Search Results

You are looking at 1 - 7 of 7 items for

  • Author or Editor: S. P. de Szoeke x
  • User-accessible content x
Clear All Modify Search
S. P. de Szoeke and C. S. Bretherton

Abstract

Using a large eddy simulation (LES), the atmospheric boundary layer (ABL) is numerically modeled along 95°W from 8°S to 4°N during boreal autumn, and compared to observations from the East Pacific Investigation of Climate Processes in the Coupled Ocean–Atmosphere System (EPIC) 2001. Since the local ABL winds are predominantly southerly in this season, a “quasi-Lagrangian” forcing is used in which the ABL air column is forced as if it were advecting northward with the mean September–October 2001 meridional wind across the equatorial cold tongue and the rapidly warming SSTs to the north. Pressure gradients and large-scale zonal advective tendencies are prescribed as a function of latitude. Where possible, observations from the EPIC 2001 experiment are used for forcing and for comparison with model results.

The ABL's modeled vertical structure accords with the conceptual model of Wallace et al. and agrees well with observations. Surface stability accounts for the minimum in surface wind over the equatorial cold tongue and the maximum over the warm water to the north. Stability of the lower ABL over the cold tongue allows a jet to accelerate at about 500-m height, relatively uncoupled to the frictional surface layer. Vertical mixing over the warm water to the north distributes this momentum to the surface.

Additional simulations were performed to explore the modeled ABL's sensitivity to pressure gradients, zonal advection, free-tropospheric humidity, and initial conditions. The model ABL was robust: changing the forcings resulted in little change in the modeled structure. The strongest sensitivity was of stratocumulus clouds over the cold tongue to cloud-top radiative cooling. Once formed at the southern edge of the cold tongue, modeled stratocumulus clouds demonstrate a remarkable ability to maintain themselves over the cold tongue in the absence of surface fluxes by radiative cooling at their tops. The persistence of thin stratocumulus clouds in this Lagrangian model suggests that horizontal advection of condensate might be an important process in determining cloudiness over the cold tongue.

Full access
Simon P. de Szoeke and Christopher S. Bretherton

Abstract

During boreal summer and fall, there is a strong southerly boundary layer flow across the equator into the east Pacific intertropical convergence zone (ITCZ). The modulation of this flow on synoptic to seasonal time scales is studied using an index of meridional pressure difference between the equator and the ITCZ along 95°W. Two complementary datasets from the East Pacific Investigation of Climate (EPIC) are used to study eastern Pacific variability. Daily measurements of sea level pressure (SLP) from Tropical Atmosphere Ocean (TOA) array buoys from May to November 2001 provide temporal coverage, and eight flights by a C-130 aircraft during September to October 2001 document the associated modulation of lower tropospheric vertical structure.

The principal mode of variability of the perturbation SLP along 95°W from 1°S to 12°N, derived by principal component analysis from either the eight flights (PC1C-130) or from daily TAO buoy observations (PC1), explains 77% of the meridional pressure gradient variability. The pressure anomalies at 1.6 km are similar to those at the surface. The time series of the first mode of the TAO observations shows that most of the variance is in the 2–7-day range. Low pressure at 12°N is associated with southerly and westerly surface wind anomalies, and enhanced precipitation in the ITCZ. The depth of ITCZ convection is more strongly correlated to meridional wind above the planetary boundary layer (PBL) than to meridional wind within the PBL. There is little correlation of PBL meridional flow across the equator with ITCZ convection.

Regression of PC1C-130 against the 95°W cross sections observed by dropwinsondes released during the eight C-130 flights shows correlations of westerlies to positive PC1C-130 (low pressure at 12°N). Between the equator and 4°N, statistically significant northerlies just above the PBL at 1–2-km height and southerlies at 4 km are correlated with negative PC1C-130, having high SLP at 12°N, an anomalously weak meridional SLP gradient, and suppressed convection in the ITCZ.

PC1 is bandpass filtered and correlated with reanalysis fields to identify the structures that modulate meridional pressure gradients along 95°W. Most of the variability at periods less than 15 days is related to easterly waves. Seasonal trends in PC1 during May–October 2001 reflect the seasonal evolution of the sea and land surface temperatures. After the seasonal trend is removed, a geostrophic westerly jet at 12°N—probably related to the Madden–Julian oscillation—dominates PC1 variability on time scales longer than 15 days.

Full access
S. P. de Szoeke, C. W. Fairall, and Sergio Pezoa

Abstract

In October 2007 the NOAA ship Ronald H. Brown sailed southward within 300 km of the coast of Ecuador and Peru, sampling surface meteorology, air–sea turbulent and radiative fluxes, cloud properties, and upper-air soundings from the equator to 20°S. Two distinct water masses characterize the coastal region: cold-pool water below 19°C in the Southern Hemisphere, and warm-pool water above 20°C to the north, with a transition between the water masses at 2.5°S. Net turbulent and radiative fluxes warm the cool water south of 2.5°S by 100 W m−2 but do not warm the equatorial water significantly. Winds blow parallel to the shore, about 5 m s−1 over the cold pool and 7 m s−1 over the equator. Stratocumulus clouds are remarkably solid over the coastal cold pool, with only brief periods of partial clearing, mostly in the afternoon. Lower aerosol concentrations and thicker clouds observed farther from the coast on 22–23 October are coincident with a pocket of open cells seen to the west and southwest of the ship. Observations from this cruise and other NOAA Stratus cruises (2001 and 2003–07) are suitable for comparison with model simulations and provide context for future field experiments. These datasets are publicly available.

