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David Atlas, Bernard Walter, Shu-Hsien Chou, and P. J. Sheu

Abstract

The combination of vertical lidar and in situ meteorological observations from two aircraft provide an unprecedented view of the marine atmospheric boundary layer (MABL) during a cold air outbreak. To a first approximation, the lidar reflectivity is associated with the concentration of sea salt aerosols. Across the capping inversion, the lidar reflectivity contours approximate isentropes and streamlines thereby defining the inversion. Within the mixed layer, high reflectivity cores are associated with updrafts carrying aerosol-rich air upward and conversely. These effects are enhanced by increasing humidity in updraft and decreasing humidity in downdrafts that operate to increase and decrease aerosol sizes. Narrow high reflectivity columns extend upward from the ocean indicating that organized flow exists all the way to the surface. Entrainment across the inversion is manifested by small scale perturbations (∼200–500 m) superimposed upon the large scale (&sim 1–2 km) undulations of the inversion. These occur where the local entrainment zone is sharpest; generally, this is on the upshear side of the, convective. domes where Kelvin-Helmholtz instability is triggered by local compression of the inversion,

The MABL on 20 January 1983 is highly organized. The organization takes the form of 1–2 km scale roll vortices and corresponding undulations of the inversion with amplitude of 150–200 m peak to trough. The roll circulation is very strong with up and downdrafts of 2–4 in s−-1 at the 210 m level. The axes of the rolls are essentially north-south along the direction of the strong northerly low-level winds. The rising arm of the roll coincides with a column of high lidar reflectivity and with the updraft which transport aerosols, moisture, and heat up from the surface. The presence of the rolls, driven mainly by the combination of strong transverse sheer and buoyancy, serves to produce low-level convergence which concentrates the small-scale buoyant eddies to form a single well-ordered updraft in the manner previously postulated by LeMone.

The fluxes measured by the covariance method in the undulating inversion are unreliable because of the sensitivity to detrending and inadequate sampling of the exchanges across the interfaces of the dames and troughs. The partitioning method of Wilczak and Businger provides improved insight as to the mechanisms responsible for the downward flux in the inversion. However, unlike Wilczak and Businger, who find the downward flux dominated by cold updrafts we find that it is due mainly to the entrainment of warm eddies which are then transported downward by the larger-scale roll circulations on the downshear side of the domes.

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K-M. Lau, P-J. Sheu, and I-S. Kang

Abstract

In this paper, fundamental multiscale circulation modes in the global atmosphere are identified with the objective of providing better understanding of atmospheric low-frequency variabilities over a wide range of spatial and temporal scales. With the use of a combination of rotated principal component technique, singular spectrum analysis, and phase space portraits, three categories of basic multiscale modes in the atmosphere are found. The first is the interannual mode (IAM), which is dominated by time scales longer than a year and can be attributed to heating and circulation anomalies associated with the coupled tropical ocean-atmosphere, in particular the E1 Niño–Southern Oscillation. The second is a set of tropical intraseasonal modes consisting of three separate multiscale patterns (ISO-1, -2, -3) related to tropical heating that can be identified with the different phases of the Madden–Julian Oscillation (MJO), including its teleconnection to the extratropics. The ISO spatial and temporal patterns suggest that the extratropical wave train in the North Pacific and North America is related to heating over the Maritime Continent and that the evolution of the MJO around the equator may require forcing from the extratropics spawning convection over the Indian Ocean. The third category represents extratropical intraseasonal oscillations arising from internal dynamics of the basic-state circulation. In the Northern Hemisphere, there are two distinct circulation modes with multiple frequencies in this category: the Pacific/North America (PNA) and the North Atlantic/Eurasia (NAE). In the Southern Hemisphere, two phase-locked modes (PSA-1 and PSA-2) are found depicting an eastward propagating wave train from eastern Australia, via the Pacific South America to the South Atlantic. The extratropical modes exhibit temporal characteristics such as phase locking and harmonic oscillations possibly associated with quadratically nonlinear dynamical systems.

