Microphysical characteristics of MJO convection over the Indian Ocean during DYNAMO

Tropospheric moisture is a key factor controlling the global climate and its variability. For instance, moistening of the lower troposphere is necessary to trigger the convective phase of a Madden-Julian oscillation (MJO). However, the relative importance of the processes controlling this moistening has yet to be quantified. Among these processes, the importance of the moistening by shallow convection is still debated. The authors use high-frequency observations of humidity and convection from the Research Vessel (R/V) Mirai that was located in the Indian Ocean ITCZ during the Cooperative Indian Ocean Experiment on Intraseasonal Variability/ Dynamics of the MJO (CINDY/DYNAMO) campaign. This study is an initial attempt to directly link shallow convection to moisture variations within the lowest 4 km of the atmosphere from the convective scale to the mesoscale. Within a few tens of minutes and near shallow convection occurrences, moisture anomalies of 0.25-0.5 g kg 21 that correspond to tendencies on the order of 10-20 g kg 21 day 21 between 1 and 4 km are observed and are attributed to shallow convective clouds. On the scale of a few hours, shallow convection is associated with anomalies of 0.5-1 g kg 21 that correspond to tendencies on the order of 1-4 g kg 21 day 21 according to two independent datasets: lidar and soundings. This can be interpreted as the resultant mesoscale effect of the population of shallow convective clouds. Large-scale advective tendencies can be stronger than the moistening by shallow convection; however, the latter is a steady moisture supply whose importance can increase with the time scale. This evaluation of the moistening tendency related to shallow convection is ultimately important to develop and constrain numerical models.

Section: B In Situ Moisture Variation Compositesmentioning

confidence: 97%

Observation of Moisture Tendencies Related to Shallow Convection

Yoneyama

Katsumata

Nishizawa

et al. 2015

“…(Only information from the S band is employed in this study.) The details of S-PolKa data quality-control and fieldinstrumentation management can be found at https:// www.eol.ucar.edu/instrumentation/remote-sensing/s-pol and in prior studies (Powell and Houze 2013;Zuluaga and Houze 2013;Rowe and Houze 2014). When viewing S-PolKa data in this study, the caveat that the S-PolKa sampling region is far smaller than and on the corner of the northern sounding array must be considered.…”

Section: B Radar Cloud Measurementsmentioning

confidence: 99%

“…Therefore, what are retained are boundary layer-based clouds and highbased clouds with contiguous detectable rain shafts. During suppressed periods, when rainfall is minimal and stratiform systems rare, these echo features are dominated by boundary layer-based convective clouds (Barnes and Houze 2013;Powell and Houze 2013;Zuluaga and Houze 2013;Rowe and Houze 2014).…”

Section: B Radar Cloud Measurementsmentioning

confidence: 99%

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Diurnally Modulated Cumulus Moistening in the Preonset Stage of the Madden–Julian Oscillation during DYNAMO*

Ruppert

Johnson

2015

120

151

Atmospheric soundings, radar, and air-sea flux measurements collected during Dynamics of the Madden-Julian Oscillation (DYNAMO) are employed to study MJO convective onset (i.e., the transition from shallow to deep convection) in the tropical Indian Ocean. The findings indicate that moistening of the low-midtroposphere during the preonset stage of the MJO is achieved by simultaneous changes in the convective cloud population and large-scale circulation. Namely, cumuliform clouds deepen and grow in areal coverage as the drying by large-scale subsidence and horizontal (westerly) advection wane. The reduction of large-scale subsidence is tied to the reduction of column radiative cooling during the preonset stage, which ultimately links back to the evolving cloud population. While net column moistening in the preonset stage is tied to large-scale circulation changes, a new finding of this study is the high degree to which the locally driven diurnal cycle invigorates convective clouds and cumulus moistening each day. This diurnal cycle is manifest in a daytime growth of cumulus clouds (in both depth and areal coverage) in response to oceanic diurnal warm layers, which drive a daytime increase of the air-sea fluxes of heat and moisture. This diurnally modulated convective cloud field exhibits prominent mesoscale organization in the form of open cells and horizontal convective rolls. It is hypothesized that the diurnal cycle and mesoscale cloud organization characteristic of the preonset stage of the MJO represent two manners in which local processes promote more vigorous daily-mean column moistening than would otherwise occur.

“…Zuluaga and Houze (2013) showed further how the number of MCSs is modulated by synoptic-scale waves during active periods. Rowe and Houze (2014) have examined the microphysical structures of the MCSs during active MJO periods. Guy and Jorgensen (2014) analyzed DYNAMO aircraft data and found broad echo-top-height distributions in MCSs during the MJO active period.…”

Section: Introductionmentioning

confidence: 99%

Variation of Lightning and Convective Rain Fraction in Mesoscale Convective Systems of the MJO

Virts

Houze

2015

Self Cite

Characteristics of mesoscale convective systems (MCSs) in regions affected by the Madden-Julian oscillation (MJO) are investigated using a database of MCSs observed by the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Advanced Microwave Scanning Radiometer for Earth Observing System (AMSR-E). Lightning occurrence detected by the World-Wide Lightning Location Network (WWLLN) is composited in a framework centered on the MCSs. During MJO active periods, MCSs are more numerous and larger, as the convective features persist and attain greater horizontal scales. Anomalies of the lifted index, derived from the European Centre for Medium-Range Weather Forecasts (ECMWF) interim reanalysis (ERA-Interim) fields, indicate that MCS environments are more stable during MJO active periods.Over the Indian Ocean, Maritime Continent, and western Pacific, lightning density in an MCS maximizes during the time that the total number of systems begins to increase as the MJO is beginning to be more active, implying both more vigorous convection and less extensive stratiform rain areas at this transitional time of the MJO. The peak in MJO precipitation coincides with peak occurrence of interconnected MCSs with larger stratiform rain fraction, shown by the Tropical Rainfall Measuring Mission satellite, while composites of lightning frequency show that during MJO active periods the zone of lightning is contracted around the centers of MCSs, and flashes are less frequent.