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- Author or Editor: Yolande L. Serra x
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Abstract
Data obtained in the eastern Pacific intertropical convergence zone (ITCZ) during the Tropical Eastern Pacific Process Study (TEPPS) show a 3–6-day variability. The NOAA ship Ronald H. Brown collected surface meteorological observations, C-band Doppler radar volumes, atmospheric soundings, and rainfall data while on station at 7.8°N, 125°W from 8–23 August 1997. The 3–6-day variability was a prominent timescale in the meridional wind and humidity data. The maxima of the surface-to-700-mb meridional wind anomalies were 5–12 m s–1. Maxima of the moisture anomalies at the same levels were ∼1–3 g kg–1, in phase with the low-level southerlies, while upper-level moisture lagged the southerlies by less than a day. The radar indicated a close connection between precipitation in the vicinity of the ship and the meridional wind, with the precipitation occurring when the surface southerlies were strongest. The meridional and zonal wind components had a maximum of 850–450-mb wind shear during the southerly events. The vertical profiles of wind, humidity, and temperature in each of the events at 3–6-day intervals were generally consistent with theoretical studies of easterly waves.
Satellite infrared (IR) data indicated significant variance at 3–6-day periods within the region 0°–20°N, 75°–150°W for July–August 1997. The variability seen by satellite was most pronounced off the coast of South America and over the warm waters north of the equator, which mark the latitude of the ITCZ. Satellite-observed cloudiness exhibited wavelike organization with disturbed areas moving westward at ∼8 m s–1 (6°–7° day–1), with wavelengths of 2000–3000 km. Consistent with previous descriptions of easterly waves, a chain of low clouds and moisture formed inverted V patterns in visible and water-vapor channels of satellite imagery. Mesoscale convective systems formed along this chain. Some of these systems appeared to have been associated with low-pressure regions that eventually led to hurricanes. However, others did not spin off tropical storms, thus implying that their existence did not necessarily depend on these events. One set of mesoscale convective systems moved eastward through the 3–6-day westward-moving waves in conjunction with a Kelvin wave.
Surface meteorological data from the Woods Hole Oceanographic Institution's Improved Meteorological Instrumentation (IMET) buoy moored at 10°N, 125°W and the NOAA Tropical Atmosphere–Ocean (TOA) moored buoys in the eastern Pacific also indicated significant 3–6-day variability in the meridional wind component during August 1997. Spectral analysis of the buoy data were consistent with spectrally analyzed satellite IR data in showing a seasonal spatial pattern of 3–6-day variability with easterly waves most active during the July–September season. Together the ship, satellite, and buoy data show that the TEPPS ship data were collected in a large-scale environment that was characterized by an interference pattern of two wave types. A regular progression of easterly waves produced the dominant 3–6-day variability, which was modified during one time period by the passage of a Kelvin wave approaching the region from the west.
Abstract
Data obtained in the eastern Pacific intertropical convergence zone (ITCZ) during the Tropical Eastern Pacific Process Study (TEPPS) show a 3–6-day variability. The NOAA ship Ronald H. Brown collected surface meteorological observations, C-band Doppler radar volumes, atmospheric soundings, and rainfall data while on station at 7.8°N, 125°W from 8–23 August 1997. The 3–6-day variability was a prominent timescale in the meridional wind and humidity data. The maxima of the surface-to-700-mb meridional wind anomalies were 5–12 m s–1. Maxima of the moisture anomalies at the same levels were ∼1–3 g kg–1, in phase with the low-level southerlies, while upper-level moisture lagged the southerlies by less than a day. The radar indicated a close connection between precipitation in the vicinity of the ship and the meridional wind, with the precipitation occurring when the surface southerlies were strongest. The meridional and zonal wind components had a maximum of 850–450-mb wind shear during the southerly events. The vertical profiles of wind, humidity, and temperature in each of the events at 3–6-day intervals were generally consistent with theoretical studies of easterly waves.
Satellite infrared (IR) data indicated significant variance at 3–6-day periods within the region 0°–20°N, 75°–150°W for July–August 1997. The variability seen by satellite was most pronounced off the coast of South America and over the warm waters north of the equator, which mark the latitude of the ITCZ. Satellite-observed cloudiness exhibited wavelike organization with disturbed areas moving westward at ∼8 m s–1 (6°–7° day–1), with wavelengths of 2000–3000 km. Consistent with previous descriptions of easterly waves, a chain of low clouds and moisture formed inverted V patterns in visible and water-vapor channels of satellite imagery. Mesoscale convective systems formed along this chain. Some of these systems appeared to have been associated with low-pressure regions that eventually led to hurricanes. However, others did not spin off tropical storms, thus implying that their existence did not necessarily depend on these events. One set of mesoscale convective systems moved eastward through the 3–6-day westward-moving waves in conjunction with a Kelvin wave.
Surface meteorological data from the Woods Hole Oceanographic Institution's Improved Meteorological Instrumentation (IMET) buoy moored at 10°N, 125°W and the NOAA Tropical Atmosphere–Ocean (TOA) moored buoys in the eastern Pacific also indicated significant 3–6-day variability in the meridional wind component during August 1997. Spectral analysis of the buoy data were consistent with spectrally analyzed satellite IR data in showing a seasonal spatial pattern of 3–6-day variability with easterly waves most active during the July–September season. Together the ship, satellite, and buoy data show that the TEPPS ship data were collected in a large-scale environment that was characterized by an interference pattern of two wave types. A regular progression of easterly waves produced the dominant 3–6-day variability, which was modified during one time period by the passage of a Kelvin wave approaching the region from the west.
