Atmospheric response to the annular solar eclipse of 26 December 2019 over Cochin, India

Highlights

• Atmospheric response to annular solar eclipse of December 2019.

• Drastic reduction in incoming solar radiation and variations in meteorological variables.

• Mean PBL and Tropopause Heights were reduced by 50% and 5.5%.

• Detected the presence of internal gravity waves with periodicities of 40–60 min.

• Study useful for radiative transfer calculation and also for testing numerical models.

Abstract

The rapid response of the atmosphere to the reduction in solar radiation during the annular solar eclipse on 26th December 2019 with maximum obscurity of 91.16% over Cochin, located in the southwest peninsular India, has been investigated. A suite of instruments encompassing an advanced VHF wind profiling Radar operating at 205 MHz, GPS radiosonde, and automatic weather station was employed to record the temporal and vertical variations in various meteorological parameters. Compared to the corresponding values averaged for two control days neighboring the eclipse day, the temperature, relative humidity, net radiation, pressure and wind speed at the surface were altered by −4.0 °C, +39.6%, −383.0 Wm⁻², +0.62 mb, and −2.6 ms⁻¹, respectively. The corresponding percentage variations are −13.6%, +71.0%, −67.0%, +0.06%, and −99.0% during the maximum phase of the eclipse, and approached close to the control-day values within about 2 h. Multiple temperature inversions as large as 1 °C were observed up to a height of about 7.3 km. The heights of mean planetary boundary layer (PBL) and tropopause were decreased by about 355 m (−50%) and 900 m (−5.5%), respectively during the peak of obscuration. Meanwhile, the vertical profiles of both the horizontal and vertical wind velocities demonstrate substantial reduction. The vertical wind dispersion, which characterizes the fluctuations in vertical air motion, was reduced by 34% in the PBL compared to its corresponding values on the control days. Subsequently, wavelet spectral analysis of the vertical wind yields a quasi-periodic oscillation indicating the presence of internal gravity waves with periodicities of 40–60 min in the lower altitudes up to 3 km during the eclipse day. Besides reporting the characteristic variations in various atmospheric parameters, the present study reveals the height-time variations in the vertical winds, a vital parameter of significance in the studies of convection, turbulence, and atmospheric waves.

Introduction

A solar eclipse is a magnificent celestial phenomenon wherein the moon occults the Sun’s disc either partially or fully, and thus, it offers the prospect of observing the changes taking place in the Earth's atmosphere as a result of the sharp reduction in the incoming solar radiation. The reduction of solar irradiance depends on the extent to which the lunar occultation obstructs the sun’s direct rays (Dani and Devara, 2002). Subsequently, the direct effect of fluctuations in irradiance will be manifested in the form of changes in meteorological parameters and photochemical as well as dynamical processes. A multitude of changes in the atmospheric state variables and physical processes during partial or total solar eclipses have been reported extensively over the past several decades (Mohanakumar and Devanarayanan, 1982, Niranjan et al., 1997, Zanis et al., 2007, Muraleedharan et al., 2011, Ratnam et al., 2012, Reddy et al., 2020, and references therein). Gerasopoulos et al. (2008) provided an overview of the total solar eclipse on March 2006 that had significant effects right from the planetary boundary layer (PBL) through the Ionosphere in terms of photochemistry, air quality, thermal fluctuations in the ozone layer (gravity waves), electron concentration, reflection heights, etc. Kazadzis et al. (2007) had investigated the spectral effect of the limb darkening to the global direct irradiance and actinic flux during a total solar eclipse and found a much-pronounced decrease in the radiation at the lower wavelengths. Earlier studies had focused on eclipsed-induced changes in total ozone concentration as well. Some of them had reported a decrease in total ozone concentration (e.g., Devanarayanan and Mohanakumar, 1982, Chakrabarty et al., 1997, Dani and Devara, 2002, Nishanth et al., 2011) while some others observed different results (Stranz, 1961, Bojkov, 1968, Imai et al., 2015). Variations observed in atmospheric aerosol concentration and size distributions during eclipses are also reported. Dani and Devara (2002) had observed an enhancement in the aerosol optical depth (AOD), attributed primarily to an increase in relative humidity and weakening of atmospheric turbulence. Based on a multiwavelength radiometer observation of spectral AODs, Niranjan and Thulasiraman (1998) deduced that the aerosol size distribution got modified into a monomodal distribution during the total solar eclipse of 24 October 1997 as compared to a bimodal distribution, ascribed to the changes in the surface meteorology.

In addition to exploring the changes in the mesospheric and stratospheric chemistry, there had been attempts to examine the changes in the planetary boundary layer (PBL) and meteorological parameters (e.g., Gerasopoulos et al., 2008). Ground based lidar observation of the PBL height over Greece showed a decrease during the March 2006 eclipse phase, indicating suppression of turbulence activity similar to that during the sunset hours (Amiridis et al., 2007). Another study over Greece reports a dramatic reduction in the incoming global radiation and subsequent significant changes in surface meteorological parameters such as air temperature and wind speed (Founda et al., 2007). A partial solar eclipse occurred in South Korea in July 2009 provided an opportunity to record a maximum of 89.9% reduction in solar radiation and associated changes in surface temperature, relative humidity, wind speed, and ground-level ozone (Chung et al., 2010). Chimonas (1970) estimated the amplitude of the internal gravity waves generated by the moon’s shielding of the solar heating and demonstrated it to form a bow wave about the shadow region. As the lunar shadow sweeps at supersonic speed across the Earth, the cooling spot acts as a continuous source of gravity waves that build up into bow waves (Chimonas and Hines, 1971). Using a VHF Mesosphere-Stratosphere-Troposphere (MST) Radar, Ratnam et al. (2012) demonstrated a reduction in mesospheric echoes and the occurrence of high frequency gravity waves (30–60 min) in the troposphere and mesosphere. There is observational evidence of gravity wave event in the stratosphere, mesosphere, and ionosphere reported over the South-Central United States (Azeem et al., 2015). A wide range of surface and near-surface meteorological observations were made during the 2015 UK partial solar eclipse as reported by Burt (2016). This study also reported a large reduction in the solar radiation, decrease in surface temperature and cloud base height, and a slight increase in atmospheric stability during the eclipse.

Approaches based on modelling of the internal gravity waves also have been carried out in the past. Founda et al. (2007) had reported the atmospheric response of the solar eclipse on 29 March 2006 by employing a Weather Research and Forecasting (WRF) Model. The model sensitivity study could simulate the strong anomalies in surface air temperature which was consistent with observations. In another study (Dang et al., 2018), the researchers investigated the global upper atmospheric response to the Great American Solar Eclipse that occurred on 21 August 2017 by employing a high-resolution coupled ionosphere-thermosphere-electrodynamics model. Their study showed a global response rather than local influence in the form of large-scale travelling atmospheric disturbances. Mahmood et al. (2020) used the WRF Model to simulate the effects of the above solar eclipse with and without the eclipse in their model. The model results were in good agreement with the observation for the reduction in solar radiation and surface air temperature. Buick et al. (1998) modelled the internal waves at the interface between two immiscible fluids by incorporating gravitational interactions. Standing internal waves are found to oscillate under the action of the gravitational interaction.

Similar to the published works reported in the literature, an observational portrayal of the immediate response of the tropopause to the drastic reduction of insolation during the annular solar eclipse of 26 December 2019 is provided in this paper. Section 2 details the solar eclipse, the experimental set-up, and data characteristics. Section 3 portrays the key results from the observations, followed by a summary and conclusion in Section 4.