Mesoscale Convective Systems as a source of electromagnetic signals registered by ground-based system and DEMETER satellite

registered by ground-based system and DEMETER satellite Karol Martynski, Jan Blecki, Roman Wronowski, Andrzej Kulak, Janusz Mlynarczyk, Rafal Iwanski Department of Electronics, AGH University of Science and Technology, Kraków, Poland Space Research Centre PAS, Warsaw, Poland Satellite Remote Sensing Department Institute of Meteorology and Water Management National Research Institute Cracow, Poland Correspondence to: Karol Martynski (karol.martynski@agh.edu.pl)


Results
From the South-West, Poland was covered by a warm tropical air masses. Their advection over colder polar maritime air caused the occurrence of a significant thermal contrasts between Western and Eastern Europe. For instance temperature difference in Benelux and Eastern Germany and Poland, was larger than 15°C. Furthermore due to temperature discrepancy in tropospheric layers a jet-stream had occurred in the middle troposphere (700 hPa) with the air flow around 15-25 m/s. Thus conditions were favourable for wind-shear to occur, which is vital for thunderstorm development. Over Southern Sweden, a local low is noticeable, that caused flow of the cold front from the Western Europe. Around 19 UTC over the Lower Silesia (Polish voivodeship) a derecho had occurred, well known for strong wind gusts often exceeding 40 m/s, the  Vertical wind shears support the separation of the updrafts from downdrafts, moreover they support processes that are responsible for development of the multicellular thunderstorms. Discussed air flow in the middle troposphere allowed the whole system to move with relatively high velocity. The convergence in the lower troposphere let the Bow Echo to form, which manifested as a squall line. Last, but not least, another significant condition for MCS development is the advection of the cold air mass from the Western regions of Europe. Thermodynamic conditions, which appeared over South-Western Poland, additionally confirm development of the strong convective phenomena. The data from atmospheric soundings show high temperature level at the ground layer (over 30°C), with dew temperature at 22°C. A strong air flow from the West is visible in the whole troposphere. Thermodynamic indicators such as CAPE (Convective Available Potential Energy) 2500 J/kg or CIN (Convective Inhibition) -100 J/kg indicates strong convective processes. A significant drying of air appears, then a dry adiabatic gradient is noticeable in the middle troposphere. Small inversion layer (CIN) favours "the gathering" of the energy beneath it, when convection is strong enough it is possible to break through the inversion, which 3 65 70 75 80 85 90 directly leads to the intensification of the convection processes. Then tropopause is penetrated by the convection and an overshooting top may appear in the lower parts of the stratosphere (Fig. 1). Apart from the discussed thermodynamic parameters, SBCAPE (Surface Based Convective Available Potential Energy) is significant. The parameter indicates the convection in the surface layer, in this case it exceeded 2500 J/kg. Furthermore DCAPE (Downward Convective Available Potential Energy) is available, which is the potential of downdrafts -1077 J/kg. Wind shear parameter in 0-6 km was higher than 20 m/s, whereas in 0-3 km the parameter was equal to 13 m/s. A significant development of the thunderstorm phenomenon is visible by the measurements of the cloud reflectivity. In many parts of the MCS a level that exceeds 50 dBZ is distinguishable that indicates strong convective processes, which supplied cloud development at the level higher than 15 km over the ground level.
Strong atmospheric discharges stem from significant MCS development. In the period of the highest thunderstorm activity 24 +CG (Cloud-to-Ground), 322 -CG and 2836 IC (Intracloud) discharges were detected (Fig. 2).
Additionally we provide data for other periods where a significant amount of discharges occurred (Table. 1   During the analysis of the DEMETER data for the whole lifespan of the discussed MCS, we have encountered a signature of a whistler -a characteristic type of waves that occurs in VLF frequency range. The whistlers are a cold plasma waves in the frequency range from the ion cyclotron up to the electron plasma frequency or electron cyclotron frequency.
These waves are common in space around Earth and may be registered in the ionosphere and the magnetosphere, by the satellite onboard receivers as well as by the ground-based systems. The characteristic shape of whistler's spectrum with falling frequency in time is a result of its dispersion feature and propagation. The group velocity is greater for waves with higher frequencies than for lower ones. The whistlers propagate along magnetic field lines from the site of the thunderstorm.
The arrival of the lower frequency waves is delayed in relation to higher frequency (Helliwell, 1965;Hayakawa, 1995). Figure 3 presents a whistler that has been detected by DEMETER overpass, which was 287 km away from the causative lightning stroke. Its precise location was provided by PERUN. During that time an impulse was caught by ground-based systems, which detected an impulse slightly ahead than the satellite, PERUN detected a signal at 20:05:49.13 and classified it as a +CG with the maximum current of 24 kA. Hylaty measurements distinguish an impulse at 20:05:49.14 with an amplitude 220 pT and a charge moment of 103 C km. The satellite registered a signal at 20:05:49.23 with an electric field 1200 µV/m the magnetic field is omitted due to high noise. The whole period of MCS activity has been presented in