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Benkui Tan
and
John P. Boyd

Abstract

The evolution, both stable and unstable, of contrarotating vortex pairs (“modons”) perturbed by upper-surface and bottom Ekman pumping is investigated using a homogeneous model with a variable free upper-surface and bottom topography. The Ekman pumping considered here differs from the classical Ekman pumping in that the divergence-vorticity term in the vorticity equation, nonlinear and omitted in previous studies, is explicitly included. Under the influence of both nonlinear Ekman pumping and the beta term, eastward- and westward-moving modons behave very differently.

Eastward-moving modons are stable to the upper-surface perturbation but westward-moving modons are not. The latter move southwestward, triggering the tilt instability: the beta effect deepens the cyclones but weakens the anticyclone, and the vortex pair disperses into wave packets.

Eastward-moving modons are stable to bottom friction in the sense that they diminish in time gradually at a rate independent of the signs of the vortices. Westward-moving modons behave differently depending on the strength of bottom friction. Cyclones decay faster than anticyclones, triggering the tilt instability in westward-moving modons, but only if the bottom friction is very weak. For sufficiently strong bottom friction, in contrast, modons decay monotonically: the cyclones still decay faster than anticyclones, but no wave packets formed before the modons completely dissipate.

Westward-moving modons are always unstable to topographic forcing. Eastward-moving modons have varying behavior controlled by the height and width of the topography. Below a critical height, determined by the width, modons survive the topographic interaction: their trajectory meanders but the two contrarotating vortices always remain bound together after escaping the topography. Above the critical height, modons disassociate: the two vortices separate and disperse into wave packets. When the width of the topography is comparable to modon width, there exists a stable window within the unstable region of the topographic height in which the modons also survive the topographic encounter.

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Jackson Tan
,
Christian Jakob
, and
Todd P. Lane

Abstract

The use of cloud regimes in identifying tropical convection and the associated large-scale atmospheric properties is investigated. The regimes are derived by applying cluster analysis to satellite retrievals of daytime-averaged frequency distributions of cloud-top pressure and optical thickness within grids of 280 km by 280 km resolution from the International Satellite Cloud Climatology Project between 1983 and 2008. An investigation of atmospheric state variables as a function of cloud regime reveals that the regimes are useful indicators of the archetypal states of the tropical atmosphere ranging from a strongly convecting regime with large stratiform cloudiness to strongly suppressed conditions showing a large coverage with stratocumulus clouds. The convectively active regimes are shown to be moist and unstable with large-scale ascending motion, while convectively suppressed regimes are dry and stable with large-scale descending winds. Importantly, the cloud regimes also represent several transitional states. In particular, the cloud regime approach allows for the identification of the “building blocks” of tropical convection, namely, the regimes dominated by stratiform, deep, and congestus convection. The availability of the daily distribution of these building blocks for more than 20 years opens new avenues for the diagnosis of convective behavior as well as the evaluation of the representation of convection in global and regional models.

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Cong Long Zhao
,
Peter S. Bakwin
, and
Pieter P. Tans

Abstract

Unattended measurements of carbon dioxide (CO2) mixing ratio at three altitudes up to 496 m above the surface on a television transmitter tower in the southeastern United States have been made for a period of 4 yr. This report describes the design of the automatic tower measuring system in detail. A nondispersive infrared (NDIR) analyzer is used to measure the CO2 concentration continuously. Real-time control and data collection uses a PC 486 running under the multitasking operating system QNX. The CO2 data show strong diurnal and seasonal variations, and large vertical gradients. A comparison of this study’s continental tower data with data from “background” sites should provide a strong constraint for regional and global models of terrestrial CO2 fluxes.

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Xi Guo
,
James P. Kossin
, and
Zhe-Min Tan

Abstract

Tropical cyclone (TC) translation speed (TCTS) can affect the duration of TC-related disasters, which is critical to coastal and inland areas. The long-term variation of TCTS and its relationship to the variability of the midlatitude jet stream and storm migration is discussed here for storms near the North Atlantic coast during 1948–2019. Our results reveal the prominent seasonality in the long-term variation of TCTS, which can be largely explained by the seasonality in the covariations of the midlatitude jet stream and storm locations. Specifically, significant increases of TCTS occur in June and October during the past decades, which may result from the equatorward displacement of the jet stream and poleward migration of storm locations. Prominent slowdown of TCTS is found in August, which is related to the weakened jet strength and equatorward storm migration. In September, the effects of poleward displacement and weakening of the jet stream on TCTS are largely compensated by the poleward storm migration, and therefore no significant change in TCTS is observed. Meanwhile, the multidecadal variability of the Atlantic may contribute to the multidecadal variability of TCTS. Our findings emphasize the significance in taking a seasonality view in discussing the variability and trends of near-coast Atlantic TCTS under climate change.

