Studies of the wind filtering effect of gravity waves observed at Allahabad (25.45°N, 81.85°E) in India
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB 2010
Received: 11 December 2008
Accepted: 29 November 2009
Published: 4 March 2010
Well-defined coherent wave sources associated with the passage of short-period gravity waves were observed in all-sky images of OH emission on a total 21 occasions during January to May 2008 at Allahabad (25.45°N, 81.85°E, dip lat ∼16.49°N) in India. The wave motions exhibited similar spatial and temporal properties during each month, but the north-east ward motions were distinctly dominant in April and May 2008. It is a well-known theory that the upward propagating gravity waves may be blocked or absorbed at a critical layer. We have computed the magnitude and direction of atmospheric gravity waves subject to blocking by horizontal winds, i.e., critical layer directional filtering. The HWM-93 model (Hedin et al., 1996) was used to compute the two components of neutral wind velocity at Allahabad for the period of observation of gravity waves during March and April 2008. Data from two components of wind velocity were then used to construct the blocking diagrams, which show the directions and apparent phase velocities of wave propagation blocked at a given altitude. The blocking diagrams were then compared with experimental observations of gravity waves in OH airglow to determine the accuracy of the wind model and explain the critical layer theory.
Key wordsGravity waves critical layer blocking wind directional filtering effect Allahabad propagation characteristics
Low light images of OH hydroxyl bands taken with CCD cameras (Taylor et al., 1997; Nakamura et al., 1999; Mukherjee, 2003) can provide excellent opportunities for remote sensing of the two-dimensional (2D) evolution of spatial and temporal characteristics of the movement of short-period gravity waves over a large geographical region with great precision. The image data provides vital statistical information on the apparent horizontal phase velocity, direction, wavelength, and period of these waves. This collective body of data together with geographical locations, shape, orientation, and time of displays has been most helpful to researchers investigating the main sources of these waves.
In the troposphere, large-amplitude gravity waves are produced by flow over mountains, severe weather events (fronts, squall lines, etc), convection, jet streams, and other processes. These waves can cause cloud banding and affect the rainfall pattern. They also break, in the same manner as the ocean waves break near the sea shore, producing turbulence which can be a hazard to aircrafts.
Gravity waves which propagate higher up into the atmosphere (e.g., into the stratosphere and mesosphere) grow in amplitude due to decreasing air densities. When gravity waves propagate through the airglow layers, the densities of atoms, molecules, and ions that cause photochemical reactions fluctuate. As a result, the airglow layers show 2D wave patterns. This means that in the horizontal airglow extent, the areas with strong and weak emission intensities change both temporarily and spatially. Consequently, temporal and spatial structures of gravity waves can be mapped by observing the airglow intensity images taken with a sensitive CCD camera, enabling researchers to study the dynamic processes occurring in the mesosphere.
It is well known that both the background wind and energy attenuation have profound effects on the characteristics of upward propagating atmospheric gravity waves (Ding et al., 2003). Hines (1960) and Cowling et al. (1971) studied the effect of background winds on the gravity ray paths. Both research groups found that changes in the intrinsic frequency of the gravity wave were dependent on the direction of the wind. If the gravity wave propagates along the wind, the intrinsic frequency is shifted downward; when the wave frequency equals to zero, wave energy is lost to the mean flow, a process known as critical coupling. Medioros et al. (2003) and Taylor et al. (1993) found that winds have a directional filtering effect on propagation of gravity waves.
Preliminary data on gravity waves collected at the Allahabad station (25.45°N, 81.85°E, dip lat ∼16.49°N) in India showed that, in 2008, the preferential direction of wave propagation was towards the north-east in the months of April and May and in the south-west direction in February—March (Pragati et al., 2010). Taylor et al. (1993) explained that the apparent asymmetry in the wave propagation direction may be caused by a wave filtering effect that can occur at any height where the horizontal wind velocity along the direction of the horizontal wave vector equals the horizontal phase velocity and the intrinsic frequency is Doppler-shifted to zero. A horizontal surface (r-θ plot) has been constructed at the critical altitude to identify the region where the gravity waves are blocked for upward propagation. Gravity waves propagating within the region would be heavily absorbed by diverse mechanisms (Ryan, 1991), and it would not be possible to observe those with horizontal phase speeds lying outside this region.
We have used the HWM-93 model for computing the two components of the background neutral wind. The model reproduces the mean wind profile for any geographic location on any day and at any time of the year. Here, we present and discuss the use of this model in computing the blocking diagrams for a given site—in this case, Allahabad—for different altitudes. The results of new imaging measurements (Pragati et al., 2010) have been compared with the model computation to study the wind filtering effect on the propagation characteristics of gravity waves. These are the first imaging observations of gravity waves from this latitude region in India.
