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Daniel M. Gilford and Susan Solomon

Abstract

Water vapor and ozone are powerful radiative constituents in the tropical lower stratosphere, impacting the local heating budget and nonlocally forcing the troposphere below. Their near-tropopause seasonal cycle structures imply associated “radiative seasonal cycles” in heating rates that could affect the amplitude and phase of the local temperature seasonal cycle. Overlying stratospheric seasonal cycles of water vapor and ozone could also play a role in the lower stratosphere and upper troposphere heat budgets through nonlocal propagation of radiation. Previous studies suggest that the tropical lower stratospheric ozone seasonal cycle radiatively amplifies the local temperature seasonal cycle by up to 35%, while water vapor is thought to have a damping effect an order of magnitude smaller. This study uses Aura Microwave Limb Sounder observations and an offline radiative transfer model to examine ozone, water vapor, and temperature seasonal cycles and their radiative linkages in the lower stratosphere and upper troposphere. Radiative sensitivities to ozone and water vapor vertical structures are explicitly calculated, which has not been previously done in a seasonal cycle context. Results show that the water vapor radiative seasonal cycle in the lower stratosphere is not sensitive to the overlying water vapor structure. In contrast, about one-third of ozone’s radiative seasonal cycle amplitude at 85 hPa is associated with longwave emission above 85 hPa. Ozone’s radiative effects are not spatially homogenous: for example, the Northern Hemisphere tropics have a seasonal cycle of radiative temperature adjustments with an amplitude 0.8 K larger than the Southern Hemisphere tropics.

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Daniel M. Gilford, Susan Solomon, and Kerry A. Emanuel

Abstract

Recent studies have investigated trends and interannual variability in the potential intensity (PI) of tropical cyclones (TCs), but relatively few have examined TC PI seasonality or its controlling factors. Potential intensity is a function of environmental conditions that influence thermodynamic atmosphere–ocean disequilibrium and the TC thermodynamic efficiency—primarily sea surface temperatures and the TC outflow temperatures—and therefore varies spatially across ocean basins with different ambient conditions. This study analyzes the seasonal cycles of TC PI in each main development region using reanalysis data from 1980 to 2013. TC outflow in the western North Pacific (WNP) region is found above the tropopause throughout the seasonal cycle. Consequently, WNP TC PI is strongly influenced by the seasonal cycle of lower-stratospheric temperatures, which act to damp its seasonal variability and thereby permit powerful TCs any time during the year. In contrast, the other main development regions (such as the North Atlantic) exhibit outflow levels in the troposphere through much of the year, except during their peak seasons. Mathematical decomposition of the TC PI metric shows that outflow temperatures damp WNP TC PI seasonality through thermodynamic efficiency by a quarter to a third, whereas disequilibrium between SSTs and the troposphere drives 72%–85% of the seasonal amplitude in the other ocean basins. Strong linkages between disequilibrium and TC PI seasonality in these basins result in thermodynamic support for powerful TCs only during their peak seasons. Decomposition also shows that the stratospheric influence on outflow temperatures in the WNP delays the peak month of TC PI by a month.

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Daniel M. Gilford, Susan Solomon, and Robert W. Portmann

Abstract

An abrupt drop in tropical tropopause layer (TTL) water vapor, similar to that observed in 2000, recently occurred in 2011, and was concurrent with reductions in TTL temperature and ozone. Previous studies have indicated that such large water vapor variability can have significant radiative impacts. This study uses Aura Microwave Limb Sounder observations, the Stratospheric Water Vapor and Ozone Satellite Homogenized dataset, and two radiative transfer models to examine the radiative effects of the observed changes in TTL water vapor and ozone on TTL temperatures and global radiative forcing (RF). The analyses herein suggest that quasi-isentropic poleward propagation of TTL water vapor reductions results in a zonal-mean structure with “wings” of extratropical water vapor reductions, which account for about half of the 2011 abrupt drop global radiative impact. RF values associated with the mean water vapor concentrations differences between 2012/13 and 2010/11 are between −0.01 and −0.09 W m−2, depending upon the altitude above which perturbations are considered. TTL water vapor and ozone variability during this period jointly lead to a transient radiative cooling of ~0.25–0.5 K in layers below the tropopause. The 2011 abrupt drop also prolonged the reduction in stratospheric water vapor that followed the 2000 abrupt drop, providing a longer-term radiative forcing of climate. Water vapor concentrations from 2005 to 2013 are lower than those from 1990 to 1999, resulting in a RF between these periods of about −0.045 W m−2, approximately 12% as large as, but of opposite sign to, the concurrent estimated CO2 forcing.

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Daniel M. Gilford, Susan Solomon, and Kerry A. Emanuel

Abstract

This study investigates relationships between observed tropical cyclone (TC) maximum intensities and potential intensity (PI) over the seasonal cycle. To directly compare observed and potential intensities, one must account for month-to-month variability in TC tracks and frequencies. Historical TC best track data and reanalysis PI calculations are combined to develop an along-track record of observed maximum and potential intensities for each storm in the satellite-era (1980–2015) across four ocean basins. Overall, observed maximum intensity seasonal cycles agree well with those of along-track PI. An extreme value theory application shows that at least 25 storms must be observed in a given month to have high confidence that the most intense wind speeds of historical TCs follow along-track PI seasonality. In the North Atlantic and Southern Hemisphere regions, there are too few observed storms outside their traditional TC seasons, limiting PI applicability across the seasonal cycle. Small intraseasonal along-track PI variabilities in these regions are driven by TC thermodynamic disequilibrium and sea surface temperatures. Thermodynamic disequilibrium drives seasonal cycles of eastern North Pacific along-track PI and observed maximum intensity, which minimize in August and maximize in June and October. Western North Pacific along-track PI and observed maximum intensity seasonal cycles are relatively flat, and have a local minimum in August because of reduced thermodynamic efficiency, which is linked to anomalously warm near-tropopause outflow temperatures. Powerful (>65 m s−1) western Pacific TCs historically occur in every month except January, due to a combination of tropopause region and SST seasonal influences.

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Daniel M. Gilford, Shawn R. Smith, Melissa L. Griffin, and Anthony Arguez

Abstract

The daily temperature range (DTR; daily maximum temperature minus daily minimum temperature) at 290 southeastern U.S. stations is examined with respect to the warm and cold phases of the El Niño–Southern Oscillation (ENSO) for the period of 1948–2009. A comparison of El Niño and La Niña DTR distributions during 3-month seasons is conducted using various metrics. Histograms show each station’s particular distribution. To compare directly the normalized distributions of El Niño and La Niña, a new metric (herein called conditional ratio) is produced and results are evaluated for significance at 95% confidence with a bootstrapping technique. Results show that during 3-month winter, spring, and autumn seasons DTRs above 29°F (16.1°C) are significantly more frequent during La Niña events and that DTRs below 15°F (8.3°C) are significantly more frequent during El Niño events. It is hypothesized that these results are associated spatially with cloud cover and storm tracks during each season and ENSO phase. Relationships between DTRs and ENSO-related relative humidity are examined. These results are pertinent to the cattle industry in the Southeast, allowing ranchers to plan for and mitigate threats posed by periods of low DTRs associated with the predicted phase of ENSO.

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