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On the Relationship Between Gravity Waves and Tropopause Height and Temperature Over the Globe Revealed by COSMIC Radio Occultation Measurements

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On the Relationship Between Gravity Waves and Tropopause Height and Temperature Over the Globe Revealed by COSMIC Radio Occultation Measurements atmosphere Article On the Relationship between Gravity Waves and Tropopause Height and Temperature over the Globe Revealed by COSMIC Radio Occultation Measurements Daocheng Yu 1, Xiaohua Xu 1,2,*, Jia Luo 1,3,* and Juan Li 1 1 School of Geodesy and Geomatics, Wuhan University, 129 Luoyu Road, Wuhan 430079, China; [email protected] (D.Y.); [email protected] (J.L.) 2 Collaborative Innovation Center for Geospatial Technology, 129 Luoyu Road, Wuhan 430079, China 3 Key Laboratory of Geospace Environment and Geodesy, Ministry of Education, 129 Luoyu Road, Wuhan 430079, China * Correspondence: [email protected] (X.X.); [email protected] (J.L.); Tel.: +86-27-68758520 (X.X.); +86-27-68778531 (J.L.) Received: 4 January 2019; Accepted: 6 February 2019; Published: 12 February 2019 Abstract: In this study, the relationship between gravity wave (GW) potential energy (Ep) and the tropopause height and temperature over the globe was investigated using COSMIC radio occultation (RO) dry temperature profiles during September 2006 to May 2013. The monthly means of GW Ep with a vertical resolution of 1 km and tropopause parameters were calculated for each 5◦ × 5◦ longitude-latitude grid. The correlation coefficients between Ep values at different altitudes and the tropopause height and temperature were calculated accordingly in each grid. It was found that at middle and high latitudes, GW Ep over the altitude range from lapse rate tropopause (LRT) to several km above had a significantly positive/negative correlation with LRT height (LRT-H)/ LRT temperature (LRT-T) and the peak correlation coefficients were determined over the altitudes of 10–14 km with distinct zonal distribution characteristics. While in the tropics, the distributions of the statistically significant correlation coefficients between GW Ep and LRT/cold point tropopause (CPT) parameters were dispersive and the peak correlation were are calculated over the altitudes of 14–38 km. At middle and high latitudes, the temporal variations of the monthly means and the monthly anomalies of the LRT parameters and GW Ep over the altitude of 13 km showed that LRT-H/LRT-T increases/decreases with the increase of Ep, which indicates that LRT was lifted and became cooler when GWs propagated from the troposphere to the stratosphere. In the tropical regions, statistically significant positive/negative correlations exist between GW Ep over the altitude of 17–19 km and LRT-H/LRT-T where deep convections occur and on the other hand, strong correlations exist between convections and the tropopause parameters in most seasons, which indicates that low and cold tropopause appears in deep convection regions. Thus, in the tropics, both deep convections and GWs excited accordingly have impacts on the tropopause structure. Keywords: gravity waves; potential energy; tropopause; COSMIC 1. Introduction The tropopause is the transition layer between the upper troposphere and the lower stratosphere, which are distinct from one another in vertical mixing timescales, static stabilities, trace constituents, and thermal balance [1]. The variations of the tropopause, which are the responses to any changes in the physical, chemical, and thermal characteristics of the two regions, are linked closely to the stratosphere-troposphere exchange as well as climate variability and change [2–4]. Atmosphere 2019, 10, 75; doi:10.3390/atmos10020075 www.mdpi.com/journal/atmosphere Atmosphere 2019, 10, 75 2 of 15 Different definitions and concepts exist for the determination of the tropopause [5]. The thermal tropopause, which is also called the lapse rate tropopause (LRT), was defined by the World Meteorological Organization (WMO) as the lowest level at which the lapse rate decreases to 2 K·km−1 or less, provided that the average lapse rate between this level and all higher levels within 2 km does not exceed 2 K·km−1. LRT can be obtained from vertical profiles of atmospheric temperature and are applied globally, both in the tropics and in the extra-tropics [6]. The cold point tropopause (CPT), which is usually applied in the tropics, is the level of the temperature minimum as the temperature decreases with height from the surface up to certain altitude and then increases at higher altitudes in the stratosphere [7]. The CPT is an import indicator of stratosphere-troposphere coupling and exchange [2]. The variations of the tropopause height and temperature show sub-seasonal, seasonal and inter-annual variabilities [8–12] and are closely related to atmospheric waves [8,10,13–16], among which the effects of Gravity waves (GWs) [10,17,18] are significant. Gravity waves (GWs) are usually excited in the troposphere and propagate upward, transferring energy, momentum, and water vapor and depositing vertical mixing of heat [19], which affects tropopause temperature directly