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Simon P. de Szoeke

Abstract

The atmospheric circulation depends on poorly understood interactions between the tropical atmospheric boundary layer (BL) and convection. The surface moist static energy (MSE) source (130 W m−2, of which 120 W m−2 is evaporation) to the tropical marine BL is balanced by upward MSE flux at the BL top that is the source for deep convection. Important for modeling tropical convection and circulation is whether MSE enters the free troposphere by dry turbulent processes originating within the boundary layer or by motions generated by moist deep convection in the free troposphere. Here, highly resolved observations of the BL quantify the MSE fluxes in approximate agreement with recent cloud-resolving models, but the fluxes depend on convective conditions. In convectively suppressed (weak precipitation) conditions, entrainment and downdraft fluxes export equal shares (60 W m−2) of MSE from the BL. Downdraft fluxes are found to increase 50%, and entrainment to decrease, under strongly convective conditions. Variable entrainment and downdraft MSE fluxes between the BL and convective clouds must both be considered for modeling the climate.

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Simon P. de Szoeke

Abstract

A small integrated oceanographic thermometer with a nominal response time of 1 s was affixed to a floating hose “sea snake” towed near the bow of a research vessel. The sensor measured the near-surface ocean temperature accurately and in agreement with other platforms. The effect of conduction and evaporation is modeled for a sensor impulsively alternated between water and air. Large thermal mass makes most sea snake thermometers insensitive to temperature impulses. The smaller 1-s thermometer cooled by evaporation, but the sensor never reached the wet bulb temperature. The cooling was less than 6% of the (~2.7 °C) difference between the ocean temperature and the wet bulb temperature in 99% of 2 s–1 samples. Filtering outliers, such as with a median, effectively removes the evaporative cooling effect from 1- or 10-minute average temperatures.

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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.

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Eric D. Skyllingstad and Simon P. de Szoeke

Abstract

Cloud-resolving large-eddy simulations (LES) on a 500 km × 500 km periodic domain coupled to a thermodynamic ocean mixed layer are used to study the effect of large-scale moisture convergence M on the convective population and heat and moisture budgets of the tropical atmosphere, for several simulations with M representative of the suppressed, transitional, and active phases of the Madden–Julian oscillation (MJO). For a limited-area model without an imposed vertical velocity, M controls the overall vertical temperature structure. Moisture convergence equivalent to ~200 W m−2 (9 mm day−1) maintains the observed temperature profile above 5 km. Increased convective heating for simulations with higher M is partially offset by greater infrared cooling, suggesting a potential negative feedback that helps maintain the weak temperature gradient conditions observed in the tropics. Surface evaporation decreases as large-scale moisture convergence increases, and is only a minor component of the overall water budget for convective conditions representing the active phase of the MJO. Cold pools generated by evaporation of precipitation under convective conditions are gusty, with roughly double the wind stress of their surroundings. Consistent with observations, enhanced surface evaporation due to cold pool gusts is up to 40% of the mean, but has a small effect on the total moisture budget compared to the imposed large-scale moisture convergence.

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Simon P. de Szoeke and Shang-Ping Xie

Abstract

Warmer SST and more rain in the Northern Hemisphere are observed year-round in the tropical eastern Pacific with southerly wind crossing the equator toward the atmospheric heating. The southerlies are minimal during boreal spring, when two precipitation maxima straddle the equator. Fourteen atmosphere–ocean coupled GCMs from the Coupled Model Intercomparison Project (CMIP3) and one coupled regional model are evaluated against observations with simple metrics that diagnose the seasonal cycle and meridional migration of warm SST and rain. Intermodel correlations of the metrics elucidate common coupled physics. These models variously simulate the climatology of SST and ITCZ rain.

In 8 out of 15 models the ITCZ alternates symmetrically between the hemispheres with the seasons. This seasonally alternating ITCZ error generates two wind speed maxima per year—one northerly and one southerly—resulting in spurious cooling in March and a cool SST error of the equatorial ocean. Most models have too much rain in the Southern Hemisphere so that SST and rain are too symmetric about the equator in the annual mean. Weak meridional wind on the equator near the South American coast (2°S–2°N, 80°–90°W) explains the warm SST error there.

Northeasterly wind jets blow over the Central American isthmus in winter and cool the SST in the eastern Pacific warm pool. In some models the strength of these winds contributes to the early demise of their northern ITCZ relative to observations. The February–April northerly wind bias on the equator is correlated to the antecedent December–February Central American Pacific wind speed at −0.88. The representation of southern-tropical stratus clouds affects the underlying SST through solar radiation, but its effect on the meridional atmospheric circulation is difficult to discern from the multimodel ensemble, indicating that errors other than the simulation of stratus clouds are also important for accurate simulation of the meridional asymmetry.

This study identifies several features to be improved in atmospheric and coupled GCMs, including the northeasterly cross–Central American wind in winter and meridional wind on the equator. Improved simulation of the seasonal cycle of meridional wind could alleviate biases in equatorial SST and improve simulation of ENSO and its teleconnections.

