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Browsing by Subject "Double intertropical convergence zone"

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  • Mom, Bernd (2023)
    The parameterization of deep convection is simulated poorly over the Central and East Pacific. This could lead to issues in predicting the annual total precipitation in the tropics, such as the existence of a double intertropical convergence zone over the equatorial Pacific. Resolving tropical deep convection instead of parameterization leads to the presence of more linear systems. Observations over Atlantic indicate that shear-perpendicular lines (squall lines) propagate faster than shear- parallel lines, mainly due to their connection with the low-level vertical wind shear (VWS). The study examines the different movement speeds of mesoscale convective systems (MCSs) over the East- and West Pacific to determine whether this could explain the reason why climate models have problems with predicting deep convection. A higher proportion of fast moving MCSs (squall lines) could contribute to the prediction problems in the tropics. The MCS motion is determined by the sum of the mean wind and propagation speed. In squall lines, the MCS motion is mainly influenced by the propagation, which is associated with the low-level VWS. Therefore, the effect of the low-level VWS on the fast- and slow moving MCSs is also investigated. The Global High-Resolution Mesoscale Convective System Database is used, which provides infor- mation about the time, location and movement of MCSs. Additionally, ERA5 wind data is used to obtain the mean wind and VWS. Two specific areas over the northern equatorial Pacific are chosen to compare the different types of MCSs. These areas are over the East Pacific (120°W - 140°W and 5°N - 12°N) and West Pacific (140°E - 160°E and 0°N - 7°N). The movement speed is used to categorize the MCSs into three groups: slow moving MCSs (< 3 m/s), moderate moving MCSs (3 m s−1 − 7 m s−1) and fast moving MCSs (> 7 m/s). The study reveals that the share of the fast moving MCSs is 9.8% over the East Pacific and 13.8% over the West Pacific. This is only a 4 percent point difference between the two areas. Therefore, it is not shown that the fast moving MCSs contribute to the existing issues that models have in predicting the annual total precipitation over the East Pacific. Moreover, approximately 85-90% are categorized as slow- to moderate moving MCSs. Hence, the influence of fast moving MCSs is relatively small when compared to the other types. A difference is seen in the mean wind and VWS over the East Pacific, but do not explain the MCS motion vector. Therefore, the difference between fast- and slow moving MCSs cannot be explained by only the monthly averaged mean wind and low-level VWS over the East Pacific. Over the West Pacific, the mean wind direction and VWS are about the same in direction and speed. Therefore, the difference between fast- and slow moving MCSs is not explained by the low-level VWS over the West Pacific.