[A-AS05] Large-scale moisture and organized cloud systems

[AAS05-P03] Large-scale Cirrus Formation Events in the ITCZ

*Noriyuki Nishi¹, Atsushi Hamada², Hiroaki Miura³, Masaki Satoh⁴ (1.Faculty of Science, Fukuoka University, 2.Faculty of Sustainable Design, University of Toyama, 3.Graduate School of Science, The University of Tokyo, 4.Atmosphere and Ocean Research Institute, The University of Tokyo)

Keywords:Cirrus, Tropics

Over the center part of the tropical Pacific, the Inter Tropical Convergence Zone (ITCZ) is detected on the northern side of the equator. In the ITCZ, we sometimes observe drastic deformation of a cloud band elongated zonally with a several thousand-kilometer scale. A meridionally narrow cloud band, which is mainly composed of deep convective systems, simultaneously expands meridionally and separates into two or three parallel cloud bands of cirriform clouds. They stay alive for one or two days (Hamada et al., 2013, JMSJ). Since this process can produce a large amount of cirriform clouds at a time, it can be a good target for studying large-scale cirrus formation. We studied this process with the ERA5 analysis and a global cloud system resolving model NICAM (Satoh et al. 2014, PEPS). This kind of separation events could not be described well in any objective reanalysis dataset, probably due to a lack of observations over the open ocean. However, in the ERA5 reanalysis, such separation events are very well described. We analyzed some typical events in this dataset and found the following aspects of these events. The speed of meridional expansion of the two or three bands is almost the same as the meridional wind speed at around 200 hPa, suggesting that the movement of cirriform clouds is explained as the advection of cloud ice by the divergent wind at the upper troposphere. During the meridional expansion, the penetrative upward motion, mainly accompanied by deep cumulonimbus, becomes much weaker than that at the formation of the one convective cloud band. The upward motion is rather related to convergence in the mid-troposphere (400-500 hPa). This results in shallow circulation in the mid-upper troposphere, which may have considerably contributed to the band's meridional expansion. Such mid-tropospheric convergence has a complex shape that is somewhat asymmetric about the center latitude of the events. We should make further analysis to reveal whether this convergence is essential to the band separation and what is the cause of this convergence. Another interesting aspect of the ERA5 data is the tilted upward motion resembling the theoretical internal gravity wave pattern emanating upward from the wave source in the mid-troposphere. Enhancement of cloud ice is detected in this upward motion; the position of phase of upward motion of the wave may interpret the movement of the cloud band. Suppose an internal gravity wave with a broad spectrum is excited in the initial stage of the cloud band with active cumulonimbi. In that case, the arrival time to 200 hPa and the angle of the ray path should differ according to the wave frequency. Wave with higher (lower) frequency would have more (less) vertical ray path and reach faster (slower) to 200 hPa. We simulated these events using NICAM with initial values from the ERA5. We succeeded in reproducing several cases in recent three years. In these cases, the meridional speed of the bands is close to the meridional wind at that level, as shown in ERA5 data. However, the gravity wave signal is much weaker than that in the ERA5. This discrepancy may indicate that advection is mainly important for the progress of the cloud band separation. We will also introduce some interesting points detected in the ERA5 and NICAM simulation results.