Additionally, the observed monthly and seasonal anomalies arise from a complex interplay of the various multiscale low-frequency modes. The relative dominance of the different modes varies widely from month to month and from year to year. On the monthly time scale, while one or two mechanisms may dominate in one year, no single mechanism seems to dominate for all years. There are indications that when the IAM, that is, ENSO heating patterns are strong, the extratropical modes may be suppressed and vice versa. For the seasonal mean, the interannual mode tends to dominate and the contribution from the PNA remains quite significant.

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K-M. Lau, P. J. Sheu, S. Schubert, D. Ledvina, and H. Weng

Abstract

An intercomparison study of the evolution of large-scale circulation features during TOGA COARE has been carried out using data from three 4D assimilation systems: the National Meteorological Center (NMC, currently known as the National Center for Environmental Prediction), the Navy Fleet Numerical Oceanography Center, and the NASA Goddard Space Flight Center. Results show that the preliminary assimilation products, though somewhat crude, can provide important information concerning the evolution of the large-scale atmospheric circulation over the tropical western Pacific during TOGA COARE. Large-scale features such as sea level pressure, rotational wind field, and temperature are highly consistent among models. However, the rainfall and wind divergence distributions show poor agreement among models, even though some useful information can still be derived. All three models shows a continuous background rain over the Intensive Flux Area (IFA), even during periods with suppressed convection, in contrast to the radar-estimated rainfall that is more episodic. This may reflect a generic deficiency in the oversimplified representation of large-scale rain in all three models.

Based on the comparative model diagnostics, a consistent picture of large-scale evolution and multiscale interaction during TOGA COARF emerges. The propagation of the Madden and Julian Oscillation (MJO) from the equatorial Indian Ocean region into the western Pacific foreshadows the establishment of westerly wind events over the COARE region. The genesis and maintenance of the westerly wind (WW) events during TOGA COARE are related to the establishment of a large-scale east-west pressure dipole between the Maritime Continent and the equatorial central Pacific. This pressure dipole could be identified in part with the ascending (low pressure) and descending (high pressure) branches of the MJO and in part with the fluctuations of the austral summer monsoon.

Accompanying the development of WW over the IFA and crucial to its maintenance is a robust meridional circulation, with strong cross-equatorial flow and rising motion near the entrance region of the WW and sinking motion in the extratropical Northern Hemisphere. The presence of a quasi-stationary equatorial heat source near the date line may have provided additional feedback mechanisms for the WWs. Surface pressure and wind surges related to cold air outbreaks off the East Asian continent play an important role in the rapid build up and/or termination of the WWs during TOGA COARE. The establishment of WWs in the near equatorial region may be linked to the modulation of North Pacific storm track activities.

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J. A. Curry, C. A. Clayson, W. B. Rossow, R. Reeder, Y.-C. Zhang, P. J. Webster, G. Liu, and R.-S. Sheu

An integrated approach is presented for determining from several different satellite datasets all of the components of the tropical sea surface fluxes of heat, freshwater, and momentum. The methodology for obtaining the surface turbulent and radiative fluxes uses physical properties of the atmosphere and surface retrieved from satellite observations as inputs into models of the surface turbulent and radiative flux processes. The precipitation retrieval combines analysis of satellite microwave brightness temperatures with a statistical model employing satellite observations of visible/infrared radiances. A high-resolution dataset has been prepared for the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE) intensive observation period (IOP), with a spatial resolution of 50 km and temporal resolution of 3 h. The high spatial resolution is needed to resolve the diurnal and mesoscale storm-related variations of the fluxes. The fidelity of the satellite-derived surface fluxes is examined by comparing them with in situ measurements obtained from ships and aircraft during the TOGA COARE IOP and from vertically integrated budgets of heat and freshwater for the atmosphere and ocean. The root-mean-square differences between the satellite-derived and in situ fluxes are dominated by limitations in the satellite sampling; these are reduced when some averaging is done, particularly for the precipitation (which is from a statistical algorithm) and the surface solar radiation (which uses spatially sampled satellite pixels). Nevertheless, the fluxes are determined with a useful accuracy, even at the highest temporal and spatial resolution. By compiling the fluxes at such high resolution, users of the dataset can decide whether and how to average for particular purposes. For example, over time, space, or similar weather events.

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