Retrieval of the vertical temperature profile of atmosphere from MST radar backscattered signal

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1 Indian Journal of Radio & Space Physics Vol. 35, August 6, pp Retrieval of the vertical temperature profile of atmosphere from MST radar backscattered signal I M L Das 1, & Pramod Kumar 1 M N Saha Centre of Space Studies, University of Allahabad, Allahabad 11, India Department of Physics, University of Allahabad, Allahabad 11, India drimldas@yahoo.com Received 8 September 4; revised 1 February 6; accepted February 6 The vertical temperature profile of the tropical lower atmosphere has been reconstructed from the backscattered signal received by Indian MST radar located at Gadanki (13.47 N, 79. E). The reconstructed temperature profile compares well with the altitude variation of temperature obtained from the simultaneously launched radiosonde flight. Keywords: Vertical temperature profile, MST radar, Tropical lower atmosphere, Backscattered signal PACS No.: 9.6.Ry; 9.6.Ta; 9.6.Ek IPC Code: G1S13/95; G1S13/ 1 Introduction The VHF radar is an excellent ground based tool to explore the dynamics of the lower and middle atmosphere. Its capability to measure all the three components of velocity with significantly good accuracy on time scales of few tens of seconds has made it particularly very useful for the investigation of atmospheric stability and turbulence structures. The knowledge of the variability of atmospheric temperature is very important for the study of atmospheric stability and turbulence structures. Gage and Green 1 presented a technique for estimating temperature from vertically looking VHF radar data. Rottger successfully demonstrated that MST radar data could be used to obtain temperature profile of the lower atmosphere as it provides high altitude resolution data of vertical winds on a continuous basis in troposphere and lower stratosphere. The method is primarily based on the identification of Brunt-Vaisala frequency in the spectra of vertical wind velocity oscillations. Following this, Revathy et al. 3 had derived temperature profile from the observed Brunt- Vaisala frequency. In the present paper, a method has been demonstrated to reconstruct the vertical temperature profile of the lower atmosphere directly from the height profile of radar backscattered echo power. The advantage of the present method is that it can yield a temperature profile using average SNR over a period of about min, whereas Revathy et al. 3 method requires about 1.5 h observations to obtain the altitude profile of Brunt-Vaisala frequency which is subsequently used for reconstructing the temperature profile. MST radar system and data base The Indian Mesosphere-Stratosphare-Troposphere (MST) radar at Gadanki (13.47 N, 79. E) is a pulse coded coherent VHF phased array radar, operating at 53 MHz with an average power aperture product of Wm and a vertical resolution of 15 m. The radar backscattered echo in vertical direction is available from an altitude of ~3.75 km to altitudes ranging from ~5 to 31 km. Rao et al. 4 have described in detail the Indian MST radar system. The Indian MST radar is routinely operated daily for about 45 min between 163 and 1715 hrs IST close to the radiosonde launch time from Chennai (13.1 N, 8. E) in a standard mode called common mode observations (CMO). The CMO is aimed at generating a long-term database for various scientific studies. Chennai is the nearest radiosonde observing station of India Meteorology Department (IMD). A campaign for simultaneous observations by MST radar and radiosonde flights was carried out at National MST Radar Facility (NMRF), Gadanki (renamed as National Atmospheric Research Laboratory), during the period 19 July-14 Aug The CMO data obtained during this campaign have been used. During the campaign period, balloons were launched from the radar site at 163 hrs LT.

