Eswar Sunkara

Abstract We have utilized the Gadanki MST Radar and Rayleigh LIDAR to understand the vertical coupling between the lower atmosphere and mesosphere through the short-period gravity waves (GWs). The short-period GWs (20 min–2 h) are noticed both in the troposphere and in the mesosphere during the deep convection. During the convection, the large vertical velocities (>5 m/s) and significant variations in the momentum flux (∼3 m2/s2) are noticed in the troposphere and higher fluxes (∼45 m2/s2) are evidenced in the mesosphere. The observations suggest the vertical coupling between the lower and middle atmosphere during convection.

ABSTRACT Using long-term data (January 1998 to December 2009) collected from Mesosphere-Stratosphere-Troposphere (MST) radar located at a tropical station, Gadanki (13.5°N, 79.2°E), India, variability of low-latitude mesospheric vertical wind is investigated for the first time. The emphasis is on sub-daily, monthly, seasonal and annual variabilities in vertical wind in the altitude region 65-85 km. Possible sources of errors in the vertical wind measurement at mesospheric altitudes from the MST radar are discussed. The observed mesospheric vertical wind is generally upward in all the seasons. During winter and summer, significant sub-daily variations are noticed followed by spring and fall equinoxes. The vertical wind can reach occasionally values as high as 5 ms-1 but most of the time (95%) it is, in general, less than ˜2.63 ms-1. The present observations are consistent with the general circulation features in recent models for low latitude locations with northward and upward wind prevailing throughout the year representing part of meridional circulation.

ABSTRACT In the present study, we mainly tested two existing hypotheses linked with the solar eclipse effect on the middle atmosphere. One is on the VHF radar mesospheric echo occurrence characteristics and another on the wave generation and propagation. Significant reduction in the echo occurrence is found during the maximum epoch of the eclipse as expected. Estimations on D-region electron density during the eclipse day and on a normal day are also made. High frequency gravity waves (30 min–1 h) in the vicinity of the eclipse path are detected in the troposphere and mesosphere, which is not expected.

The recent communication satellite systems tend to employ higher frequency (18-60 GHz) bands to satisfy the growing capacity requirements. Such wide bandwidths are valuable in supporting applications such as high speed data transmission and video distribution. The attenuation of Satellite signals due to water vapor absorption is very essential for high frequency (>10GHz) satellite communication. Using the formulae referred in Recommendations ITU-R, P.676-5, predictions of specific attenuation and path attenuation due to water vapor absorption are calculated and presented and also compared with the water vapor attenuation estimated from Radiosonde data collected from the India Meteorological Department (IMD), for the first time over tropics. The important findings of the current study includes the observation of slant path attenuation value, which is high for frequencies like 20 GHz, 24 GHz, 26 GHz, 47 GHz and () low for frequencies 18 GHz, 16 GHz, 14 GHz, and 12 GHz etc. It is also observed that for low elevation angles the slant path attenuation is maximum and its value decreases with the increase of elevation angle. For the 1200 GMT similar results are obtained indicating that there is significant diurnal change in the observed attenuation.

In the present study, the effect of total solar eclipse, occurred on 22 July 2009, on water vapour in the troposphere, refractivity and temperature in the troposphere and the stratosphere using the observations available from COSMIC GPS RO, is reported. The investigation is extended to the entire middle atmosphere using SABER aboard TIMED satellite to study the response in the temperature and ozone. A significant enhancement in the water vapour and the refractivity in the lower and middle troposphere are noticed on the eclipse day when compared to non-eclipse days. Using the GPS RO observations, it is also found that the temperature responds differently at different altitudes, i.e. cooling in the troposphere and warming in the stratosphere. Similar features in temperature are also noticed in SABER observations below 40 km. Above 40 km altitude, cooling is observed up to an altitude of 70 km, therein again warming is noticed. An increase in ozone concentration is found throughout the middle atmosphere except near 30 km. Tropopause altitude is also observed to vary significantly during the solar eclipse with decrease (increase) in the altitude (temperature) of about 1-1.5 km (3-5 K). Large perturbations in the temperature, due to gravity waves in the stratosphere and the mesosphere, are noticed on the eclipse day and found westward propagating as expected. For the first time, evidences of solar eclipse in the entire lower and middle atmosphere is presented using ground based and satellite borne observations.