Full access
Simon P. de Szoeke, Christopher S. Bretherton, Nicholas A. Bond, Meghan F. Cronin, and Bruce M. Morley

Abstract

The atmospheric boundary layer (ABL) along 95°W in the eastern equatorial Pacific during boreal autumn is described using data from the East Pacific Investigation of Climate (EPIC) 2001, with an emphasis on the evolution of the thermodynamic ABL properties from the cold tongue to the cold-advection region north of the sea surface temperature (SST) front. Surface sensible and latent heat fluxes and wind stresses between 1°S and 12°N are calculated from data from eight NCAR C-130 research aircraft flights and from Tropical Atmosphere Ocean (TAO) buoys. Reduced surface wind speed and a 10 m s−1 jet at a height of 500 m are found over the equatorial cold tongue, demonstrating the dependence of the surface wind speed on surface stability.

The ABL exhibits a maximum in cloud cover on the north (downwind) side of the warm SST front, at 1°–3°N. Turbulent mixing driven by both surface buoyancy flux and radiative cooling at the cloud tops plays a significant role in maintaining the depth and structure of the ABL. The ABL heat budget between the equator and 3°N is balanced by comparable contributions from advective cooling, radiative cooling, surface warming, and entrainment warming. Entrainment drying is a weak contributor to the moisture budget, relative to dry advection and surface evaporation. Both the heat and moisture budgets are consistent with a rapid entrainment rate, 12 ± 2 mm s−1, deduced from the observed rise of the inversion with latitude between 0° and 4°N.

Full access
David B. Mechem, Carly S. Wittman, Matthew A. Miller, Sandra E. Yuter, and Simon P. de Szoeke

Abstract

Marine boundary layer clouds are modified by processes at different spatial and temporal scales. To isolate the processes governing aerosol–cloud–precipitation interactions, multiday synoptic variability of the environment must be accounted for. Information on the location of low clouds relative to the ridge–trough pattern gives insight into how cloud properties vary as a function of environmental subsidence and stability. The technique of self-organizing maps (SOMs) is employed to objectively classify the 500-hPa geopotential height patterns for 33 years of reanalysis fields (ERA-Interim) into pretrough, trough, posttrough, ridge, and zonal-flow categories. The SOM technique is applied to a region of prevalent marine low cloudiness over the eastern North Atlantic Ocean that is centered on the Azores island chain, the location of a long-term U.S. Department of Energy observation site. The Azores consistently lie in an area of substantial variability in synoptic configuration, thermodynamic environment, and cloud properties. The SOM method was run in two ways to emphasize multiday and seasonal variability separately. Over and near the Azores, there is an east-to-west sloshing back and forth of the western edge of marine low clouds associated with different synoptic states. The different synoptic states also exhibit substantial north–south variability in the position of high clouds. For any given month of the year, there is large year-to-year variability in the occurrence of different synoptic states. Hence, estimating the climatological behavior of clouds from short-term field campaigns has large uncertainties. This SOM approach is a robust method that is broadly applicable to characterizing synoptic regimes for any location.

Full access
Simon P. de Szoeke, Eric D. Skyllingstad, Paquita Zuidema, and Arunchandra S. Chandra

Abstract

Cold pools dominate the surface temperature variability observed over the central Indian Ocean (0°, 80°E) for 2 months of research cruise observations in the Dynamics of the Madden–Julian Oscillation (DYNAMO) experiment in October–December 2011. Cold pool fronts are identified by a rapid drop of temperature. Air in cold pools is slightly drier than the boundary layer (BL). Consistent with previous studies, cold pools attain wet-bulb potential temperatures representative of saturated downdrafts originating from the lower midtroposphere.

Wind and surface fluxes increase, and rain is most likely within the ~20-min cold pool front. Greatest integrated water vapor and liquid follow the front. Temperature and velocity fluctuations shorter than 6 min achieve 90% of the surface latent and sensible heat flux in cold pools. The temperature of the cold pools recovers in about 20 min, chiefly by mixing at the top of the shallow cold wake layer, rather than by surface flux.

Analysis of conserved variables shows mean BL air is composed of 51% air entrained from the BL top (800 m), 22% saturated downdrafts, and 27% air at equilibrium with the ocean surface. The number of cold pools, and their contribution to the BL heat and moisture, nearly doubles in the convectively active phase compared to the suppressed phase of the Madden–Julian oscillation.

Full access
C. R. Mechoso, R. Wood, R. Weller, C. S. Bretherton, A. D. Clarke, H. Coe, C. Fairall, J. T. Farrar, G. Feingold, R. Garreaud, C. Grados, J. McWilliams, S. P. de Szoeke, S. E. Yuter, and P. Zuidema

The present paper describes the Variability of the American Monsoon Systems (VAMOS) Ocean–Cloud–Atmosphere–Land Study (VOCALS), an international research program focused on the improved understanding and modeling of the southeastern Pacific (SEP) climate system on diurnal to interannual time scales. In the framework of the SEP climate, VOCALS has two fundamental objectives: 1) improved simulations by coupled atmosphere–ocean general circulation models (CGCMs), with an emphasis on reducing systematic errors in the region; and 2) improved estimates of the indirect effects of aerosols on low clouds and climate, with an emphasis on the more precise quantification of those effects. VOCALS major scientific activities are outlined, and selected achievements are highlighted. Activities described include monitoring in the region, a large international field campaign (the VOCALS Regional Experiment), and two model assessments. The program has already produced significant advances in the understanding of major issues in the SEP: the coastal circulation and the diurnal cycle, the ocean heat budget, factors controlling precipitation and formation of pockets of open cells in stratocumulus decks, aerosol impacts on clouds, and estimation of the first aerosol indirect effect. The paper concludes with a brief presentation on VOCALS contributions to community capacity building before a summary of scientific findings and remaining questions.

Full access