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Raul P. Lejano
,
Joyce Melcar Tan
, and
A. Meriwether W. Wilson

Abstract

As the world’s urban poor increase in numbers, they become acutely vulnerable to hazards from extreme weather events. On 8 November 2013, Typhoon Haiyan struck the province of Leyte, Philippines, with casualties numbering in the thousands, largely because of the ensuing storm surge that swept the coastal communities. This study investigates the role and dynamics of risk communication in these events, specifically examining the organizational processing of text within a complex institutional milieu. The authors show how the risk communication process failed to convey meaningful information about the predicted storm surge, transmitting and retransmitting the same routine text instead of communicating authentic messages in earnest. The key insight is that, rather than focus solely on the verbatim transmission of a scripted text, risk communication needs to employ various modes of translation and feedback signals across organizational and institutional boundaries. Adaptation will require overcoming organizational rigidities in order to craft proportionate responses to extreme weather events that may lie outside personal and institutional memory. Future work should build upon the textual processing approach to risk communication, expanding it into a comprehensive relational model of environmental cognition.

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Britton B. Stephens
,
Peter S. Bakwin
,
Pieter P. Tans
,
Ron M. Teclaw
, and
Daniel D. Baumann

Abstract

A commercially available differential fuel-cell analyzer has been adapted to make field-based ppm-level measurements of atmospheric O2 variations. With the implementation of rapid calibrations and active pressure and flow control, the analysis system described here has a 1σ precision of ±2.5 per meg (≈0.5 ppm) for a 2-min measurement. Allowing for system stabilization after switching inlet lines, a 6-min measurement with a precision of ±1.4 per meg (≈0.3 ppm) every 20 min is obtained. The elimination of biases in any atmospheric O2 measurement depends critically on careful gas-handling procedures, and after screening for known sources of bias a comparability of ±10 per meg (≈2 ppm) with the present setup is estimated. In comparison to existing techniques, the relatively small size, low cost, fast response, motion insensitivity, and ease of implementation of the fuel-cell analyzer make it particularly useful for a wide range of unattended field applications. This system has been used to measure atmospheric O2 concentrations at the WLEF tall-tower research site in northern Wisconsin semicontinuously from June 2000 to December 2003. These measurements represent the first extended O2 record in and above a forest ecosystem, and are being used to investigate global carbon budgeting, plant physiology, continental boundary layer mixing and synoptic transport, and potential means of industrial emission verification. In this paper, the measurement technique is described in detail and several weeks of data are presented to illustrate its performance.

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Haobo Tan
,
Jietai Mao
,
Huanhuan Chen
,
P. W. Chan
,
Dui Wu
,
Fei Li
, and
Tao Deng

Abstract

This paper discusses the application of principal component analysis and stepwise regression in the retrieval of vertical profiles of temperature and humidity based on the measurements of a 35-channel microwave radiometer. It uses the radiosonde data of 6 yr from Hong Kong, China, and the monochromatic radiative transfer model (MonoRTM) to calculate the brightness temperatures of the 35 channels of the radiometer. The retrieval of the atmospheric profile is then established based on principal component analysis and stepwise regression. The accuracy of the retrieval method is also analyzed. Using an independent sample, the root-mean-square error of the retrieved temperature is less than 1.5 K, on average, with better retrieval results in summer than in winter. Likewise, the root-mean-square error of the retrieved water vapor density reaches a maximum value of 1.4 g m−3 between 0.5 and 2 km, and is less than 1 g m−3 for all other heights. The retrieval method is then applied to the actual measured brightness temperatures by the 35-channel microwave radiometer at a station in Nansha, China. It is shown that the statistical model as developed in this paper has better retrieval results than the profiles obtained from the neural network as supplied with the radiometer. From numerical analysis, the error with the water vapor density retrieval is found to arise from the treatment of cloud liquid water. Finally, the retrieved profiles from the radiometer are studied for two typical weather phenomena during the observation period, and the retrieved profiles using the method discussed in the present paper is found to capture the evolution of the atmospheric condition very well.