In the present study, the effect of total solar eclipse, occurred on 22 July 2009, on water vapour in the troposphere, refractivity and temperature in the troposphere and the stratosphere using the observations available from COSMIC GPS RO, is reported. The investigation is extended to the entire middle atmosphere using SABER aboard TIMED satellite to study the response in the temperature and ozone. A significant enhancement in the water vapour and the refractivity in the lower and middle troposphere are noticed on the eclipse day when compared to non-eclipse days. Using the GPS RO observations, it is also found that the temperature responds differently at different altitudes, i.e. cooling in the troposphere and warming in the stratosphere. Similar features in temperature are also noticed in SABER observations below 40 km. Above 40 km altitude, cooling is observed up to an altitude of 70 km, therein again warming is noticed. An increase in ozone concentration is found throughout the middle atmosphere except near 30 km. Tropopause altitude is also observed to vary significantly during the solar eclipse with decrease (increase) in the altitude (temperature) of about 1-1.5 km (3-5 K). Large perturbations in the temperature, due to gravity waves in the stratosphere and the mesosphere, are noticed on the eclipse day and found westward propagating as expected. For the first time, evidences of solar eclipse in the entire lower and middle atmosphere is presented using ground based and satellite borne observations.

A minor stratospheric sudden warming (SSW) event was noticed in the southern hemisphere (SH) during September (day 259) 2010 along with two episodic warmings in early August (day 212) and late October (day 300) 2010. Among the three warming events, the signature of mesosphere response was detected only for the September event in the mesospheric wind dataset from both meteor radar and MF radar located at King Sejong Station (62°S, 59°W) and Rothera (68°S, 68°W), Antarctica, respectively. The zonal winds in the mesosphere reversed approximately a week before the September SSW event, as has been observed in the 2002 major SSW. Signatures of mesospheric cooling (MC) in association with strato-spheric warmings are found in temperatures measured by the Microwave Limb Sounder (MLS). Simulations of specified dynamics version of Whole Atmosphere Community Climate Model (SD-WACCM) are able to reproduce these observed features. The mesospheric wind field was found to differ significantly from that of normal years probably due to enhanced planetary wave (PW) activity before the SSW. From the wavelet analysis of wind data of both stations, we find that strong 14–16 day PWs prevailed prior to the SSW and disappeared suddenly after the SSW in the mesosphere. Our study provides evidence that minor SSWs in SH can result in significant effects on the mesospheric dynamics as in the northern hemisphere.

We have investigated the coupling between the stratosphere and mesosphere–lower thermosphere (MLT) in the Southern Hemisphere (SH) during 2010 minor sudden stratospheric warmings (SSWs). Three episodic SSWs were noticed in 2010. Mesospheric zonal winds between 82 and 92 km obtained from King Sejong Station (62.22°S, 58.78°W) meteor radar showed the significant difference from usual trend. The zonal wind reversal in the mesosphere is noticed a week before the associated SSW similar to 2002 major SSW. The mesosphere wind reversal is also noticed in " Specified Dynamics " version of Whole Atmosphere Community Climate Model (SD-WACCM) and Ground-to-top-side model of Atmosphere and Ionosphere for Aeronomy (GAIA) simulations. The similar zonal wind weakening/rever-sal in the lower thermosphere between 100 and 140 km is simulated by GAIA. Further, we observed the mesospheric cooling in consistency with SSWs using Microwave Limb Sounder data. However, the GAIA simulations showed warming between 130 and 140 km after few days of SSW. Thus, the observation and model simulation indicate for the first time that the 2010 minor SSW also affects dynamics of the MLT region over SH in a manner similar to 2002 major SSW.

In the present study, we mainly tested two existing hypotheses linked with the solar eclipse effect on the middle atmosphere. One is on the VHF radar mesospheric echo occurrence characteristics and another on the wave generation and propagation. Significant reduction in the echo occurrence is found during the maximum epoch of the eclipse as expected. Estimations on D-region electron density during the eclipse day and on a normal day are also made. High frequency gravity waves (30 min–1 h) in the vicinity of the eclipse path are detected in the troposphere and mesosphere, which is not expected.

This short communication reports on an attempt which has been made to study mesospheric echo occurrence duration for the first time, using a large data base (1998 to 2004) collected from MST radar located at Gadanki (13.5 • N, 79.2 • E), a tropical station in India. It is well known that VHF radar echoes in the mesosphere are due to both the presence of irregularities with scales of half the radar wavelengths, and also the production and loss rates of electrons in the lower D-region. The main aim of the present paper is to estimate the duration of mesospheric echoes at each altitude and to establish the preference of their occurrence, if any, in altitude and time. In general, mesospheric echoes show tilted layers, a sporadic nature above/below the main layer (70–80 km), and a seasonal shift in the height of occurrence. Based on the time and height of echo occurrence, we broadly divide the periods of mesospheric echo occurrence (duration) into 5–20 minutes (T1), 20–40 minutes (T2), and ≥40 minutes (T3), and the height of occurrence into 65–70 km (R1), 70–80 km (R2), and 80–85 km (R3). It is observed that long duration echoes (T2 and T3) occur mostly in the R2 region and are highly sporadic in the R1 and R3 regions. In addition, the solar zenith angle dependence on the duration of mesospheric echoes is also studied and no significant variation was found in any of the above-mentioned categories. This study will contribute to a better understanding of the dynamical aspects of the mesosphere using VHF radar observations.

