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  • Author or Editor: David H. Bromwich x
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Zhong Liu
and
David H. Bromwich

Abstract

The blocking effect of Ross Island and Hut Point peninsula, Antarctica, has been investigated since the early part of this century. Due to lack of continuous measurements of boundary-layer winds, the investigations were limited to an overall description of the blocking effect with no information on the diurnal variation or the detailed vertical structure of the approaching airflow.

An acoustic sounder (sodar) was deployed during the 1990/91 austral summer season at Williams Field in the upwind area south of Ross Island, Antarctica. Such equipment can continuously measure three-dimensional winds from a few tens of meters above the surface up to an altitude of several hundred meters, thus providing a new opportunity to study the dynamics of the stably stratified planetary boundary layer.

In addition to confirming earlier work, the sodar winds show a significant diurnal variation of the blocking effect, which amplifies with height. Such variation is dominated by the changes in the upstream air mass in which katabatic airflow from Byrd, Mulock, and Skelton glaciers plays a central role.

Through case studies, the breakdown of the prevailing wind regime in the Ross Island area was associated with the influence of meso- and synoptic-scale pressure gradients on the katabatic airflow approaching from the south and with very localized geostrophic winds deflected around the topography of Ross Island.

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Zhong Liu
and
David H. Bromwich

Abstract

The surface wind pattern over the ice sheets of Antarctica is irregular with marked areas of airflow confluence near the coastal margins. Where cold air from a large interior area of the ice sheet converges (a confluence zone), an anomalously large supply of air is available to feed the coastal katabatic winds, which, as a result, are intensified and more persistent. The confluence zone inland of Siple Coast, West Antarctica, differs from its East Antarctic counterparts in that the terrain slopes become gentler rather than steeper as the coast is approached. In addition, synoptic processes exert substantially more impact on the behavior of the surface winds.

A month-long field program to study the dynamics of the springtime katabatic wind confluence zone has been carried out near Siple Coast. Two sites, Upstream B (83.5°S, 136.1°W) and South Camp (84.5°S, 134.3°W), were established roughly perpendicular to the downslope direction. The field program involved the use of the ground-based remote sensing equipment (sodar and RASS) along with conventional surface and balloon observations. Previous analyses revealed the cross-sectional structure of the confluence zone as consisting of a more buoyant West Antarctic katabatic airflow overlying a less buoyant katabatic airflow originating from East Antarctica.

The force balances inside the confluence zone are here investigated for three situations: mean (all available wind profiles from balloon launches), and two extreme cases (light and strong winds). A linear regression method is used to estimate the mean vertical wind shears and horizontal temperature gradients. The vertical wind shears are used to examine whether or not the airflows are in geostrophic balance. The results are 1) the airflow above the surface at both sites is in geostrophic balance for the three situations; 2) inside the West Antarctic katabatic wind zone, there are three forces in the north–south direction—the restoring pressure gradient force associated with blocking of the katabatic and synoptic winds, the downslope buoyancy force, and the synoptic pressure gradient force associated with the time-averaged low in the South Pacific Ocean; 3) above the West Antarctic katabatic wind layer, the observed easterly wind is due to the synoptic pressure gradient force associated with the low; 4) inside the East Antarctic katabatic wind zone, in addition to the above three forces, there is the downslope buoyancy force associated with the inversion; and 5) large-scale transient synoptic systems strongly influence the downslope wind speed and the boundary layer depth, resulting in the light and strong wind cases.

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Jonathan D. Wille
,
David H. Bromwich
,
John J. Cassano
,
Melissa A. Nigro
,
Marian E. Mateling
, and
Matthew A. Lazzara

Abstract

Accurately predicting moisture and stability in the Antarctic planetary boundary layer (PBL) is essential for low-cloud forecasts, especially when Antarctic forecasters often use relative humidity as a proxy for cloud cover. These forecasters typically rely on the Antarctic Mesoscale Prediction System (AMPS) Polar Weather Research and Forecasting (Polar WRF) Model for high-resolution forecasts. To complement the PBL observations from the 30-m Alexander Tall Tower! (ATT) on the Ross Ice Shelf as discussed in a recent paper by Wille and coworkers, a field campaign was conducted at the ATT site from 13 to 26 January 2014 using Small Unmanned Meteorological Observer (SUMO) aerial systems to collect PBL data. The 3-km-resolution AMPS forecast output is combined with the global European Centre for Medium-Range Weather Forecasts interim reanalysis (ERAI), SUMO flights, and ATT data to describe atmospheric conditions on the Ross Ice Shelf. The SUMO comparison showed that AMPS had an average 2–3 m s−1 high wind speed bias from the near surface to 600 m, which led to excessive mechanical mixing and reduced stability in the PBL. As discussed in previous Polar WRF studies, the Mellor–Yamada–Janjić PBL scheme is likely responsible for the high wind speed bias. The SUMO comparison also showed a near-surface 10–15-percentage-point dry relative humidity bias in AMPS that increased to a 25–30-percentage-point deficit from 200 to 400 m above the surface. A large dry bias at these critical heights for aircraft operations implies poor AMPS low-cloud forecasts. The ERAI showed that the katabatic flow from the Transantarctic Mountains is unrealistically dry in AMPS.

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Jonathan D. Wille
,
David H. Bromwich
,
Melissa A. Nigro
,
John J. Cassano
,
Marian Mateling
,
Matthew A. Lazzara
, and
Sheng-Hung Wang

Abstract

Flight operations in Antarctica rely on accurate weather forecasts aided by the numerical predictions primarily produced by the Antarctic Mesoscale Prediction System (AMPS) that employs the polar version of the Weather Research and Forecasting (Polar WRF) Model. To improve the performance of the model’s Mellor–Yamada–Janjić (MYJ) planetary boundary layer (PBL) scheme, this study examines 1.5 yr of meteorological data provided by the 30-m Alexander Tall Tower! (ATT) automatic weather station on the western Ross Ice Shelf from March 2011 to July 2012. Processed ATT observations at 10-min intervals from the multiple observational levels are compared with the 5-km-resolution AMPS forecasts run daily at 0000 and 1200 UTC. The ATT comparison shows that AMPS has fundamental issues with moisture and handling stability as a function of wind speed. AMPS has a 10-percentage-point (i.e., RH unit) relative humidity dry bias year-round that is highest when katabatic winds from the Byrd and Mulock Glaciers exceed 15 m s−1. This is likely due to nonlocal effects such as errors in the moisture content of the katabatic flow and AMPS not parameterizing the sublimation from blowing snow. AMPS consistently overestimates the wind speed at the ATT by 1–2 m s−1, in agreement with previous studies that attribute the high wind speed bias to the MYJ scheme. This leads to reduced stability in the simulated PBL, thus affecting the model’s ability to properly simulate the transfer of heat and momentum throughout the PBL.

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