Drought monitoring and early detection have improved greatly in recent decades through the development and refinement of numerous indices and indicators. However, a lack of guidance, based on user experience, exists as to which drought-monitoring tools are most appropriate in a given location. This review paper summarizes the results of targeted user engagement and the published literature to improve the understanding of drought across North America and to enhance the utility of drought-monitoring tools. Workshops and surveys were used to assess and make general conclusions about the perceived performance of drought indicators, indices and impact information used for monitoring drought in the five main Köppen climate types (Tropical, Temperate, Continental, Polar Tundra, Dry) found across Canada, Mexico, and the United States. In Tropical, humid Temperate, and southerly Continental climates, droughts are perceived to be more short-term (less than 6 months) in duration rather than long-term (more than 6 months). In Polar Tundra climates, Dry climates, Temperate climates with dry warm seasons, and northerly Continental climates, droughts are perceived to be more long-term than short-term. In general, agricultural and hydrological droughts were considered to be the most important drought types. Drought impacts related to agriculture, water supply, ecosystem, and human health were rated to be of greatest importance. Users identified the most effective indices and indicators for monitoring drought across North America to be the U.S. Drought Monitor (USDM) and Standardized Precipitation Index (SPI) (or another measure of precipitation anomaly), followed by the Normalized Difference Vegetation Index (NDVI) (or another satellite-observed vegetation index), temperature anomalies, crop status, soil moisture, streamflow, reservoir storage, water use (demand), and reported drought impacts. Users also noted the importance of indices that measure evapotranspiration, evaporative demand, and snow water content. Drought indices and indicators were generally thought to perform equally well across seasons in Tropical and colder Continental climates, but their performance was perceived to vary seasonally in Dry, Temperate, Polar Tundra, and warmer Continental climates, with improved performance during warm and wet times of the year. The drought indices and indicators, in general, were not perceived to perform equally well across geographies. This review paper provides guidance on when (time of year) and where (climate zone) the more popular drought indices and indicators should be used. The paper concludes by noting the importance of understanding how drought, its impacts, and its indicators are changing over time as the climate warms and by recommending ways to strengthen the use of indices and indicators in drought decision making.

The Vegetation Impact Program (VIP) is a monitoring, assessment, and networking program hosted by the Midwestern Regional Climate Center. Major impacts on vegetation are often driven by weather and climate conditions. For example, damaging frost events, drought, and even flooding can impact vegetation in areas of agriculture, horticulture, nurseries, or home gardening. Pests and disease are also driven by environmental conditions.

Widespread frost or freeze events can cause extreme economic losses to the agriculture, horticulture, and nursery industries. Coordinated advance notice of an imminent freeze event can help minimize these losses. Forecasters can issue headlines ahead of these events if damage to susceptible vegetation is possible. Combining expertise among the forecasters and the vegetation specialists can provide a community collaborative opportunity that will inform the risks, susceptibility, and environmental conditions associated with frost and freeze impacts. The Midwestern Regional Climate Center has become the facilitator of this community collaboration effort through the development of the online Vegetation Impact Program and Frost/Freeze Guidance Project. This paper presents the development of these initiatives along with early results and findings.

The shortwave radiative effect of an ice cloud observed over the Atmospheric Radiation Measurement program's Southern Great Plains site in Oklahoma is investigated. Airborne microphysical data from a cloud particle imager, optical array probes, and forward scattering probes are used to construct vertical profiles of the size and shape distributions of ice crystals. Due to uncertainties associated with measuring the sizes and shapes of small ice crystals with maximum dimensions less than 120 mu m, five alternate size-shape distributions are derived and combined with existing databases of wavelength-dependent single-scattering properties of idealized ice crystals to obtain vertical profiles of optical properties. The dependence of the surface and the top-of-the-atmosphere fluxes on these uncertainties is simulated with a radiative transfer model. In addition, surface fluxes are compared against measurements at the surface. It is found that the differences between the modeled and measured fluxes are too large to be explained by uncertainties in the shape and concentrations of small ice crystals. Sensitivity tests suggest that the discrepancies occur because the real optical thickness is larger than that derived from the aircraft profiles most of the time. When the optical thickness was derived based on modeled and measured direct fluxes, the modeled total downward flux agreed well with the measurements. Slightly (less than 10%) reducing the asymmetry parameter, which is possibly associated with the presence of surface roughness, air bubble inclusions or other nonidealities in ice crystals, may further improve the agreement with observations.