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C. Potter
,
S. Klooster
,
P. Tan
,
M. Steinbach
,
V. Kumar
, and
V. Genovese

Abstract

Seventeen years (1982–98) of net carbon flux predictions from a simulation model based on satellite observations of monthly vegetation cover have been analyzed. The NASA–CASA model was driven by vegetation cover properties derived from the Advanced Very High Resolution Radiometer and radiative transfer algorithms that were developed for the Moderate Resolution Imaging Spectroradiometer (MODIS). It is found that although the terrestrial ecosystem sink for atmospheric CO2 on the North American continent has been fairly consistent at between +0.2 and +0.3 Pg C yr−1, high interannual variability in net ecosystem production (NEP) fluxes can be readily identified at locations across the continent. Five major areas having the highest variability were detected: 1) along the extreme northern vegetated zones of Canada and Alaska, 2) the northern Rocky Mountains, 3) the central-western U.S. Great Plains and central farming region, 4) across the southern United States and Mexico, and 5) in coastal forest areas of the United States and Canada. Analysis of climate anomalies over this 17-yr time period suggests that variability in precipitation and surface solar irradiance could be associated with trends in carbon sink fluxes within regions of high NEP variability.

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C. Potter
,
S. Klooster
,
P. Tan
,
M. Steinbach
,
V. Kumar
, and
V. Genovese

Abstract

Seventeen years (1982–98) of net carbon flux predictions for Southern Hemisphere continents have been analyzed, based on a simulation model using satellite observations of monthly vegetation cover. The NASA Carnegie Ames Stanford Approach (CASA) model was driven by vegetation-cover properties derived from the Advanced Very High Resolution Radiometer and radiative transfer algorithms that were developed for the Moderate Resolution Imaging Spectroradiometer (MODIS). The terrestrial ecosystem flux for atmospheric CO2 for the Amazon region of South America has been predicted between a biosphere source of –0.17 Pg C per year (in 1983) and a biosphere sink of +0.64 Pg C per year (in 1989). The areas of highest variability in net ecosystem production (NEP) fluxes across all of South America were detected in the south-central rain forest areas of the Amazon basin and in southeastern Brazil. Similar levels of variability were recorded across central forested portions of Africa and in the southern horn of East Africa, throughout Indonesia, and in eastern Australia. It is hypothesized that periodic droughts and wildfires associated with four major El Niño events during the 1980s and 1990s have held the net ecosystem carbon sink for atmospheric CO2 in an oscillating pattern of a 4–6-yr cycle, despite observations of increasing net plant carbon fixation over the entire 17-yr time period.

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É. Gerard
,
D. G. H. Tan
,
L. Garand
,
V. Wulfmeyer
,
G. Ehret
, and
P. Di Girolamo

The need for an absolute standard for water vapor observations, in the form of a global dataset with high accuracy and good spatial resolution, has long been recognized. The European Space Agency's Water Vapour Lidar Experiment in Space (WALES) mission aims to meet this need by providing high-quality water vapor profiles, globally and with good vertical resolution, using a differential absorption lidar (DIAL) system in a low earth-orbit satellite. WALES will be the first active system to measure humidity from space routinely. With launch envisaged in the 2008–2010 time frame and a minimum duration of two years, the primary mission goals are to (a) contribute to scientific research and (b) demonstrate the feasibility of longer-term operational missions. This paper assesses the benefits of the anticipated data to NWP through quantitative analysis of information content. Good vertical resolution and low random errors are shown to give substantial improvements in analysis error in one-dimensional variational data assimilation (1DVAR) comparisons with advanced infrared sounders. In addition, the vertical extent of the profiles is shown to reach 16.5 km or ~100 hPa, well above the limit of radiance assimilation (13 km or 200 hPa). Also highlighted are important applications in atmospheric sciences and climate research that would benefit from the low bias promised by spaceborne DIAL data and their complementarity to other types of humidity observations.

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