Abstract
Outgoing longwave radiation (OLR) and low-level wind fields in the Atlantic and Pacific intertropical convergence zone (ITCZ) are dominated by variability on synoptic time scales primarily associated with easterly waves during boreal summer and fall. This study uses spectral filtering of observed OLR data to capture the convective variability coupled to Pacific easterly waves. Filtered OLR is then used as an independent variable to isolate easterly wave structure in wind, temperature, and humidity fields from open-ocean buoys, radiosondes, and gridded reanalysis products. The analysis shows that while some Pacific easterly waves originate in the Atlantic, most of the waves appear to form and strengthen within the Pacific. Pacific easterly waves have wavelengths of 4200–5900 km, westward phase speeds of 11.3–13.6 m s−1, and maximum meridional wind anomalies at about 600 hPa. A warm, moist boundary layer is observed ahead of the waves, with moisture lofted quickly through the troposphere by deep convection, followed by a cold, dry signal behind the wave. The waves are accompanied by substantial cloud forcing and surface latent heat flux fluctuations in buoy observations. In the central Pacific the horizontal structure of the waves appears as meridionally oriented inverted troughs, while in the east Pacific the waves are oriented southwest–northeast. Both are tilted slightly eastward with height. Although these tilts are consistent with adiabatic barotropic and baroclinic conversions to eddy energy, energetics calculations imply that Pacific easterly waves are driven primarily by convective heating. This differs from African easterly waves, where the barotropic and baroclinic conversions dominate.
Abstract
Outgoing longwave radiation (OLR) and low-level wind fields in the Atlantic and Pacific intertropical convergence zone (ITCZ) are dominated by variability on synoptic time scales primarily associated with easterly waves during boreal summer and fall. This study uses spectral filtering of observed OLR data to capture the convective variability coupled to Pacific easterly waves. Filtered OLR is then used as an independent variable to isolate easterly wave structure in wind, temperature, and humidity fields from open-ocean buoys, radiosondes, and gridded reanalysis products. The analysis shows that while some Pacific easterly waves originate in the Atlantic, most of the waves appear to form and strengthen within the Pacific. Pacific easterly waves have wavelengths of 4200–5900 km, westward phase speeds of 11.3–13.6 m s−1, and maximum meridional wind anomalies at about 600 hPa. A warm, moist boundary layer is observed ahead of the waves, with moisture lofted quickly through the troposphere by deep convection, followed by a cold, dry signal behind the wave. The waves are accompanied by substantial cloud forcing and surface latent heat flux fluctuations in buoy observations. In the central Pacific the horizontal structure of the waves appears as meridionally oriented inverted troughs, while in the east Pacific the waves are oriented southwest–northeast. Both are tilted slightly eastward with height. Although these tilts are consistent with adiabatic barotropic and baroclinic conversions to eddy energy, energetics calculations imply that Pacific easterly waves are driven primarily by convective heating. This differs from African easterly waves, where the barotropic and baroclinic conversions dominate.
Abstract
The 2014–15 Observations and Modeling of the Green Ocean Amazon (GOAmazon) field campaign over the central Amazon near Manaus, Brazil, occurred in coordination with the larger Cloud Processes of the Main Precipitation Systems in Brazil: A Contribution to Cloud-Resolving Modeling and to the Global Precipitation Measurement (CHUVA) project across Brazil. These programs provide observations of convection over the central Amazon on diurnal to annual time scales. In this study, we address the question of how Kelvin waves, observed in satellite observations of deep cloud cover over the GOAmazon region during the 2014–15 time period, modulate the growth, type, and organization of convection over the central Amazon. The answer to this question has implications for improved predictability of organized systems over the region and representation of convection and its growth on local to synoptic scales in global models. Our results demonstrate that Kelvin waves are strong modulators of synoptic-scale low- to midlevel free-tropospheric moisture, integrated moisture convergence, and surface heat fluxes. These regional modifications of the environment impact the local diurnal cycle of convection, favoring the development of mesoscale convective systems. As a result, localized rainfall is also strongly modulated, with the majority of rainfall in the GOAmazon region occurring during the passage of these systems.
Abstract
The 2014–15 Observations and Modeling of the Green Ocean Amazon (GOAmazon) field campaign over the central Amazon near Manaus, Brazil, occurred in coordination with the larger Cloud Processes of the Main Precipitation Systems in Brazil: A Contribution to Cloud-Resolving Modeling and to the Global Precipitation Measurement (CHUVA) project across Brazil. These programs provide observations of convection over the central Amazon on diurnal to annual time scales. In this study, we address the question of how Kelvin waves, observed in satellite observations of deep cloud cover over the GOAmazon region during the 2014–15 time period, modulate the growth, type, and organization of convection over the central Amazon. The answer to this question has implications for improved predictability of organized systems over the region and representation of convection and its growth on local to synoptic scales in global models. Our results demonstrate that Kelvin waves are strong modulators of synoptic-scale low- to midlevel free-tropospheric moisture, integrated moisture convergence, and surface heat fluxes. These regional modifications of the environment impact the local diurnal cycle of convection, favoring the development of mesoscale convective systems. As a result, localized rainfall is also strongly modulated, with the majority of rainfall in the GOAmazon region occurring during the passage of these systems.
