5:15 PM - 7:15 PM
[AAS08-P04] Fine Structure of the Zonally-Extended Upper Cloud Bands in the ITCZ
Keywords:cirrus, tropics
Over the central part of the tropical Pacific, the Intertropical Convergence Zone (ITCZ) is detected on the northern side of the equator. In the ITCZ, we sometimes observe drastic deformation of a zonally-elongated cloud band with a several thousand-kilometer scale. The narrow cloud band, composed of deep convective systems (hereafter, initial band), simultaneously expands meridionally and separates into two or three parallel cloud bands confined in the upper troposphere (hereafter separated bands). They persist for one or two days (Hamada et al., 2013, JMSJ). This process can produce many cirriform clouds at once; it can be an interesting target for studying large-scale cloud formation. We studied this process with observational data and the global cloud-resolving model NICAM (Satoh et al. 2014, PEPS). We used ERA5 reanalysis dataset (Hersbach et al. 2020, QJRMS) and globally-merged full-resolution (~4 km) IR data (Janowiak et al. 2001, BAMS).
In this presentation, we report on the results of an analysis of the mechanisms by which the separated bands continuously move away from the position of the initial band and are maintained over about one day. The shallow meridional circulations with two different scales have crucial roles in this process.
First, the upper cloud in the separated bands is associated with a vertical circulation characterized by meridional convergence around 300 hPa and divergence around 150 hPa. An upward motion is observed within this cloud, with a velocity ranging from -0.1 to -0.2 Pa/s (-2.5 to -5 km/day). This value is an order of magnitude larger than typical upper-level clouds in the tropics, suggesting the presence of a unique mechanism for maintaining the upward motion. In these separation events, the cloud region expands only in the meridional direction while maintaining its band-like structure. Compared to nearly circular cloud clusters that gradually expand in two horizontal dimensions, the separated bands are more likely to retain a high density of condensates. This retention may enhance cloud formation and contribute to the continuous development of updrafts.
Next, we analyzed a larger-scale shallow circulation. The separated bands appear to be advected by the meridional wind at the central altitude of the cloud (approximately 200 hPa). It is necessary to investigate why this meridionally divergent wind persists for more than a day. The deep and strong convective upward motion near the initial band, which are associated with cumulonimbus clouds, weakens rapidly. However, in many cases, upward motion confined mainly at mid-to-upper levels remains near the initial band. A large-scale shallow circulation with a horizontal scale of approximately 1000 km may form, with convergence around 300-400 hPa and divergence around 200 hPa near the initial band. However, in some cases, this circulation is not well detected in meridional cross-sections of the longitude range of the bands. It suggests that this circulation may not be a simple, longitudinally uniform system. Based on these two scales of shallow circulation, we further discuss the mechanisms that sustain the separation phenomenon.
In this presentation, we report on the results of an analysis of the mechanisms by which the separated bands continuously move away from the position of the initial band and are maintained over about one day. The shallow meridional circulations with two different scales have crucial roles in this process.
First, the upper cloud in the separated bands is associated with a vertical circulation characterized by meridional convergence around 300 hPa and divergence around 150 hPa. An upward motion is observed within this cloud, with a velocity ranging from -0.1 to -0.2 Pa/s (-2.5 to -5 km/day). This value is an order of magnitude larger than typical upper-level clouds in the tropics, suggesting the presence of a unique mechanism for maintaining the upward motion. In these separation events, the cloud region expands only in the meridional direction while maintaining its band-like structure. Compared to nearly circular cloud clusters that gradually expand in two horizontal dimensions, the separated bands are more likely to retain a high density of condensates. This retention may enhance cloud formation and contribute to the continuous development of updrafts.
Next, we analyzed a larger-scale shallow circulation. The separated bands appear to be advected by the meridional wind at the central altitude of the cloud (approximately 200 hPa). It is necessary to investigate why this meridionally divergent wind persists for more than a day. The deep and strong convective upward motion near the initial band, which are associated with cumulonimbus clouds, weakens rapidly. However, in many cases, upward motion confined mainly at mid-to-upper levels remains near the initial band. A large-scale shallow circulation with a horizontal scale of approximately 1000 km may form, with convergence around 300-400 hPa and divergence around 200 hPa near the initial band. However, in some cases, this circulation is not well detected in meridional cross-sections of the longitude range of the bands. It suggests that this circulation may not be a simple, longitudinally uniform system. Based on these two scales of shallow circulation, we further discuss the mechanisms that sustain the separation phenomenon.