2 DAS & PRAMOD KUMAR: VERTICAL TEMPERATURE PROFILE FROM MST RADAR BACKSCATTERED SIGNAL 81 These radiosonde observations are used to validate the temperature profile obtained from MST radar data. 3 Methodology for estimating temperature The backscattered signal power received by the radar is related to the power reflection coefficient of the probing volume and radar parameters 1 as α Pt Ae Pr = ρ 4λ r (1) where α is the efficiency factor, P t is the peak transmitted power, λ the radar wavelength, A e effective area of the antenna and r the range of the target. The power reflection coefficient ρ of MST radar can be described 5 in terms of the mean radio refractive index gradient ( M ) as ( ) ( λ) ( ) Ζ -Ζ 1 ρ = F r M e 4 ( λ) Η, F 1 ( λ ) = F r () where Z o and H are the reference altitude and scale height, respectively. The parameter F ( λ) for Indian MST radar (λ=5.66 m) is estimated as The atmospheric pressure can be expressed as ( zo z)/ H o P = P e (3) In upper troposphere, the humidity content is almost absent and the atmosphere can be considered as dry and M can be expressed as 6 P Τ M = ( ) + Γ d Τ Ζ (4) where P is atmospheric pressure in mbar, T the absolute temperature and Γ is dry adiabatic lapse rate. The expression for range corrected echo S = Ρ r can be obtained using power v ( r ) Eqs (1)-(4) as 6 d Τ Γ d + 3 Z ln ( S v ) ln Ζ = ς+ + Τ Η where ς = ln BP and 6 α Ae F( λ)( r)( ) = Pt ( Ζ ) (5) B. 8λ The constant ς depends on meteorological conditions as well as on radar parameters. At the reference altitude, Ζ = Ζ, we have Τ ς = ln S v ln Γ + + ln Z o Ζ ( ) d ( Τo ) (6) Thus, the constant ς can be estimated by taking the appropriate values of the meteorological parameters T T, at the reference height Ζ. Once the value of Z ς is known, we get a non-linear first order differential equation in Τ which can be solved to estimate temperature for each range-bin above and below the reference height Z numerically. The estimated temperature T depends critically on the constant ς. The constant ς depends on the temperature and its gradient at the reference height Z apart from other parameters. But it is very difficult to know their exact values as they depend on the instantaneous local T weather conditions. Any deviation in results in a Z small deviation in ς which further produces a sharp deviation in the estimated temperature. Thus, even a small deviation in the value of ς can lead to large error in the estimated temperature, T. The backscattered signal power received by the Indian MST radar has been used by Rao and Jain 6 to determine the height of the tropical tropopause and by Revathy et al. 3,7,8 for the deduction of temperature profile using the reference atmospheric model for Indian equatorial zone as developed by Sasi 9. The reference atmospheric model of Sasi 9 is actually the revised version of the model proposed by Sasi and Sen Gupta 1. In the height region of our interest both these models are identical.

3 8 INDIAN J RADIO & SPACE PHYS, AUGUST 6 The focus of the present study is to get a meaningful temperature profile near the stable layer of tropopause and in the lower stratosphere. We arbitrarily fix the reference height (one below the tropopause and one in the lower stratosphere) and corresponding temperatures using the model of Sasi and Sen Gupta 1. The model is also used to find the T value of at the reference height. The value of ς Z is adjusted by successive reiteration method till the estimated temperature profile passes through the temperatures corresponding to the chosen reference heights. This gives the correct temperature profile. Figure 1 shows the average vertical temperature profiles for the months of July and August from Sasi and Sen Gupta model 1. When this model was used to find the temperature, the estimated temperature from MST radar was found to be high by 4 K below the tropopause as compared to the simultaneous observations of temperature from radiosonde flights as shown in Fig.. The reference atmosphere for Indian equatorial zone was evolved using IMD balloon data from surface to 17 km for stations Chennai, Port Blair, Trivandrum and Minicoy and M-1 rocket data from 17 to 8 km for Thumba. Since the mean temperature differences between Trivandrum and other IMD stations were found to be less than K, only the temperature data over Trivandrum was used to evolve the model 9. The aerial distance between Trivandrum (85 N, 77 E) and Gadanki is more than 55 km. This may be one of the possible reasons for the difference between the estimated temperature from MST radar and temperature inferred from simultaneous observations of radiosonde flights. Therefore, the radiosonde temperature profile available at a nearby IMD station of Chennai, which is located 1 km south-east of the radar site, has been used. The average height profile of temperature as obtained from the Chennai radiosonde observations for the months of July and August is also shown in Fig. 1 for comparison. Use of meteorological parameters corresponding to Chennai station gives a very good agreement between the estimated temperature from MST radar and temperature inferred from simultaneous observations of radiosonde flights as shown in Fig.. Thus, application of the meteorological parameters corresponding to Chennai radosonde station is more appropriate for obtaining the vertical temperature profile of the tropical lower atmosphere from Indian MST radar. Therefore, the average height profile of temperature as obtained from the Chennai radiosonde observations has been used for this study. Fig. 1 Average vertical temperature profiles from Sasi- Sen Gupta model and Chennai radiosonde observations for the month of July and August Results and discussion The vertically backscattered echo power was analyzed using atmospheric data processor (ADP) software developed by NMRF. The analysis involves (i) removal of interference, if any, (ii) incoherent integration and (iii) computation of three low order moments. These three moments represent the received signal power, the weighted mean Doppler shift and half power width of the Doppler power spectrum. Figure 3 shows the comparison of MST radar retrieved temperatures with the radiosonde measured temperatures on different days during the campaign period. From Fig. 3 it is clear that radar retrieved temperature matches fairly well with the radiosonde measured profiles on fair weather days, such as, 6 and 8 July. On other days, there are some discrepancies between the radar retrieved and

