Search Results

You are looking at 1 - 10 of 121 items for

  • Author or Editor: S. Lee x
  • Refine by Access: All Content x
Clear All Modify Search
Lee S. Elson

Abstract

Infrared and radio observations of the upper cloud region of Venus indicate that the north polar region contains Features of large thermal contrast. A cold collar, encompassing a region of temperature inversions, lies between latitudes of ∼65 and 75°C, and a pair of warm features, separated by ∼180° of longitude and centered near 80° latitude, rotate about the pole with a period of ∼2.9 days. It is shown that the cold temperatures associated with the inversions lead to an enhancement in the mean zonal wind in a localized area near the pole, and that this enhancement makes the mean flow barotropically unstable. Since data for this region are limited a model for the thermal structure has been used for calculating growth times and phase periods of the unstable modes. Choosing model parameters to agree as closely as possible with available data, it has been determined that the rotating warm features are likely to be manifestations of barotropically unstable waves.

Full access
Lee S. Elson

Abstract

Geopotential height fields, based on high vertical resolution radiometer measurements, have been used to infer the circulation in the stratosphere from 100 to 1 mb, in the Northern Hemisphere, on a daily basis during February and March of 1979. Initial calculations were based on geostrophy in the traditional way. In addition to demonstrating the benefits of high vertical resolution, these calculations show that for the disturbed conditions present at this time, many of the terms in the momentum equation which were neglected would have made nonnegligible contributions to the balance of terms. In particular, the convergence of meridional wave flux can affect the zonally averaged meridional component of the momentum budget. Ageostrophic terms can affect the zonally varying part of the momentum budgets as well, although an accurate assessment of their importance is complicated by nonlinear processes. These and other results suggest that studies using diagnostically derived winds should include a scale analysis of the momentum budget to verify that the approximations used are valid. Revised estimates have been made of both the zonally averaged and zonally varying components of the wind that include important ageostrophic contributions.

Full access
Lee S. Elson

Abstract

Based on the success of several 2-D (latitude, longitude) linear barotropic instability models at matching some of the observed characteristics of the cloud level, polar region of the Venus atmosphere, a more realistic, linear, 3-D (height, latitude and longitude) model has been developed to further test the hypothesis that the observed features can be described by linear instability theory. The approach taken is to vary the model input parameters to see whether it is possible to produce modes that resemble the observations of wave activity and to compare those input parameters with other observations of the mean state.

Sensitivity studies show that in addition to a well-documented dependence on the mean zonal wind, the growth and propagation of unstable modes depends on the latitude variation of the mean temperature (and hence static stability). These studies have lead to the specification of a model basic state wind and temperature field that produces modes which are matched to observations of spatial structure, preferred wavenumber and phase speed of the polar disturbances. Wavenumber 2 is found to have the shortest growth time and unlike the 2-D results wavenunibers 1–3 share a nearly common period of about 3 days. The derived basic state has a temperature field that is quite similar to Pioneer Venus observations; however, in some regions the model basic state wind field departs from cyclostrophic values based on temperature observations.

Full access
Lee S. Elson

Abstract

Data from the Limb Infrared Monitor of the Stratosphere (LIMS) have been used to define zonally averaged basic-state temperature and zonal wind fields in the middle atmosphere for several periods during the winter of 1978–79. This basic state has been used to calculate the phase speeds, growth rates, and spatial structures of unstable modes using a linear, quasigeostrophic model. These results have been compared with temperature and ozone variance amplitudes from a spectral analysis of the same LIMS data. The comparison indicates that there is a close match between phase speeds for the most rapidly growing modes predicted by the model and phase speeds for statistically significant temperature and ozone variances. Both calculated and observed modes tend to be limited in latitudinal extent to a few tens of degrees and in vertical extent to about 10 km. These modes also tend to be nondispersive. Examples are given for the Southern Hemisphere near 0.25 mb (60 km) and for low latitudes of the Northern Hemisphere near 15 mb (30 km).