2. The All-sky Imaging System
3. Image Processing for Spectral Analysis
In the case presented here, we have converted the fisheye lens image into an image in geographic coordinates assuming 87 km (hm) to be the altitude of the OH layer. After processing the all-sky images, from a warped image to an unwarped one, the gravity wave parameters, horizontal wavelength, period, phase velocity, and propagation direction are retrieved from successive images by knowing the size of the images transformed into the geographical coordinates and the time difference of two sequential images, respectively (Maekawa, 2000). The imager field of view with 75. zenith angle is 500 km in diameter at an altitude of 87 km.
4. Observations and Results
4.1 OH nightglow observations
4.2 OI 557.7-nm nightglow observations
Figure 4 shows the signature of gravity waves observed at Allahabad in the OI 557.7-nm images obtained on the night of April 25, 2009. These waves were prominent in the north-east portion of the sky and were moving in the north-east direction. Based on consecutive images, we inferred the phase velocity of the waves to be 50 m/s; the wavelength and periodicity of the waves were 38 km and 13 min, respectively. These values are the signature of short-period atmospheric gravity waves at an altitude of 97 km.
4.3 Wind profiles for the Allahabad station
The model describes the transition from predominantly diurnal variation in the upper thermosphere to semi-diurnal variation in the lower thermosphere and the transition from summer to winter flow above 140 km to winter to summer flow below this height. This HWM model provides the components for the zonal and meridional winds for a specified latitude, longitude, time, and Ap index. A comparison of the HWM values with winds derived from IRI parameters and from ionosonde measurements have generally been in good agreement. Figure 5 shows two components (zonal and meridional) of wind velocity computed from the HWM-93 model during the period of observation of gravity waves at 21:00, 22:00, 23:00, and 24:00 hours at Allahabad during April 2008. To examine the effect of changing time on the blocking surfaces in the diagrams, we plotted the zonal and meridional wind profile as a function of altitude at 1-h intervals between 2100 hours and 2400 hours, which is the period during which most of the gravity wave data were recorded. As expected, the zonal winds showed maximum variations below an altitude of 80 km, and minimal changes occurred above 80 km; in contrast, the meridional winds varied substantially. The maximum value for the zonal component of the wind was ∼15 m/s around 60 km, and the maximum value of the meridional component was ∼-20 m/s around 90 km.
4.4 The model for critical level blocking
The observations revealed the spatial and temporal properties for the wave, but with a distinct tendency toward preferential directions of motion which varied from month to month. The apparent asymmetry in the propagation headings of the gravity waves is due to the presence of critical layers, which can occur at any height level where the horizontal wave vector equals the horizontal phase velocity and the intrinsic frequency is Doppler-shifted to zero. A horizontal surface region can be constructed at the critical layer height to obtain a polar plot (“a blocking diagram”) showing the range of azimuthal angles and speeds of gravity waves that are blocked from further upward propagation (Ryan, 1991). Gravity waves with horizontal phase speeds and directions within this region would encounter heavy absorption from a large number of diverse and relatively unstructured mechanisms (Booker and Bretherton, 1967; Hazel, 1967; Jones, 1968; Fritts and Geller, 1976; Fritts, 1978; He et al., 1990) as they approach the critical layer. A horizontal surface can be constructed at the critical layer altitude to obtain a polar plot. Gravity waves with horizontal phase velocities outside this region would not encounter any critical layer and should be observable.
Figure 6 shows the blocking diagram for April at local midnight for the observation site. The figure was computed for a height of 85 km, which is just below the nominal peak of the OH emission (∼87 km, thickness 8 km), and the plot therefore represents the cumulative effects of wave blocking for all heights up to and including the base of the OH layer. The circular nature of the plot indicates the restricted regions for wave propagation. Added to this plot is another polar plot which represents the horizontal propagation direction of gravity waves in the north-east direction along with their magnitudes recorded during the month of April.
In March, four wave displays and four velocity measurements were made (see Table 1). Figure 7 represents the blocking diagram for the month of March. It can be seen that the waves propagate in the north-east direction for only one day; for the remaining three days of observation the propagation is in the south-west direction. On two occasions (March 2 and 4), localized gravity waves were observed. The horizontal phase velocity and horizontal wavelength could not be determined on March 4 because the image obtained was faint.
The spatial and temporal properties of the wave motions observed are typical of the low-latitude nightglow displays. These displays have occassionally been observed with a time period of <19 min and having a horizontal phase velocity <48 m/s and a horizontal wavelength <39 km. Although individual sources for the waves have been difficult to locate, there are a number of factors that satisfactorily explain the sources. There is almost a complete agreement between the image measurements and the model predictions, and between the predicted and observed directions of wave propagation. This agreement suggests that the middle atmospheric winds can play an important role in governing the flux and azimuthal propagation distribution of short-period wave energy reaching the upper atmosphere.
