Two prominent Madden-Julian Oscillations (MJOs) occurred during the SOP. Rainfall over - PowerPoint Presentation

1. Two prominent Madden-Julian Oscillations (MJOs) occurred during the SOP. Rainfall over the Northern Sounding Array (NSA) was strongly modulated by these MJO events while the Southern Sounding Array (SSA) experienced more frequent episodes of rainfall related primarily to ITCZ convection.To examine times with a preference for convection to occur predominately over one sounding array, the following periods were defined:SSA daily rainfall > 3 X NSA daily rainfall: SSA heavy rain days (green shading)NSA daily rainfall > 3 X SSA daily rainfall: NSA heavy rain days (orange shading) Based on these criteria the top panel shows that 5 days were classified as NSA heavy rain days, 17 days as SSA heavy rain days, and 39 remaining days (white shading). The remainder of this presentation will focus on SSA heavy rain days. Diurnal Cycle of ITCZ Convection during DYNAMO Paul E. Ciesielski and Richard H. Johnson Colorado State UniversityIntroductionDuring the special observing period (SOP) of the DYNAMO/CINDY/AMIE field campaign, conducted over the Indian Ocean from 1 October to 30 November 2011, two sounding networks one north and one south of the equator, took 4-8 soundings/day. This dataset with 3-h time resolution offers a unique opportunity to investigate the diurnal cycle of convection during DYNAMO. SOP time series of TRMM rainfall Rain map for SSA heavy rain daysAcknowledgments: NSF Grant AGS-1360237, NASA Grant NNX13AF74G, and DOE Grant DE-SC0008582SSA heavy rain daysDiurnal cycle of rainfallSSA heavy rain days characterized by ITCZ convection over SSA which occur during the pre-onset (i.e., suppressed) MJO phases. Characteristcs of SSA heavy rain days: Diurnal cycle of rainfall is largely out of phase between arrays.NSA rainfall (black curve) has a peak in late afternoon/evening and a minimum in early morning.SSA rainfall (red curve) has minimum in late afternoon/evening and a peak in early morning.SSA diurnal rainfall range is ~40% of mean.North-south cross sections (72°- 80°E) of derived fields on SSA heavy rain daysConclusionsDiurnal cycle of low-level divergence on SSA heavy rain daysdivergenceEarly morning (05LT)Late afternoon (17LT)vertical motionapparent heatingapparentdryingCross-equatorial Hadley cell exhibits a diurnal pulsing: stronger in early morning when radiative effects may play a role to drive stronger convection in ITCZ (Gray and Jacobson 1977).On SSA heavy rain days (i.e., MJO pre-onset state), ITCZ convection over SSA shows max (min) of low-level convergence, upward motion and rainfall in early morning (late afternoon). consistent with Deser and Smith (1998) findings in East Pacific.Fields over NSA out of phase with those over SSA. Diurnal pulsing of ITCZ impacts conditions on opposite side of equator.Daytime reduction of subsidence over the NSA within the subsiding branch of the cross-equatorial Hadley cell may allow shallow convection and associated moistening to deepen during the pre-onset stage of the MJO. This large-scale diurnal modulation of convection may assist the local mechanisms described in Ruppert and Johnson (2015, 2016) which aid in the moistening of lower-mid troposphere during the pre-onset suppressed stage of the MJO. Diurnal cycle of low-level divergence is out phase between arrays.Over NSA, nighttime low-level divergence reverts to weak daytime convergence.Time series of TRMM rainfall averaged over NSA (black) and SSA (red). For explanation of shading see text below. NSASSANSASSANSASSAMap of low-level divergence diurnal haromonic Complex diurnal pattern over I0 (Dai and Deser 1999)12-hour time difference in Tmax (i.e. local time of diurnal harmonic peak in low-level divergence) between NSA and SSA. N S N S