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Convection and Easterly Wave Structures Observed in the Eastern Pacific Warm Pool during EPIC-2001

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  • 1 NSSTC/ESSC, University of Alabama in Huntsville, Huntsville, Alabama
  • | 2 Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado
  • | 3 NASA Marshall Space Flight Center, Huntsville, Alabama
  • | 4 Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado
  • | 5 NOAA/ETL, Boulder, Colorado
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Abstract

During September–October 2001, the East Pacific Investigation of Climate Processes in the Coupled Ocean–Atmosphere System (EPIC-2001) intertropical convergence zone (ITCZ) field campaign focused on studies of deep convection in the warm-pool region of the eastern Pacific. This study combines C-band Doppler radar, sounding, and surface heat flux data collected aboard the R/V Ronald H. Brown during EPIC to describe the kinematic and thermodynamic states of the ITCZ environment, together with tendencies in convective structure, lightning, rainfall, and surface heat fluxes as a function of 3–5-day easterly wave phase.

Three easterly waves were observed at the location of the R/V Brown during EPIC-2001. Wind and thermodynamic data reveal that the wave trough axes exhibited positively correlated u and υ winds, a slight westward phase tilt with height, and relatively strong (weak) northeasterly tropospheric shear following the trough (ridge) axis. Temperature and humidity perturbations exhibited mid- to upper-level cooling (warming) and drying (moistening) in the northerly (trough and southerly) phase. At low levels, warming (cooling) and moistening (drying) occurred in the northerly (southerly) phase.

Composited radar, sounding, lightning, and surface heat flux observations suggest the following systematic behavior as a function of wave phase: zero to one-quarter wavelength ahead of (behind) the wave trough in northerly (southerly) flow, larger (smaller) convective available potential energy (CAPE), lower (higher) convective inhibition (CIN), weaker (stronger) tropospheric shear, larger (smaller) convective rain fractions, higher (lower) conditional mean rain rates, higher (lower) lightning flash densities, and more (less) robust convective vertical structure occurred. Latent and sensible heat fluxes reached a minimum in the northerly phase and then increased through the trough, reaching a peak during the ridge phase (leading the peak in CAPE). Larger areas of light convective and stratiform rain and slightly larger (10%) area-averaged rain rates occurred in the vicinity of, and just behind, the trough axes in southerly and ridge flow. Importantly, the transition in convective structure observed across the trougth axis when considered with the relatively small change in area mean rain rates suggests the presence of a transition in the vertical structure of diabatic heating across the easterly waves examined. The inferred transition in heating structure is supported by radar-diagnosed divergence profiles that exhibit convective (stratiform) characteristics ahead of (behind) the trough.

Corresponding author address: Dr. Walter A. Petersen, NSSTC/ESSC, University of Alabama in Huntsville, Huntsville, AL 35899. Email: walt.petersen@msfc.nasa.gov

Abstract

During September–October 2001, the East Pacific Investigation of Climate Processes in the Coupled Ocean–Atmosphere System (EPIC-2001) intertropical convergence zone (ITCZ) field campaign focused on studies of deep convection in the warm-pool region of the eastern Pacific. This study combines C-band Doppler radar, sounding, and surface heat flux data collected aboard the R/V Ronald H. Brown during EPIC to describe the kinematic and thermodynamic states of the ITCZ environment, together with tendencies in convective structure, lightning, rainfall, and surface heat fluxes as a function of 3–5-day easterly wave phase.

Three easterly waves were observed at the location of the R/V Brown during EPIC-2001. Wind and thermodynamic data reveal that the wave trough axes exhibited positively correlated u and υ winds, a slight westward phase tilt with height, and relatively strong (weak) northeasterly tropospheric shear following the trough (ridge) axis. Temperature and humidity perturbations exhibited mid- to upper-level cooling (warming) and drying (moistening) in the northerly (trough and southerly) phase. At low levels, warming (cooling) and moistening (drying) occurred in the northerly (southerly) phase.

Composited radar, sounding, lightning, and surface heat flux observations suggest the following systematic behavior as a function of wave phase: zero to one-quarter wavelength ahead of (behind) the wave trough in northerly (southerly) flow, larger (smaller) convective available potential energy (CAPE), lower (higher) convective inhibition (CIN), weaker (stronger) tropospheric shear, larger (smaller) convective rain fractions, higher (lower) conditional mean rain rates, higher (lower) lightning flash densities, and more (less) robust convective vertical structure occurred. Latent and sensible heat fluxes reached a minimum in the northerly phase and then increased through the trough, reaching a peak during the ridge phase (leading the peak in CAPE). Larger areas of light convective and stratiform rain and slightly larger (10%) area-averaged rain rates occurred in the vicinity of, and just behind, the trough axes in southerly and ridge flow. Importantly, the transition in convective structure observed across the trougth axis when considered with the relatively small change in area mean rain rates suggests the presence of a transition in the vertical structure of diabatic heating across the easterly waves examined. The inferred transition in heating structure is supported by radar-diagnosed divergence profiles that exhibit convective (stratiform) characteristics ahead of (behind) the trough.

Corresponding author address: Dr. Walter A. Petersen, NSSTC/ESSC, University of Alabama in Huntsville, Huntsville, AL 35899. Email: walt.petersen@msfc.nasa.gov

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