Contrast in Lightning Activity over Land and Sea – Further Analyses of Thermodynamic Conditions

The present study is directed to test whether data over Bulgaria (land) and Black Sea (sea) confirms the established in other geographical regions difference in thermodynamic characteristics over land and sea, which may explain the land-ocean contrast in lightning activity. The territory of investigation includes maritime area (western part of the Black Sea – up to 36°E) and continental area (Bulgaria). The necessary temperature and humidity profiles are taken from GFS proximity sounding for the center of each grid box where at least one flash is recorded. Several surface and at higher levels atmospheric data, convective available potential energy (CAPE) and normalize CAPE (NCAPE) (calculated for different layers) are considered. The output of 1-D cloud model with parameterized microphysics is also used. The results indicate that the shapes of CAPE over land and sea are almost similar up to –40C; only CAPE (–40C to EL) is indistinguishably more ―elegant‖ over the sea in comparison with a ―fat‖ CAPE (–40C to EL) over the land. As a result of the difference in temperature and humidity profiles over land and sea, the simulated clouds that developed over land are more vigorous and with significantly larger amount of ice particles (ice crystals and graupel) in comparison to the simulated clouds over sea, which may explain the land-ocean contrast in lightning activity. INTRODUCTION The analysis of lightning data shows a predominance of global annual lightning activity over land versus over maritime areas Christian et al. [2003]. More detailed studies revealed that the land-sea lightning contrast has seasonal and diurnal variations [e.g. Petrova et al. 2014, Blakeslee et al. 2014]. The physical reasons for this contrast have not been definitively established yet. It is  Contact information: Rumjana Mitzeva, Faculty of Physics, University of Sofia, Sofia 1164, Bulgaria. Email: rumypm@phys.uni-sofia.bg XV International Conference on Atmospheric Electricity, 15-20 June 2014, Norman, Oklahoma, U.S.A. 2 accepted that the difference in thermodynamic conditions (thermal hypothesis) and/or difference in numbers and size of CCN-IN (aerosol hypothesis) over land and sea may be responsible for difference in lightning activity. Our previous studies [Mitzeva et al. 2011] showed a significant difference between the mean values of surface temperature, relative humidity and mixing ratio, over the territory of Bulgaria (land) and the Black Sea in the grid boxes where lightning were detected. However the difference between mean values of CAPE over Bulgaria (land) and Black Sea in the grid boxes with lightning were not well pronounced Markova [2013]. This result is in agreement with the results from other authors [see e.g. Lucas et al.1994] that the estimated values of CAPE over land and warm ocean regions show little contrast. Since CAPE depends on both the magnitude of buoyancy and the depth of the integration (from the level of free convection to the equilibrium level) one may assume that еnvironments with similar CAPE may have different degrees of instability if one environment is characterized by tall and thin CAPE and the other by short and wide CAPE. The analyses of Lucas et al. [1994] revealed that the positive area over the ocean is generally ―tall and slim‖ (i.e., CAPE is a result of a small instability, but distributed through a large fraction of the troposphere), while the positive area over the continental regions is ―short and fat‖ (i.e. CAPE is a result of large instability distributed over a shallower depth of the troposphere). Based on this it was concluded that the difference in the CAPE shape over land and sea may explain why oceanic convection has weaker vertical velocities than continental convection for environments with equivalent CAPE. To take into account the shape of CAPE Blanchard [1998] suggested to consider NCAPE which is a normalized CAPE by the depth over which the integration takes place and to consider vertically partition CAPE and NCAPE into multiple layers. However based on Halverson et al. [2002], Williams and Stanfill [2002] concluded that the shape of CAPE might not explain the regional differences in tropical lightning. Based on the theoretical consideration and observations Williams and Stanfill [2002] concluded that due to the higher relative humidity over sea, the cloud base height is systematically lower over the sea than over the land. As a result of this the thermal sizes at cloud base over the sea are smaller and the impact of mixing with the environmental air is higher. As a consequence, the clouds over the sea are with warmer cloud top, smaller depth and lower updraft velocity that are of primary importance to cloud microphysics and electrification. The present study is a continuation of the work presented in Mitzeva et al. [2011] and is focused to test whether the data over Bulgaria (land) and the Black Sea (sea) confirms i) the XV International Conference on Atmospheric Electricity, 15-20 June 2014, Norman, Oklahoma, U.S.A. 3 established by Lucas et al. [1994] and Blanchard et al. [2010] difference in the shape of CAPE over land and sea; ii) the anticipated by Williams and Stanfill [2002] impact of surface conditions on cloud base height and in turn on cloud microphysics and dynamics that are strongly related to cloud electrifications. It is worthy to note that our work is directed to establish if there is a difference in various thermodynamic characteristics of the environment over land and sea when thunderstorms developed and not to answer the question why the locations of lightning detection over land are more abundant than over sea. DATA AND METHODOLOGY OF STUDY In the present study the territory of investigation includes maritime area (western part of the Black Sea – up to 36°E) and continental area (Bulgaria). Our previous study [Mitzeva at al. 2011] revealed that during summer over Bulgaria, the maximum of flash density is in the 3-hours afternoon time interval centred at 1200 UTC. The results also showed that the difference between land-sea lightning activities is largest in these afternoon hours. For this reasons the analyses of environmental conditions are carried out for the above mentioned afternoon interval (herein it is denoted by 1200 UTC). The information about lightning for 74 days during the summer of 2006 (JJA) is provided from the ZEUS network operated by the National Observatory of Athens (NOA). The number of recorded flashes has been calculated for each grid box (0.25x0.25 degrees) in the 3-hours afternoon time interval (1200 UTC). Additionally each grid box is characterized as continental or maritime depending on the surface cover of the area it represents. It is worth mentioning that for the analyzed afternoon period the number of grid boxes where lightning was detected over Bulgaria are 1031 and over the western part of the Black Sea 164, although the analyzed maritime area is ~ 2,4 times larger than the continental area. After the exclusion of the grid boxes with calculated CAPE < 250 J kg -1 (based on the established by Boorman et al. [2010] threshold) the number of boxes over Bulgaria and Black Sea is reduced to 916 and 160 correspondingly. The necessary temperature and humidity profiles at 1200 UTC are taken from proximity sounding for the center of each grid box where at least one flash is recorded. The proximity soundings are obtained by the numerical model GFS (http://ready.arl.noaa.gov/). Several surface and at higher levels atmospheric data, CAPE and NCAPE (calculated for different layers) are considered (see Appendix A). XV International Conference on Atmospheric Electricity, 15-20 June 2014, Norman, Oklahoma, U.S.A. 4 The output of 1-D cloud model with parameterized microphysics is also used. A short description of the numerical model is given in Appendix B. The list of analyzed microphysical and thermodynamic in-cloud characteristics (obtained by model simulations) is also presented in Appendix A. The mean values, medians, and 10 th , 25 th , 75 th and 90 th percentiles of the corresponding parameters in the environment as well the values of some in-cloud characteristics (obtained by the simulation with 1-D model) over sea and land where lightning were detected are compared and discussed.