No abstract

Motivated by recent developments in tropical-cyclone dynamics, this paper reexamines a basic aspect of tropical-cyclone behaviour, namely, the sensitivity of tropical-cyclone models to the surface drag coefficient. Previous theoretical and numerical studies of the sensitivity in axisymmetric models have found that the intensity decreases markedly with increasing drag coefficient. Here we present a series of three-dimensional convection-permitting numerical experiments in which the intensification rate and intensity of the vortex increase with increasing surface drag coefficient until a certain threshold value is attained and then decrease. In particular, tropical depression-strength vortices intensify to major hurricane intensity for values of C K /C D as small as 0.1, significantly smaller than the critical threshold value of about 0.75 for major hurricane development predicted by Emanuel using an axisymmetric balance model. Moreover, when the drag coefficient is set to zero, no system-scale intensification occurs, despite persistent sea-to-air fluxes of moisture that maintain deep convective activity. This result is opposite to that found in a prior axisymmetric study by Craig and Gray.The findings are interpreted using recent insights obtained on tropical-cyclone intensification, which highlight the intrinsically unbalanced dynamics of the tropicalcyclone boundary layer. The reasons for the differences from earlier axisymmetric studies and some potential ramifications of our findings are discussed.The relative insensitivity of the intensification rate and intensity found for drag coefficients typical of high wind speeds over the ocean calls into question the need for coupled ocean wave-atmospheric models to accurately forecast tropical-cyclone intensity. Copyright c 2010 Royal Meteorological Society Key Words: hurricane; typhoon; surface exchange; frictional drag

The research program of the Center for Economic Studies (CES) produces a wide range of theoretical and empirical economic analyses that serve to improve the statistical programs of the U.S. Bureau of the Census. Many of these analyses take the form of CES research papers. The papers are intended to make the results of CES research available to economists and other interested parties in order to encourage discussion and obtain suggestions for revision before publication. The papers are unofficial and have not undergone the review accorded official Census Bureau publications. The opinions and conclusions expressed in the papers are those of the authors and do not necessarily represent those of the U.S. Bureau of the Census. Republication in whole or part must be cleared with the authors.

The link between information technology and productivity is clear. Yet how computers affect productivity is not well understood. Ours is the first study using data for approximately 30,000 U.S. manufacturing plants to examine the effect of computer networks on productivity. We find a positive and significant relationship between computer networks and plant labor productivity. Plants with lower productivity in earlier periods are also more likely to have a computer network, supporting the hypothesis that plants use networks to catch up. The positive network effect remains significant when we account for endogenous computer networks. (JEL L6, O3)

The research program of the Center for Economic Studies produces a wide range of theoretical and empirical economic analyses that serve to improve the statistical programs of the U.S. Bureau of the Census. Many of these analyses take the form of research papers. The purpose of the Discussion Papers is to circulate intermediate and final results of this research among interested readers within and outside the Census Bureau. The opinions and conclusions expressed in the papers are those of the authors and do not necessarily represent those of the U.S. Bureau of the Census. All papers are screened to ensure that they do not disclose confidential information.