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Kathryn L. Verlinden and Simon P. de Szoeke

ABSTRACT

Time series of solar and thermal infrared radiative flux profiles are simulated with the Rapid Radiative Transfer Model (RRTM) using a hierarchy of constraints from radar reflectivity and passive microwave cloud remote sensing measurements collected over a ship in the southeastern tropical Pacific Ocean (20°S) during the second leg of the Variability of American Monsoon Systems (VAMOS) Ocean–Cloud–Atmosphere–Land Study Regional Experiment (VOCALS-REx). Incorporating additional constraints results in simulations of physically consistent radiative profiles throughout the atmosphere, especially within the cloud, where they are difficult to observe precisely. Simulated surface radiative fluxes are compared with those observed on the ship and by aircraft.

Due to the strong Rayleigh scattering of drizzle drops compared to cloud droplets that absorb, emit, and scatter natural radiation, cloud radar reflectivity overestimates cloud liquid water content (LWC). As a result, clouds are optically too thick and transmission ratios are too low in simulations using radar LWC. Imposing a triangular (increasing linearly with height from zero at cloud base) LWC profile in agreement with microwave liquid water path (LWP) improves the simulation of the transmission ratio. Constraining the corresponding microphysical cloud effective radius to that retrieved from optical depth, LWP, and cloud thickness results in additional improvements to the simulations. Time series, averages, and composite diurnal cycles of radiative fluxes, heating rates, and cloud radiative forcing are presented.

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Simon P. de Szoeke and Eric D. Maloney

ABSTRACT

The Madden–Julian oscillation (MJO) dominates tropical weather on intraseasonal 30–90-day time scales, yet mechanisms for its generation, maintenance, and propagation remain unclear. Although surface moist static energy (MSE) flux is greatest under strong winds in the convective phase, sea surface temperature (SST) warms by ~0.3°C in the clear nonconvective phase of the MJO. Winds converging into the hydrostatic low pressure under warm air over the warm SST increase the vertically integrated MSE. We estimate column-integrated MSE convergence using a model of mixed layer (ML) winds balancing friction, planetary rotation, and hydrostatic pressure gradients. Small (0.3 K) SST anomalies associated with the MJO drive 7 W m−2 net column MSE convergence averaged over the equatorial Indian Ocean ahead of MJO deep convection. The MSE convergence is in the right phase to contribute to MJO generation and propagation. It is on the order of the total MSE tendency previously assessed from reanalysis, and greater than surface heat flux anomalies driven by intraseasonal SST fluctuations.

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David B. Mechem, Sandra E. Yuter, and Simon P. de Szoeke

Abstract

A near-large-eddy simulation approach with size-revolving (bin) microphysics is employed to evaluate the relative sensitivity of southeast Pacific marine boundary layer cloud properties to thermodynamic and aerosol parameters. Simulations are based on a heavily drizzling cloud system observed by the NOAA ship Ronald H. Brown during the Variability of the American Monsoon Systems (VAMOS) Ocean–Cloud–Atmosphere–Land Study—Regional Experiment (VOCALS-Rex) field campaign. A suite of numerical experiments examines the sensitivity of drizzle to variations in boundary layer depth and cloud condensation nuclei (CCN) concentration in a manner consistent with the variability of those parameters observed during VOCALS-Rex. All four simulations produce cellular structures and turbulence characteristics of a circulation driven predominantly in a bottom-up fashion. The cloud and subcloud layers are coupled by strong convective updrafts that provide moisture to the cloud layer. Distributions of reflectivity calculated from model droplet spectra agree well with reflectivity distributions from the 5-cm-wavelength scanning radar aboard the ship, and the statistical behavior of cells over the course of the simulation is similar to that documented in previous studies of southeast Pacific stratocumulus. The simulations suggest that increased aerosol concentration delays the onset of drizzle, whereas changes in the boundary layer height are more important in modulating drizzle intensity.

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Simon P. de Szoeke, James B. Edson, June R. Marion, Christopher W. Fairall, and Ludovic Bariteau

Abstract

Dynamics of the Madden–Julian Oscillation (DYNAMO) and Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) observations and reanalysis-based surface flux products are used to test theories of atmosphere–ocean interaction that explain the Madden–Julian oscillation (MJO). Negative intraseasonal outgoing longwave radiation, indicating deep convective clouds, is in phase with increased surface wind stress, decreased solar heating, and increased surface turbulent heat flux—mostly evaporation—from the ocean to the atmosphere. Net heat flux cools the upper ocean in the convective phase. Sea surface temperature (SST) warms during the suppressed phase, reaching a maximum before the onset of MJO convection. The timing of convection, surface flux, and SST is consistent from the central Indian Ocean (70°E) to the western Pacific Ocean (160°E).

Mean surface evaporation observed in TOGA COARE and DYNAMO (110 W m−2) accounts for about half of the moisture supply for the mean precipitation (210 W m−2 for DYNAMO). Precipitation maxima are an order of magnitude larger than evaporation anomalies, requiring moisture convergence in the mean, and on intraseasonal and daily time scales. Column-integrated moisture increases 2 cm before the convectively active phase over the Research Vessel (R/V) Roger Revelle in DYNAMO, in accordance with MJO moisture recharge theory. Local surface evaporation does not significantly recharge the column water budget before convection. As suggested in moisture mode theories, evaporation increases the moist static energy of the column during convection. Rather than simply discharging moisture from the column, the strongest daily precipitation anomalies in the convectively active phase accompany the increasing column moisture.

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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.

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