Full access
Lee S. Elson

Abstract

A diagnostic, quasi-linear model has been developed which uses observed solar-related temperatures and a specified zonal mean circulation and thermal structure to find the solar-related circulation above the clouds of Venus. Because there are no observations of the mean circulation above the clouds, it has been calculated using an independent model. Although the model-derived, solar-related circulation depends on the mean flow to a much greater degree than is the case for terrestrial tides, and although there is uncertainty in this mean flow, several important conclusions have been drawn concerning the solar-related circulation and thermal structure. Given that the solar forcing is likely to have a maximum in equatorial regions. there is an anomalously large response in the polar regions. It is primarily because of this unusual polar thermal structure that the model requires some process, such as dissipation, to act as an important sink for momentum. In the model, dissipation is specified as a Rayleigh friction whose coefficient is an unknown, free parameter. If such a formalism is correct, it is concluded that either the dissipation is extremely efficient by terrestrial standards and the solar-related circulation is small, or the dissipation is similar to that of the earth and the circulation is likely to be large enough to have an impact on the mean circulation.

Full access
C. I. Lin and S. C. Lee

Abstract

No abstract available.

Full access
Paul S. Lee and Fred M. Snell

Abstract

An annual, zonally averaged, steady-state global climate model is developed which represents an extension of the hemispherical model reported by Temkin and Snell (1976). The model employs twelve 15° zones and incorporates the diffuse thin cloud tropospheric structure of Weare and Snell (1974) as a feedback mechanism calculated separately over land and ocean employing the appropriate mean land height temperatures and ocean level temperatures. Radiative calculations are also carried separately over land and ocean. Meridional energy transport is parameterized very similar to that of Temkin and Snell but also includes transport across the equator linearly related to the temperature difference.

The response of the model to variations in various climatic determinants is studied, including global variations in carbon dioxide, aerosol and solar constant as well as additions of chlorofluorocarbons to the atmosphere. The model reveals that the Southern Hemisphere is more stable than the Northern Hemisphere by a factor of about 1.4 due to the dominance of ocean between 30° and 65°S with very little land-snow positive feedback. The Northern Hemisphere response is very similar to that reported by Temkin and Snell; therefore on a global basis the model shows somewhat greater stability. The response to additions of chlorofluorocarbons is about one-half of that reported by Ramanathan, giving an increase of 0.2 K in global average temperature at 1 ppbv for both CF2Cl2 and CFCI3.

Full access
C. L. Lin and S. C. Lee

Abstract

A study has been made of the growth through collision of water drops in the atmosphere. The method of superposition of flow fields obtained from the numerical solution of the Navier-Stokes equations was used for the calculations, and only inertia, gravity and drag force were considered.

The calculated linear collision efficiency is significantly less than the geometric collision efficiency between a large collector drop and a small collecting drop because of the effect of hydrodynamic forces. The linear collision efficiency is substantially higher than the geometric collision efficiency between similarly sized drops because of the wake effect.

To verify the validity of the calculations, the analytical results were compared with available experimental data. Satisfactory agreement was obtained for most drop sizes. It is concluded that in the absence of electricity and turbulence the dominant factor in the formation of precipitation is the collisional growth of similarly sized drops.

Full access
Chia-Ying Lee and Shuyi S. Chen

Abstract

The atmospheric boundary layer (BL) in tropical cyclones (TCs) connects deep convection within rainbands and the eyewall to the air–sea interface. Although the importance of the BL in TCs has been widely recognized in recent studies, how physical processes affect TC structure and intensity are still not well understood. This study focuses on a particular physical mechanism through which a TC-induced upper-ocean cooling within the core circulation of the TC can affect the BL and TC structure. A coupled atmosphere–ocean model forecast of Typhoon Choi-Wan (2009) is used to better understand the physical processes of air–sea interaction in TCs. A persistent stable boundary layer (SBL) is found to form over the cold wake within the TC’s right-rear quadrant, which influences TC structure by suppressing convection in rainbands downstream of the cold wake and enhancing the BL inflow into the inner core by increasing inflow angles over strong SST and pressure gradients. Tracer and trajectory analyses show that the air in the SBL stays in the BL longer and gains extra energy from surface heat and moisture fluxes. The enhanced inflow helps transport air in the SBL into the eyewall. In contrast, in the absence of a TC-induced cold wake and an SBL in an uncoupled atmosphere model forecast, the air in the right-rear quadrant within the BL tends to rise into local rainbands. The SBL formed over the cold wake in the coupled model seems to be a key feature that enhances the transport of high energy air into the TC inner core and may increase the storm efficiency.

Full access