In the present study, the effect of total solar eclipse, occurred on 22 July 2009, on water vapour in the troposphere, refractivity and temperature in the troposphere and the stratosphere using the observations available from COSMIC GPS RO, is reported. The investigation is extended to the entire middle atmosphere using SABER aboard TIMED satellite to study the response in the temperature and ozone. A significant enhancement in the water vapour and the refractivity in the lower and middle troposphere are noticed on the eclipse day when compared to non-eclipse days. Using the GPS RO observations, it is also found that the temperature responds differently at different altitudes, i.e. cooling in the troposphere and warming in the stratosphere. Similar features in temperature are also noticed in SABER observations below 40 km. Above 40 km altitude, cooling is observed up to an altitude of 70 km, therein again warming is noticed. An increase in ozone concentration is found throughout the middle atmosphere except near 30 km. Tropopause altitude is also observed to vary significantly during the solar eclipse with decrease (increase) in the altitude (temperature) of about 1-1.5 km (3-5 K). Large perturbations in the temperature, due to gravity waves in the stratosphere and the mesosphere, are noticed on the eclipse day and found westward propagating as expected. For the first time, evidences of solar eclipse in the entire lower and middle atmosphere is presented using ground based and satellite borne observations.

Objective: It is theoretically observed that atmospheric gravity waves play a key role in vertical coupling during the Mesosphere Temperature Inversion (MTI). Therefore, the present paper describes the observational evidence for vertical coupling between the stratosphere and mesosphere through the short-period gravity waves (GWs), during the Mesosphere Temperature Inversion (MTI) over a tropical region, Gadanki (13.5 o N, 79.2 o E), India. Method: The combined observations of Mesosphere-Stratosphere-Troposphere (MST) Radar and Rayleigh LIDAR located at Gadanki is utilized to study the vertical coupling. We used a unique experimental design from the two ground-based instruments that scan the lower and middle atmosphere simultaneously during the observational campaign. This kind of combined instruments are very sparsely located on the same site to make the observations unique to understand the vertical coupling processes of GWs. Result: The vertical flux of the horizontal momentum of GWs of periods in the range 20 min. to 2h is investigated in the meso-sphere using the MST Radar winds. The emphasis is made on the variability of zonal and meridional momentum fluxes in the mesosphere and possible reasons for the variability of fluxes during MTI. It is observed that raise in momentum fluxes of ~7 m 2 / s 2 in the eastward flux and ~10 m 2 /s 2 in southward flux at mesospheric altitudes during the MTI. Conclusion: The gravity wave (GW) analysis using the LIDAR temperature profiles indicate the connection between GW breaking at mesosphere altitudes and temperature inversion and thus the turbulence caused mesospheric echoes. The study suggests the prospect of coupling between stratosphere and mesosphere during the MTI.

A B S T R A C T Synoptic-scale systems like cyclones can generate broad spectrum of waves, which propagate from its source to the middle atmosphere. Coupling between the lower and middle atmosphere over Tirupati (13.6 N, 79.4 E) is studied during a very severe cyclonic storm 'Madi' (06–13 December 2013) using Weather Research and Forecast (WRF) model assimilated fields and simultaneous meteor radar observations. Since high temporal and spatial measurements are difficult to obtain during these disturbances, WRF model simulations are obtained by assimilating conventional and satellite observations using 3DVAR technique. The obtained outputs are validated for their consistency in predicting cyclone track and vertical structure by comparing them with independent observations. The good agreement between the assimilated outputs and independent observations prompted us to use the model outputs to investigate the gravity waves (GWs) and tides over Tirupati. GWs with the periods 1–5 h are observed with clear downward phase propagation in the lower stratosphere. These upward propagating waves obtained from the model are also noticed in the meteor radar horizontal wind observations in the MLT region (70–110 km). Interestingly, enhancement in the tidal activity in both the zonal and meridional winds in the mesosphere and lower thermosphere (MLT) region is noticed during the peak cyclonic activity except the suppression of semi-diurnal tide in meridional wind. A very good agreement in the tidal activity is also observed in the horizontal winds in the troposphere and lower stratosphere from the WRF model outputs and ERA5. These results thus provide evidence on the vertical coupling of lower and middle atmosphere induced by the tropical cyclone.