometer and a tilting filter photometer. The tilting filter photometer is calibrated for absolute sensitivity using a constant brightness source traceable to National Institute of Standards and Techniques (NIST) standards and is used to cross‐calibrate the Fabry‐Perot interferometer. Since the observational technique has not changed since 1983, and since the data analyses technique are uniform, these data provide a measure of the solar cycle variation of Hα brightness at Arecibo. Unlike earlier studies, we discern no systematic discrepancy between the Hα brightness and estimates of the solar Lyman‐beta flux that pumps the resonant fluorescent Hα emission. Rather, we conclude that geocoronal hydrogen abundance is generally (but not always) larger than models suggest, although not systematically so. The largest Hα emission is measured during solar minimum conditions, when brightness is approximately 50% greater than during solar maximum conditions, for measurements at solar depression angles from 15° to 35°. Above about 40° solar depression (corresponding to an illuminated column with a base at approximately 2000 km), no solar cycle variation is evident, and the brightness there is persistently greater than models. Intricacies of the Lyman‐beta radiative transport problem make detection of an early morning maximum of hydrogen density near the exobase difficult using brightness data alone, although the Hα brightness is generally asymmetric with respect to local midnight, with brighter emission in the postmidnight sector. Studies of the semidiurnal (nighttime) variation demonstrate that the Hα brightness can vary by a factor of 2 over a period of days.