El Nino/Southern Oscillation (ENSO) remains the most important coupled ocean-atmosphere phenomenon to cause global climate variability on seasonal to interannual time scales. This paper addresses the need for a reliable ENSO index that allows for the historical definition of ENSO events in the instrumental record back to 1871. The Multivariate ENSO Index (MEI) was originally defined as the first seasonally varying principal component of six atmosphere-ocean (COADS) variable fields in the tropical Pacific basin. It provides for a more complete and flexible description of the ENSO phenomenon than single variable ENSO indices such as the SOI or Nino 3.4 SST. Here we describe our effort to boil the MEI concept down to its most essential components (based on SLP, SST) to enable historical analyses that more than double its period of record to 1871-2005. The new MEI. ext confirms that ENSO activity went through a lull in the early-to mid-20th century, but was just about as prevalent one century ago as in recent decades. We diagnose strong relationships between peak amplitudes of ENSO events and their duration, as well as between their peak amplitudes and their spacing (periodicity). Our effort is designed to help with the assessment of ENSO conditions through as long a record as possible to be able to differentiate between 'natural' ENSO behaviour in all its rich facets, and the 'Brave New World' of this phenomenon under evolving GHG-related climate conditions. So far, none of the behaviour of recent ENSO events appears unprecedented, including duration, onset timing, and spacing in the last few decades compared to a full century before then. Copyright (C) 2011 Royal Meteorological Society

The impact of reduced Arctic summer sea ice on the atmosphere is investigated by forcing an atmospheric general circulation model, the Community Climate Model (CCM 3.6), with observed sea ice conditions during 1995, a low-ice year. The 51 experiments, which spanned April to October of 1995, were initiated with different states from a control simulation. The 55-year control was integrated using a repeating climatological seasonal cycle of sea ice. The response was obtained from the mean difference between the experiment and control simulations. The strongest response was found during the month of August where the Arctic displays a weak local thermal response, with wanner surface air temperatures and lower sea level pressure (SLP). However, there is a significant remote response over the North Pacific characterized by an equivalent barotropic (anomalies are collocated with height and increase in magnitude) structure, with anomalous high SLP collocated with a ridge in the upper troposphere. The ice anomalies force an increase (decrease) in precipitation north of (along) the North Pacific storm track. A linear baroclinic model forced with the transient eddy vorticity fluxes, transient eddy heat fluxes, and diabatic heating separately demonstrated that transient eddy vorticity fluxes are key to maintaining the anomalous high over the North Pacific. The model's sensitivity to separately imposed ice anomalies in the Kara, Laptev-East Siberian, or Beaufort seas includes SLP, geopotential height, and precipitation changes that are similar to but weaker than the response to the full sea ice anomaly.

Changes in extreme temperatures and pressures in the Arctic have received little attention in the context of climate change. Here we examine the distributions and extremes of surface air temperature and pressure in the Arctic for the late 20th century, using Alaskan weather station data, an atmospheric reanalysis, and general circulation models (GCMs). There is good agreement among these sources for the late 20th century, with broader distributions for both temperature and pressure in winter as compared to summer, and over land as compared to over ocean. We used the output from 21st-century greenhouse simulations by the GCMs to address the occurrence of extremes in the coming decades. The model projections of the 21st-century extremes largely agree with changes in the mean state, with record low temperatures decreasing in frequency and record high temperatures increasing in frequency. The changes in 21st-century extremes are more pronounced over the ocean, where the present-day distributions are narrower. The projected decreases of mean pressure result in more frequent occurrences of extreme low pressure, especially over the Arctic Ocean, although the extremes of pressure are less affected by changes of the means than are the extremes of temperature. Lastly, we find that the transition from sea ice to open water, and associated changes in the salinity of the surface water, can cause changes in the temperature distribution that are more complex than simple shifts in the distribution, leading to unexpected changes in the occurrence of extreme temperatures.

