Search Results

You are looking at 1 - 10 of 32 items for

  • Author or Editor: W. Timothy Liu x
  • Refine by Access: All Content x
Clear All Modify Search
W. Timothy Liu

Abstract

Wind speeds from the scatterometer (SASS) on the ocean observing satellite SEASAT averaged over 2° latitude by 2° longitude and a 92-day period are compared with wind speeds from ship reports in the western North Atlantic and the eastern North Pacific, where the concentrations of ship reports are high and the ranges of atmospheric stability and sea surface temperature are large. The comparison results are consistent for each region and for the combined data. Scatterometer winds are found to be generally higher than ship winds. The systematic dependence of the difference between scatterometer winds and ship winds on sea surface temperature and atmospheric stability are identified. The quality of ship reports is not ideal but should not depend on atmospheric stability or sea surface temperature. The systematic dependences, therefore may reflect the characteristics of scatterometer winds.

Multivariate regressions are used to extract the independent effects of different factors on the wind speed differences The difference between scatterometer winds and neutral winds from drip reports increases with increasing atmospheric stability and the trend is more prominent under stable than unstable conditions BY replacing neutral winds with observed winds, the stability dependence is reduced. At low wind speeds, the wind difference is found to depend also on sea surface temperature, probably due to temperature dependent factors, such as water viscosity, which are not included in the SASS model function. This dependence is greatly reduced at wind speeds higher than 8 m s−1. After the systematic dependence is removed from the scatterometer winds, the rms difference between the scatterometer winds and the neutral winds from ship reports was reduced from 1.7 to 0.9 m s−1,

The scatterometer measures backscatter from ocean surface short waves. The results of this study call for better understanding of the energy input from the atmosphere to the short waves which may depend on atmospheric stability and the dissipation of this energy through processes that may be affected by temperature dependent fluid properties (e.g., viscosity and surface tension).

Full access
Full access
W. Timothy Liu

Abstract

Monthly summaries of atmospheric soundings taken over 17 years from 49 midocean stations at small islands and weather ships distributed over major oceans are examined. Over tropical oceans, precipitable water is found to be a better predictor of surface-level humidity than surface-level air temperature. A statistical relation in the form of a polynomial is derived; from this relation, the monthly-mean, surface-level mixing ratio can be computed from monthly-mean precipitable water. The root-mean-square differences between the measured and derived values were found to be less than 8 × 10−4 over most ocean areas. Such a relation is useful in deriving large-scale evaporation and latent heal flux data from the ocean, using spaceborne observations.

The temporal and spatial variabilities of data deviations from this relation are examined. This relation is found to be applicable to all major ocean basins and can be used to monitor interannual variability. Boundary-layer thermodynamics of different air masses are suggested as an explanation of some characteristics of this relation.

Full access
W. Timothy Liu and Xiaosu Xie

Abstract

Satellite observations between 2007 and 2015 are used to characterize the annual occurrence of the premonsoon drought (PMD), which causes human death and economic hardship in India, and to postulate its scientific causes. The PMD is identified as the driest and hottest weeks in central India just before the summer monsoon onset. The onset is marked by a sharp increase in precipitation and soil moisture and a decrease in air temperature. The difference between integrated moisture transported in from the Arabian Sea and out to the Bay of Bengal is largely deposited as rain over land during the summer monsoon. The PMD occurs during the short period when moisture is drawn out to the Bay of Bengal before it can be replenished from the Arabian Sea. The time gap is caused by the earlier start of summer monsoon (southwest) winds in the Bay of Bengal than in the Arabian Sea. Sea surface temperature rise precedes the start of summer monsoon wind in both the Arabian Sea and the Bay of Bengal, and it has the potential to give advance warning of the PMD and thus allow mitigation of the adverse effects.