or indirectly [18]. GW activities play important roles in the global circulation and the temperature and constituent structures, such as water vapor, ozone concentrations, and other chemical constituents [20,21]. Although there are a number of works on the variations of the structure of tropopause [22–25] and GW activities [21,26,27], studies on the relationship between GWs and the tropopause are meager. Reference [10] investigated the structure and variability of temperature in the tropical upper troposphere and lower stratosphere (UTLS) using the Global Positioning System Meteorology (GPS/MET) data during April 1995 to February 1997. They found that much of the sub-seasonal variability in CPT temperature and height appeared to be related to GWs or Kelvin waves. Using ~114 h mesosphere-stratosphere-troposphere (MST) radar data at Gadanki, references [17,28] studied the wind disturbances, tropopause height, and inertial gravity wave (IGW) associated with a tropical depression passage, and they found that the tropopause height and IGW had similar periodograms, which clearly showed that the tropopause height was modulated by inertial GW. Reference [18] investigated the relationship between GWs and the temperature and height of CPT and water vapor over Tibet using the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) Radio Occultation (RO) temperature data during June 2006 to February 2014. Their results showed that GW potential energy (Ep), CPT temperature, and water vapor had good correlation with each other and that GWs affected the CPT temperature and water vapor concentration in the stratosphere. These works about the relationship between GWs and tropopause were mainly focused on certain geographic regions. The effects of GW activity on the tropopause structure over the globe needs further investigation. The COSMIC RO temperature profiles with high vertical resolution, high accuracy, long-term stability, and global coverage are ideal data sources to study the tropopause structures [4,25,29] and are applicable to analyze the global characteristics of GW activity [21,30,31]. In this study, we used COSMIC level 2 dry temperature (atmPrf) profiles during September 2006 to May 2013 to investigate the relationship between GWs and the tropopause height and temperature over the globe. Data and methods are introduced in Section2. The results and analyses are presented in Section3. Section4 discusses the possible underlying mechanism. Finally, conclusions are given in Section5. 2. Experiments 2.1. COSMIC RO Data The COSMIC dry temperature profile is from near the ground up to 60 km with a good vertical resolution (~1 km); however, due to the a priori information used in the inversion process and the residual ionospheric effects, it typically exhibits increased noise at upper levels [27,32]. Although COSMIC RO dry temperature data is used to analyze GW activity up to 50 km, it is indicated that Atmosphere 2019, 10, 75 3 of 15 the upper height level of the COSMIC temperature profiles most appropriate for GW study is below 40 km [27]. The vertical wavelengths of GW derived from COSMIC temperature are equal or greater than 2 km [32]. This work uses COSMIC post-processed level 2 dry temperature profiles (atmPrf files) of the version 2010.2640 from September 2006 to May 2013 produced by the COSMIC Data Analysis and Archive Center (CDAAC) of the University Corporation for Atmospheric Research (UCAR) to analyze the relationship between GWs and the tropopause. 2.2. GW Ep The potential energy (Ep) can well represent the feature of GW and is given by: 1 g 2 T0 2 Ep = (1) 2 N T g ¶T g N2 = + (2) T ¶z cP where g is the gravitational acceleration, N is the Brunt-Väisälä frequency, cp is the isobaric heating capacity, z is the height, and T, and T0 is the background temperature and the temperature perturbations caused by GWs, respectively. It is important to separate T and T0 from the raw COSMIC 0 temperature (T). The accurate Ep is based on the extraction of T , which is given by: T0 = T − T (3) We extracted Ep values from COSMIC RO temperature profiles following closely the method used by references [21,32]. At first, the daily COSMIC temperature profiles between 8 km and 38 km are gridded to 10◦ × 15◦ latitude and longitude resolution with a vertical resolution of 0.2 km, based on which the mean temperature of each grid is calculated for each height level. Then, the S-transform was used for each latitude and altitude, obtaining the zonal wave number 0–6 which represents the background temperature for zonal mean temperature. The S-transform is unique in that it provides frequency-dependent resolution while maintaining a direct relationship with the Fourier spectrum [33]. In the next step, this background temperature was interpolated back to the positions of raw COSMIC RO profiles and subtracted from T using Equation (3) to get the temperature perturbations T0. Finally, GW Ep was calculated by Equations (1) and (2). To further analyze the relationship between GW Ep and tropopause parameters, daily Ep values were binned and averaged in 5◦ × 5◦ longitude–latitude grid cells with a vertical resolution of 1 km.
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