Diurnal propagating precipitating systems to the north of New Guinea Island: In-situ observation of internal structure and ambient atmosphere

The diurnal cycle is the one of the significant and important characteristics of the precipitation on both ocean and land in the tropics. As the mechanism of the diurnal cycle, recently, some studies reported that the precipitating features propagate diurnally. However, there are few studies to investigate the internal structure of these diurnally propagating precipitating systems, especially over the ocean. The ocean around the New Guinea Island is one of the area with significant diurnally-propagating clouds, as shown in Liberti (2001). In the past four boreal autumns (or winters) from 2000 to 2004, we carried out the observational study of the equatorial precipitating systems and the ambient atmosphere over the warm pool by deploying research vessel MIRAI at (2N, 138E), stationary for weeks to a month. In this study, we investigated the diurnally-appearing precipitating systems around the point by using data from the shipborne Doppler radar, which can detect the behavior of the precipitating systems directly. The diurnal variation of the ambient atmosphere is obtained by 3-hourly radiosonde observation. The diurnal variation of the radar echo area for each observation period shows double peaks, in local afternoon (12-15LT) and in local midnight to dawn (3-6LT). Among them, the former afternoon peak is significant in MR01-K05 (including convectively active period) and MR02-K06 (inactive period), while the latter is significant in MR00-K07 (including both active and inactive period). The former peak corresponds to that in Liberti (2001). The northward propagation from New Guinea island to the north, including the observation point (2N, 138E), is also observed in GMS (or GOES) infrared (IR) images. The all of observed MCSs are categorized from their morphological characteristics into four: (1) arc-shaped with about 100km horizontal scale, (2) linear-shaped but not reached (2N, 138E), (3) linear-shaped and passed over (2N, 138E), and (4) northward propagation of precipitating area without significant linear shape. The latter three types, types (2), (3) and (4), are frequently appeared in local afternoon, diurnal peak of the echo area, while type (1) appeared non-diurnally. Among the diurnal MCSs, type (2) has linear convective leading edge and trailing stratiform region, while type (4) consists of widespread echo area and no significant convective leading edge. The type (3) is the hybrid type of them, i.e. the band-shaped with the width of a few to several kilometers. The environmental wind is also different: Type (2) tends to be organized perpendicular to the low-level wind shear, as in LeMone et al. (1998), while (3) and (4) do not. The moving / propagating directions are also affected by the low-level (800 hPa) wind in (2), while upper-level (300 hPa) wind in (4). The internal structure, only obtained for types (3) and (4), shows clear difference in vertical profile of divergence. The results indicate that the type (2) has the mechanism like the squall-line-type MCS, while type (4) is the resultant precipitation of propagation of wave, like gravity wave. The hybrid type is type (3). With the fact that type (2) appeared principally in the convectively inactive period while type (4) appeared only in the convectively active phase of MJO, the dynamic, thermodynamic and humidity profile could control the appearance of the diurnal MCSs. The impacts of these diurnal MCSs to the ambient atmosphere are also examined. The composite analyses show the significant anomaly around 12 to 15LT, when the afternoon diurnal MCS appeared. The anomaly is northerly (toward the MCS) in the layer around melting level, and southerly (away from the MCS) below and above. This relationship between MCS and ambient atmosphere is similar to the result of the numerical simulation on the gravity wave response around MCS (e.g. Nicholls, 1991). On the other hand, the diurnal anomaly of humidity (and mixing ratio of water vapor) shows that the appearance of positive anomaly in middle and upper layer (3 to 8 km) was earlier in the MJO convectively active phase than in than inactive phase, in the afternoon when the diurnal MCSs appeared. These indicates that the diurnal MCSs play the role of “messengers” of the land effect of New Guinea Island to modify the surrounding oceanic area both dynamically and thermodynamically.