4 DAS & PRAMOD KUMAR: VERTICAL TEMPERATURE PROFILE FROM MST RADAR BACKSCATTERED SIGNAL 83 Fig. Temperature estimated from radar using Sasi-Sen Gupta model, Chennai radosonde observations and simulataneous radiosonde flights from Gadanki ( Sasi-Sen Gupta model; Chennai radiosonde data; Gadanki radiosonde observations). radiosonde measured profiles when the sky is cloudy or overcast. The temperature measured by the radiosonde is point observation, whereas radar retrieved temperature is an integrated profile over a time. Therefore, this discrepancy between the radar retrieved and radiosonde measured temperatures is expected, and depending on the local weather conditions, it varies by 1- C. Above the tropopause, minor deviations are seen between the temperature profiles deduced from MST radar data and that observed from radiosonde. This may be due to the comparatively poor quality of data received by the MST radar from the altitudes above the stable layer of tropopause and the differences in the resolution of MST radar and radiosonde observations. However, it may be pointed out that similar trends can also be seen in the temperature profile obtained by Gage and Green 1 and Mohan et al. 11 using completely different techniques. The profile can be continued to lower altitudes simply by assuming a uniform lapse rate between the surface and the tropopause 1. The method outlined in the present study gives good temperature profile in the height range 1- km. In the height range below ~1 km the discrepancy between the radar retrieved and radiosonde measured temperatures is more pronounced. This is because the present assumption of dry atmosphere is not valid in this height range. The height range below 1 km is the most difficult one, as humidity plays an important role in this height range. It is intended to include humidity in future calculations for deducing temperature profile from surface up to 1 km altitude.

5 84 INDIAN J RADIO & SPACE PHYS, AUGUST 6 Fig. 3 Reconstructed temperature profiles from MST radar ( radar; radiosonde).

6 DAS & PRAMOD KUMAR: VERTICAL TEMPERATURE PROFILE FROM MST RADAR BACKSCATTERED SIGNAL 85 Acknowledgements The NMRF (now re-named as NARL) is a joint venture between Council of Scientific and Industrial Research (CSIR), Defence Research and Development Organization (DRDO), Department of Environment and Department of Space of the government of India. The authors express their thanks to NMRF for providing the data, to UGC-SVU Centre for MST Radar Applications and to SV University, Tirupati (A.P.) for help in radar data analysis and other related works. The IMD, Pune, provided the Chennai radiosonde data. This work was carried out under research project grant funded by Indian Space Research Organization. The constructive suggestions provided by the anonymous referees is thankfully acknowledged. References 1 Gage K S & Green J L, A technique for determining the temperature profile from VHF radar observations, J Appl Meteor (USA), 1 (198) Rottger J, Determination of Brunt-Vaisala frequency from vertical velocity spectra, MAP Handbook, Vol., 1986, pp Revathy K, Prabhakaran Nair S R & Krishna Murthy B V, Deduction of temperature profile from MST radar observations of vertical wind, Geophys Res Lett (USA), 3 (1996) Rao P B, Jain A R, Kishore P, Balamuralidhar P, Damle S H & Vishwanathan G, Indian MST radar: Part 1 System description and sample vector wind measurements in ST mode, Radio Sci (USA), 3 (1995) Gage K S, Ecklund W L & Balsley B B, A modified Fresnel scattering model for the parameterisation of Fresnel returns, Radio Sci (USA), (1985) Jaya Rao Y & Jain A R, Observations of tropical tropopause using Indian MST radar, Proceedings of 9 th International Workshop on Technical & Scientific Aspects of MST Radar (MST9), Toulouse, France,, pp Revathy K, Prabhakaran Nair S R & Krishna Murthy B V, Estimation of error in the determination of temperature using MST radar, Indian J Radio & Space Phys, 7 (1998) Revathy K, Prabhakaran Nair S R & Krishna Murthy B V, Diurnal variation of tropospheric temperature at a tropical station, Ann Geophys(France), 19 (1) Sasi M N, A reference atmosphere for the Indian equatorial zone, Indian J Radio & Space Phys, 3 (1994) Sasi M N & Sen Gupta K, A reference atmosphere for the Indian equatorial zone from surface to 8 km, Sci Rep, SPL:SR:6:85 (available from Space Physics Laboratory, Vikram Sarabhai Space Centre, Trivandrum 695 ), Mohan K, Narayana Rao D, Narayana Rao T & Raghavan S, Estimation of temperature and humidity from MST radar observations, Ann Geophys (France), 19 (1) 855.