During the Bow Echo and Mesoscale Convective Vortex Experiment, the NOAA P-3 research aircraft executed 17 spiral descents to the rear of convective lines to document the vertical variability of hydrometeors above, within, and below the stratiform melting layer. Ten spirals were behind lines that exhibited bowing, at some stage in their evolution. Although quick descents on some spirals forced sampling of different particle zones, clear trends with respect to temperature were seen. For 16 spirals, the ambient relative humidity with respect to ice was in the range of 100% +/- 4% at temperatures between -10 degrees C and the melting layer, but exhibited steady decreases below the melting layer to an average relative humidity with respect to water of 77% +/- 15% at 9 degrees C. In contrast, one spiral conducted on 29 June 2003 directly behind a developing bow echo had a relative humidity with respect to ice averaging 85% at heights above the 0 degrees C level and relative humidity with respect to ice further decreased below the 0 degrees C level to a minimum relative humidity with respect to water of 48% at 9 degrees C. Vertical profiles of particle shapes, size distributions (SDs), total mass contents (TMC), number concentrations, and parameters of gamma distributions fit to SDs were computed using optical array probe data in conjunction with measurements of radar reflectivity from the P-3 X-band tail radar. For spirals with humidity at or near saturation above the melting layer, melting particles occurred through about 300 in of cloud depth between 0 degrees and 2 degrees or 3 degrees C. Above the melting layer, number concentrations, dominated by smaller crystals, decreased at 19% +/- 10% degrees C-1, faster than the 10% +/- 7% degrees C-1 decrease of TMC dominated by larger particles. Increases in the numbers of crystals with a maximum dimension < 2 mm (N-< 2) and in the slope parameter with temperature also occurred. To the extent that in-cloud heterogeneity did not complicate observed trends, these trends suggest aggregation dominated the evolution of SDs. Observations on 29 June differ from other days and are explained by the unique position and timing of the spiral in subsaturated air behind a developing bow. On 29 June the presence of an isothermal layer at 2.5 degrees C suggested that sublimative cooling delayed the onset of melting. Ice at 7 degrees C showed that melting particles were present through 500 m of cloud depth. A slight decrease in N-< 2, but no decrease in the slope parameter, with temperature suggested that sublimation modified the impact of aggregation. Sublimative cooling would only have been significant at the location of the 29 June spiral. For other spirals, evaporative cooling below the melting layer in subsaturafed regions was the most important diabatic processes in the stratiform regions at the time of the observations.

McFarquhar et al. (2002) developed a parameterization for the single-scattering properties of tropical clouds and McFarquhar et al. (2003) found that both the representation of cloud microphysical and single-scattering properties affect estimates of cloud radiative forcing for single column model (SCM) simulations over the ARM TWP site. Thus it is important to explore how representations of microphysics and single-scattering properties vary for clouds forming in different geographical locations and associated with different formation mechanisms. Here, we combine state-of-theart libraries of single scattering properties for idealized crystals with in-situ measurements of ice crystal number concentrations derived from standard optical array probes and with size-dependent habit distributions determined from the cloud particle imager (CPI) during the 2000 Cloud IOP to develop similar parameterizations for mid-latitude clouds. We also examine differences from the parameterizations developed for tropical clouds.

