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Siri Jodha Singh Khalsa

Abstract

Data from the 1972–73 El Niño event are used to study relationships between sea surface temperature (SST), surface wind, surface heat fluxes and rainfall in the tropical Pacific cast of the dateline. It is found that the maximum in SST anomaly is not correlated spatially or temporally in any consistent manner with the sum of latent and sensible heat fluxes (QE + QS) or rainfall as estimated from highly reflective cloud. Varying anomalies in wind speed and sea-air temperature and humidity differences all influence the QE + QS anomaly in ways which may be unrelated to the SST anomaly. Factors influencing SST, namely zonal wind stress and net surface heating, are found to vary with longitude. The position of the rainfall maximum and the time variation of rainfall in the near-equatorial convergence zone are more closely related to moisture convergence than to SST. Implications to modeling efforts which equate SST anomalies with diabatic heating anomalies are discussed.

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Siri Jodha Singh Khalsa

Abstract

Atmospheric static stability ahs a role in determining where and when deep convection can occur. Owing to a sparsity of upper-air stations through most of the tropics, satellites must be relied upon for stability information. Data from the TIROS Operational Vertical Sounder (TOVS) aboard NOAA's polar-orbiting satellites are evaluated for suitability in computing atmospheric stability. A stability index is derived and compared with both monthly means and individual soundings from in situ instruments. The results suggest that, although TOVS has several shortcomings, a good measure of atmospheric stability can be obtained.

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Siri Jodha Singh Khalsa and Gary K. Greenhut

Abstract

Data from two aircraft flights in the marine atmospheric boundary layer are analyzed in an investigation of entrainment processes at the top of the well-mixed layer. Both days had strong wind shear across the inversion, which is reflected in the horizontal wind statistics. Conditional sampling is used to show that near the top of the mixed layer most updrafts are cool, moist and deficient in along-wind momentum (slow) with respect to their environment. About half of these updrafts are still positively buoyant. Downdrafts that are warm, dry and have an excess of along-wind momentum (fast) occupy the greatest area of any downdraft type. Most of them are positively buoyant. Also found near the top of the mixed layer are large numbers of warm/dry/fast updrafts and cool/moist/slow downdrafts, i.e., drafts that have overturned. Time series from this level reveal large masses of cool/moist/slow and warm/dry/fast air, usually containing both upward-moving and downward-moving elements. These observations are related to features seen in returns from clear convective boundary layers, using various remote sensing systems.

Conditional sampling is also used to determine the net buoyant production or consumption of turbulence kinetic energy by each convective element. The results support the process partitioning method of entrainment closure but deviate from the assumption of linear buoyancy flux profiles for each proem.

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Gary K. Greenhut and Siri Jodha Singh Khalsa

Abstract

The properties of updrafts and downdrafts through the entire depth of the marine atmospheric boundary layer are investigated using conditional sampling based on an indicator function derived from the vertical velocity time series. Statistics on event size, number density and area occupied are obtained, along with conditional averages of the meteorological variables and percent contributions to the fluxes by updrafts, downdrafts and the environment. A single profile is obtained for convective mass flux based on conditional averaging of updrafts and downdrafts applicable to the fluxes of latent and sensible heat and momentum.

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Siri Jodha Singh Khalsa and Ellen J. Steiner

Abstract

Temperature and moisture data from the TIROS Operational Vertical Sounder (TOVS) archives are examined for applicability to studies of the tropical atmosphere on time scales ranging from intraseasonal to interannual. Comparisons with monthly mean radiosonde data from island stations confirm the ability of TOVS to track short- and long-term atmospheric variability. Biases and rms errors are generally different for near-equatorial and subtropical stations. Inclusion of soundings derived under cloudy conditions increases negative temperature bias while improving or leaving unchanged the rms temperature error at all levels.

The large volume of TOVS data (up to 16 000 soundings per day) is reduced to manageable form by the creation of a gridded product. A structure function analysis is performed to assist in the choice of gridding parameters. The objective analysis routines used are designed to handle data voids common in satellite data fields.

A time-longitude diagram of 5-day mean precipitable water (PW) in the 1000–700 mb layer shows a strong interannual (El Niño) signal as well as time variability in the 30–60 day range in the western Pacific and Indian oceans. On the shorter time scales, maximum PW is generally coincident with axes of minimum 250 mb velocity potential.

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Siri Jodha Singh Khalsa and Gary K. Greenhut

Abstract

The properties of updrafts and downdrafts in the lower third of the marine atmospheric boundary layer (MABL) over the central Pacific Ocean are investigated using a conditional sampling technique. When the drafts are classified according to their heat and moisture content, the properties of the major classes (moist updrafts and dry downdrafts) are in agreement with a parcel displacement model of vertical mixing. The minor class events appear to be the result of the reversal of motion of the major class events. Drafts that consume turbulent kinetic energy (TKE), although small in number in the lower third of the MABL, have spatial scales comparable with drafts that produce TKE. At the lowest level, the area occupied by positively buoyant downdraft exceeds the area occupied by negatively buoyant updrafts by a factor of 2. Updrafts and downdrafts produce a large fraction of the total fluxes of heat moisture and momentum, increasing from 75% at 0.07zi to 85% at 0.32zi. Of the draft contribution to the fluxes, 95% is due to the mean properties of the events and only 5% is due to the correlated fluctuations within the events. A convective man flux parameterization, based on the mean conditions within updraft and downdrafts, is obtained for the lower third of the mixed layer.

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Gary K. Greenhut and Siri Jodha Singh Khalsa

Abstract

Conditional sampling has been applied to aircraft turbulence measurements in order to study updrafts and downdrafts over the central equatorial Pacific Ocean. Average event size, number density and proportion of time series occupied are obtained for the drafts, along with conditional averages of horizontal momentum, moisture and virtual temperature and the draft contributions to the total fluxes. It is found that updrafts are usually cool/moist and warm/moist while downdrafts are most often warm/dry. Convective mass flux parameterizations of the sensible and latent heat flux are tested. Results consistent with previous workers are obtained when the flux production is mainly by updrafts. However, when downdrafts dominate, they must be taken into account explicitly in order for the parameterization to be accurate.

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