Search Results

You are looking at 1 - 4 of 4 items for :

  • Author or Editor: Robert H. Johns x
  • Journal of Climate x
  • All content x
Clear All Modify Search
Addison L. Sears-Collins, David M. Schultz, and Robert H. Johns
Full access
Addison L. Sears-Collins, David M. Schultz, and Robert H. Johns

Abstract

A climatology of nonfreezing drizzle is created using surface observations from 584 stations across the United States and Canada over the 15-yr period 1976–90. Drizzle falls 50–200 h a year in most locations in the eastern United States and Canada, whereas drizzle falls less than 50 h a year in the west, except for coastal Alaska and several western basins. The eastern and western halves of North America are separated by a strong gradient in drizzle frequency along roughly 100°W, as large as about an hour a year over 2 km. Forty percent of the stations have a drizzle maximum from November to January, whereas only 13% of stations have a drizzle maximum from June to August. Drizzle occurrence exhibits a seasonal migration from eastern Canada and the central portion of the Northwest Territories in summer, equatorward to most of the eastern United States and southeast Canada in early winter, to southeastern Texas and the eastern United States in late winter, and back north to eastern Canada in the spring. The diurnal hourly frequency of drizzle across the United States and Canada increases sharply from 0900 to 1200 UTC, followed by a steady decline from 1300 to 2300 UTC. Diurnal drizzle frequency is at a maximum in the early morning, in agreement with other studies.

Drizzle occurs during a wide range of atmospheric conditions at the surface. Drizzle has occurred at sea level pressures below 960 hPa and above 1040 hPa. Most drizzle, however, occurs at higher than normal sea level pressure, with more than 64% occurring at a sea level pressure of 1015 hPa or higher. A third of all drizzle falls when the winds are from the northeast quadrant (360°–89°), suggesting that continental drizzle events tend to be found poleward of surface warm fronts and equatorward of cold-sector surface anticyclones. Two-thirds of all drizzle occurs with wind speeds of 2.0–6.9 m s−1, with 7.6% in calm wind and 5% at wind speeds ⩾ 10 m s−1. Most drizzle (61%) occurs with visibilities between 1.5 and 5.0 km, with only about 20% occurring at visibilities less than 1.5 km.

Full access
T. C. Johns, C. F. Durman, H. T. Banks, M. J. Roberts, A. J. McLaren, J. K. Ridley, C. A. Senior, K. D. Williams, A. Jones, G. J. Rickard, S. Cusack, W. J. Ingram, M. Crucifix, D. M. H. Sexton, M. M. Joshi, B.-W. Dong, H. Spencer, R. S. R. Hill, J. M. Gregory, A. B. Keen, A. K. Pardaens, J. A. Lowe, A. Bodas-Salcedo, S. Stark, and Y. Searl

Abstract

A new coupled general circulation climate model developed at the Met Office's Hadley Centre is presented, and aspects of its performance in climate simulations run for the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4) documented with reference to previous models. The Hadley Centre Global Environmental Model version 1 (HadGEM1) is built around a new atmospheric dynamical core; uses higher resolution than the previous Hadley Centre model, HadCM3; and contains several improvements in its formulation including interactive atmospheric aerosols (sulphate, black carbon, biomass burning, and sea salt) plus their direct and indirect effects. The ocean component also has higher resolution and incorporates a sea ice component more advanced than HadCM3 in terms of both dynamics and thermodynamics. HadGEM1 thus permits experiments including some interactive processes not feasible with HadCM3. The simulation of present-day mean climate in HadGEM1 is significantly better overall in comparison to HadCM3, although some deficiencies exist in the simulation of tropical climate and El Niño variability. We quantify the overall improvement using a quasi-objective climate index encompassing a range of atmospheric, oceanic, and sea ice variables. It arises partly from higher resolution but also from greater fidelity in modeling dynamical and physical processes, for example, in the representation of clouds and sea ice. HadGEM1 has a similar effective climate sensitivity (2.8 K) to a CO2 doubling as HadCM3 (3.1 K), although there are significant regional differences in their response patterns, especially in the Tropics. HadGEM1 is anticipated to be used as the basis both for higher-resolution and higher-complexity Earth System studies in the near future.

Full access
Jonathan Spinoni, Paulo Barbosa, Edoardo Bucchignani, John Cassano, Tereza Cavazos, Jens H. Christensen, Ole B. Christensen, Erika Coppola, Jason Evans, Beate Geyer, Filippo Giorgi, Panos Hadjinicolaou, Daniela Jacob, Jack Katzfey, Torben Koenigk, René Laprise, Christopher J. Lennard, M. Levent Kurnaz, Delei Li, Marta Llopart, Niall McCormick, Gustavo Naumann, Grigory Nikulin, Tugba Ozturk, Hans-Juergen Panitz, Rosmeri Porfirio da Rocha, Burkhardt Rockel, Silvina A. Solman, Jozef Syktus, Fredolin Tangang, Claas Teichmann, Robert Vautard, Jürgen V. Vogt, Katja Winger, George Zittis, and Alessandro Dosio

Abstract

Two questions motivated this study: 1) Will meteorological droughts become more frequent and severe during the twenty-first century? 2) Given the projected global temperature rise, to what extent does the inclusion of temperature (in addition to precipitation) in drought indicators play a role in future meteorological droughts? To answer, we analyzed the changes in drought frequency, severity, and historically undocumented extreme droughts over 1981–2100, using the standardized precipitation index (SPI; including precipitation only) and standardized precipitation-evapotranspiration index (SPEI; indirectly including temperature), and under two representative concentration pathways (RCP4.5 and RCP8.5). As input data, we employed 103 high-resolution (0.44°) simulations from the Coordinated Regional Climate Downscaling Experiment (CORDEX), based on a combination of 16 global circulation models (GCMs) and 20 regional circulation models (RCMs). This is the first study on global drought projections including RCMs based on such a large ensemble of RCMs. Based on precipitation only, ~15% of the global land is likely to experience more frequent and severe droughts during 2071–2100 versus 1981–2010 for both scenarios. This increase is larger (~47% under RCP4.5, ~49% under RCP8.5) when precipitation and temperature are used. Both SPI and SPEI project more frequent and severe droughts, especially under RCP8.5, over southern South America, the Mediterranean region, southern Africa, southeastern China, Japan, and southern Australia. A decrease in drought is projected for high latitudes in Northern Hemisphere and Southeast Asia. If temperature is included, drought characteristics are projected to increase over North America, Amazonia, central Europe and Asia, the Horn of Africa, India, and central Australia; if only precipitation is considered, they are found to decrease over those areas.

Open access