The influence of realistic Arctic sea ice anomalies on the atmosphere during winter is investigated with version 3.6 of the Community Climate Model (CCM3.6). Model experiments are performed for the winters with the most (1982/83) and least (1995/96) Arctic ice coverage during 1979-99, when ice concentration estimates were available from satellites. The experiments consist of 50-member ensembles: using large ensembles proved critical to distinguish the signal from noise.The local response to ice anomalies over the subpolar seas of both the Atlantic and Pacific is robust and generally shallow with large upward surface heat fluxes (>100 W m(-2)), near-surface warming, enhanced precipitation, and below-normal sea level pressure where sea ice receded, and the reverse where the ice expanded. The large-scale response to reduced (enhanced) ice extent to the east (west) of Greenland during 1982/83 resembles the negative phase of the Arctic Oscillation/North Atlantic Oscillation (AO/NAO) with a ridge over the poles and a trough at midlatitudes. The large-scale response was distinctly different in the Pacific, where ice extent anomalies in the Sea of Okhotsk generate a wave train that extends downstream over North America but the wave train response is greatly diminished when the model is driven by ice concentration rather than ice extent anomalies. Comparing the AGCM response to observations suggests that the feedback of the ice upon the atmospheric circulation is positive (negative) in the Pacific (Atlantic) sector. The magnitude of the wintertime response to ice extent anomalies is modest, on the order of 20 m at 500 mb. However, the 500-mb height anomalies roughly double in strength over much of the Arctic when forced by ice concetration anomalies. Furthermore, the NAO-like response increases linearly with the aerial extent of the Atlantic ice anomalies and thus could be quite large if the ice edge retreats as a result of global warming.

The study described here is a synthesis of global climate model projections of Northern Hemisphere sea ice through the end of the 21st century. The synthesis includes an enhancement of the informational content of the projections from a set of five global atmosphere–ocean–ice models. The adjustments are based on the systematic errors in the models’ present-day simulations relative to the HadISST observational data set. All models show decreases of sea ice through the 21st century when forced by the B2 scenario of greenhouse gas and aerosol concentrations. However, the differences in the present-day ice coverage simulated by the models are sufficiently large that they dominate the across-model variances of the projected ice extents. The adjustments based on the present-day biases remove much of the spread among the projections. The decreases of the adjusted ice extent by the year 2100 range from about 12 % to about 46 %. The percentage decreases are larger in summer than in winter; much of the Arctic Ocean is ice-free at the time of the summer ice minimum by the year 2099.

An extension of the simple stochastic climate model of Frankignoul and Hasselman that includes the effects of seasonal variations in upper-ocean mixed layer depth upon the persistence of winter sea surface temperature (SST) anomalies is proposed. Seasonal variations in mixed layer depth allow for the "reemergence mechanism,'' whereby thermal anomalies stored in the deep winter mixed layer persist at depth through summer and become partially reentrained into the mixed layer during the following winter. In this way, SST anomalies can recur from winter to winter without persisting through the intervening summer. Reformulating the simple stochastic climate model in terms of an effective ocean thermal capacity given by the depth of the winter mixed layer, thereby implicitly taking into account reemergence, is shown to provide a favorable fit to the observed winter-to-winter SST autocorrelations in the North Atlantic and Pacific, and represents a considerable improvement over the original model. The extended model also compares favorably with results from an entraining bulk ocean mixed layer model coupled to an atmospheric general circulation model. The authors propose that the extended model be adopted as the new "null hypothesis'' for interannual SST variability in middle and high latitudes.

Cloud radiative feedback is the most important effect determining climate response to human activity. Ice clouds reflect solar radiation and absorb thermal emission from the ground and the lower atmosphere and emit infrared radiation to space. The representation of these processes in models affects future climate predictions and there is much uncertainty in the representation of these processes. The size and shape of ice crystals has a major impact on how ice crystals affect radiation. Although the singlescattering properties of pristine ice crystals are generally well known (e.g., Yang et al. 2000), the singlescattering properties of irregular ice crystals and those of clouds composed of mixtures of various shaped crystals are not well known. Here, relationships between cloud and radiative properties, and their effects on the Earth’s energy budget, are explored.