Full access
W. Timothy Liu and Wenqing Tang

Abstract

Ocean surface stress, the turbulent transport of momentum, is largely derived from wind through a drag coefficient. In tropical cyclones (TCs), scatterometers have difficulty measuring strong wind and there is large uncertainty in the drag coefficient. This study postulates that the microwave backscatter from ocean surface roughness, which is in equilibrium with local stress, does not distinguish between weather systems. The reduced sensitivity of scatterometer wind retrieval algorithms under the strong wind is an air–sea interaction problem that is caused by a change in the behavior of the drag coefficient rather than a sensor problem. Under this assumption, a stress retrieval algorithm developed over a moderate wind range is applied to retrieve stress under the strong winds of TCs. Over a moderate wind range, the abundant wind measurements and the more established drag coefficient value allow for sufficient stress data to be computed from wind to develop a stress retrieval algorithm for the scatterometer. Using 0.9 million coincident stress and wind pairs, the study shows that the drag coefficient decreases with wind speed at a much steeper rate than previously revealed, for wind speeds over 25 m s−1. The result implies that the ocean applies less drag to inhibit TC intensification, and that TCs cause less ocean mixing and surface cooling than previous studies indicated.

Full access
W. Timothy Liu and W. G. Large

Abstract

The values of sea surface stress determined with the dissipation method and those determined with a surface-layer model from observations on F.S. Meteor during the Joint Air-Sea Interaction (JASIN) Experiment are compared with the backscatter coefficients measured by the scatterometer SASS on the satellite Seasat. This study demonstrates that SASS can be used to determine surface stress directly as well as wind speed. The quality of the surface observations used in the calibration of the retrieval algorithms, however, is important. This sample of measurements disagrees with the predictions by the existing wind retrieval algorithm under non-neutral conditions and the discrepancies depend on atmospheric stability.

Full access
W. Timothy Liu and Pearn P. Niiler

Abstract

A simple statistical technique is described to determine monthly mean marine surface-layer humidity, which is essential in the specification of surface latent heat flux, from total water vapor in the atmospheric column measured by space-borne sensors. Good correlation between the two quantities was found in examining the humidity sounding from radiosonde reports of mid-ocean island stations and weather ships. The relation agrees with that obtained from satellite (Seasat) data and ship reports averaged over 2° areas and a 92-day period in the North Atlantic and in the tropical Pacific. The results demonstrate that, by using a local regression in the tropical Pacific, total water vapor can be used to determine monthly mean surface layer humidity to an accuracy of 0.4 g kg−1. With a global regression, determination to an accuracy of 0.8 g kg−1 is possible. These accuracies correspond to approximately 10 and 20 W m−2 in the determination of latent heat flux with the bulk parameterization method, provided that other required parameters are known.

Full access
Yi-Hui Wang and W. Timothy Liu

Abstract

This study investigates the regional atmospheric response to the Kuroshio Extension (KE) using a combination of multiple satellite observations and reanalysis data from boreal winter over a period of at least a decade. The goal is to understand the relationship between KE variations and atmospheric responses at low frequencies. A climate index is used to measure the interannual to decadal KE variability, which leaves remarkable imprints on the mesoscale sea surface temperature (SST). Clear spatial coherence between the SST signals and frontal-scale atmospheric variables, including surface wind convergence, vertical velocity, precipitation, and clouds, is identified by linear regression analysis. Consistent with previous studies, the penetrating effect of the KE variability on the free atmosphere is found. The westward tilt of the atmospheric response above the KE near 500 hPa is revealed. The difference in the associations of frontal-scale air temperature and geopotential height with the KE variability between the satellite observations and the reanalysis data suggests an imperfect interpretation of frontal-scale oceanic forcing on the overlying atmosphere in the reanalysis assimilation system.

Full access
R. A. Brown and W. Timothy Liu

Abstract

An operational planetary boundary layer model (Brown, 1974,1978, 1981) for determining surface winds and stress from free-stream flow has been modified for the marine layer by including surface roughness feedback, variable humidity and interfacial layer effects. The surface winds determined from synoptic-scale pressure and temperature fields are compared to surface measurements in GOASEX and JASIN.

Full access
W. Timothy Liu, Kristina B. Katsaros, and Joost A. Businger

Abstract

Abstract not available.

Full access