The two factors which govern the wave propagation direction in the mesopause region are (1) its source location in the lower atmosphere relative to the observer and (2) the background mean wind field in the stratosphere and mesosphere. Gravity waves propagating upward from the lower atmosphere are absorbed into the mean flow as they approach a critical layer where the intrinsic frequency of the wave is Doppler-shifted to zero. This situation may occur at any height when the local horizontal wind speed along the direction of propagation equals the apparent horizontal phase speed of the gravity wave. Gravity waves with horizontal phase velocities outside of this region would not meet, by chance, a critical layer and should be observable. The absence of waves toward the south-west in April indicates that there was little wave source to the north-east of the observer.
There are many sources of gravity waves. One of the most important sources suggested for the wave generation mechanism is the tropospheric convention (Taylor and Hapgood, 1988; Medeiros et al., 2004a, 2004b). As the preferential directions of wave propagation were found to be mainly towards the north and north-east in April and May at the Allahabad observation site, one can assume that the generation source of the gravity waves should be over the continent 250–300 km away in the south and south-west direction. Some of the waves could be also be produced from ducting and/or from waves generated in situ due to reflection at the mesopause. Nakamura et al. (2003) reported that the propagation direction of short-period gravity waves at the low-latitude MLT region was consistent with the spatial distribution of tropospheric clouds. Based on a study of imaging observations carried out in Indonesia, Nakamura et al. (2003) reported that the propagation direction of the observed gravity waves was closely associated with the cloud distribution and that the observed gravity waves correlated well with the horizontal distribution of GSM images. Nakamura et al. (1988) and Taylor and Hapgood (1988) also identified thunderstorms as a source of short-period gravity waves.
6. Summary and Conclusions
Atmospheric gravity waves play an important role in the dynamics and energetics of the mesosphere, thermosphere, and ionosphere. To understand these wave phenomena we carried out imaging observations with a CCD-based All-sky imager at Allahabad and Kolhapur on clear moonless nights between January and May 2008 and in April 2009. The characteristics of the horizontal propagation parameters (time period, phase velocity, and wavelength) of the gravity waves have been inferred from OH and OI 557.7-nm im-ages. The effect of attenuation or the filtering of gravity waves by neutral wind flow has been also studied.
(1) The theory of gravity wave absorption at critical layers is well-developed. Gravity waves propagating energy upward from the lower atmosphere are absorbed into the mean flow as they approach the critical layer where the intrinsic frequency of the wave is Doppler-shifted to zero. Gravity waves can propagate upwards only when the propagation direction of gravity wave is opposite to the neutral wind direction. All-sky images of the near-infrared OH emission is well suited for this study as they provide reasonable estimates of the direction of motion and the apparent speed of the waves reaching the upper atmosphere. The observations reported here show very good agreement between the observed and the permitted wave azimuths and speeds. For the month of April 2008, the blocking region was in south-west direction, and the waves were propagating towards the north-east direction. The anisotropy in propagation direction in March and April may be due to a wave-filtering effect, which is in agreement with the findings of Taylor et al. (1993). The blocking region also varied as a function of time and altitude. There is, however, an overall consistency between the observed GW propagation and the blocking diagrams. The theory of the filtering effect of a gravity wave by its absorption into the mean wind flow at a critical layer is valid only when the background wind through which wave is propagating is somewhat stable (Richardson’s number, Ri, of the background medium is greater than 1/4). When there is a steep vertical gradient of wind flow, wind profiles may become unstable, causing refraction or over-reflection of the upward propagating gravity wave (Taylor et al., 1993).
(2) As most of the wave propagation was in the north-east direction, it is possible that for few months each year the direction is reversed and waves may propagate in other directions (south-west). Thus, in order to obtain a lucid understanding of the gravity wave propagation throughout the year, we need a larger data base. It is important to have a large number of measurements of gravity waves spanning different months at the observation site, Allahabad, to confirm the findings of the current wave filter theory. This would enable the apparent change in the drift speed directions to be studied for both the summer and winter period. In future studies, it may also be possible to study the signature of gravity wave simultaneously in OI 557.7-nm emissions (∼97 km). The OI data can provide support and evidence for wave filtering by winds. Sun et al. (2007) recently studied the filter effects of the background winds on the propagation of gravity waves using a 3D transfer function model. They concluded that the gravity waves travel easily in the anti-windward direction, primarily because the propagation distance of gravity wave packets along the winds is longer than that against the winds. This leads to a higher energy loss when gravity waves are propagating along the winds than against the winds in the same altitude range. The results showed that the atmospheric winds may act as a directional filter that would permit gravity wave packets propagating against the winds to reach evene ionospheric height with a minimum energy loss and minimum travel time. Therefore, with directional filtering by the winds, the action of the atmospheric processes seems to favor those gravity wave packets that take the minimum time to reach the ionosphere. Other components of gravity waves are also filtered by the viscosity and thermal conduction of the atmosphere as well as by the background winds when the gravity waves propagate upward to higher altitudes.
The Department of Science and Technology (DST), Govt. of India, New Delhi funded the research in Upper Atmospheric Sciences in IIG. The night airglow observations at Kolhapur were carried out under the scientific collaboration program (MoU) between IIG, Panvel, and Shivaji University, Kolhapur.
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