GC11A-0123
Extreme Rainfall Events Over Southern Africa: Assessment of a Climate Model to Reproduce Daily Extremes
It is increasingly accepted that any possible climate change will not only have an influence on mean climate but may also significantly alter climatic variability. This issue is of particular importance for environmentally vulnerable regions such as southern Africa. The subcontinent is considered especially vulnerable extreme events, due to a number of factors including extensive poverty, disease and political instability. Rainfall variability and the identification of rainfall extremes is a function of scale, so high spatial and temporal resolution data are preferred to identify extreme events and accurately predict future variability. The majority of previous climate model verification studies have compared model output with observational data at monthly timescales. In this research, the assessment of a state-of-the-art climate model to simulate climate at daily timescales is carried out using satellite derived rainfall data from the Microwave Infra-Red Algorithm (MIRA). This dataset covers the period from 1993-2002 and the whole of southern Africa at a spatial resolution of 0.1 degree longitude/latitude. Once the model's ability to reproduce extremes has been assessed, idealised regions of SST anomalies are used to force the model, with the overall aim of investigating the ways in which SST anomalies influence rainfall extremes over southern Africa. In this paper, results from sensitivity testing of the UK Meteorological Office Hadley Centre's climate model's domain size are firstly presented. Then simulations of current climate from the model, operating in both regional and global mode, are compared to the MIRA dataset at daily timescales. Thirdly, the ability of the model to reproduce daily rainfall extremes will be assessed, again by a comparison with extremes from the MIRA dataset. Finally, the results from the idealised SST experiments are briefly presented, suggesting associations between rainfall extremes and both local and remote SST anomalies.
GC11A-0124
Remote sources of water vapor forming precipitation on the Norwegian west coast at 60 degree N - a tale of hurricanes and an atmospheric river
Precipitation amounts have increased in most areas of Norway during the last 100 years, consistent with an expected spin-up of the water cycle in a warming climate. Along with the monthly mean precipitation, heavy precipitation events have also become more frequent. In southwestern Norway where precipitation amounts are largest, the precipitation trends are strongest in fall. Climate models predict this trend to be continuing over the next few decades. In this context, we studied the most extreme precipitation event in recent years on the Norwegian southwest coast that occurred in September 2005, producing flooding and landslides. We found that this event was triggered by the transport of tropical and subtropical moisture associated with two former hurricanes, Maria and Nate, which both underwent transition into extratropical cyclones. The two former hurricanes generated a large stream of (sub)tropical air which extended over more than 40 degrees of latitude and across the North Atlantic Ocean and carried a large amount of moisture originally associated with hurricane Nate - a so-called atmospheric river or moisture conveyor belt. The mountains along the Norwegian coast caused a strong orographic enhancement of the precipitation associated with the moist air. A Lagrangian moisture tracking algorithm was employed to show that the evaporative source of the precipitation falling over Norway was distributed over large parts of the North Atlantic Ocean, and indeed included large contribution from the subtropics and smaller ones from the tropics. The moisture tracking algorithm was also applied over a 5-year period and it was found that (sub)tropical sources contributed substantially to the precipitation falling in southwestern Norway also during other events. It is, thus, likely that one reason for the strong observed and predicted positive trends of precipitation in southwestern Norway is an increased transport of (sub)tropical moisture in a warming climate. This might be particularly the case if hurricanes are becoming more frequent, as suggested by some scholars. It would also explain why the strongest trends occur in fall.
GC11A-0125
Changes in Central European Soil Moisture Availability and Atmospheric Circulation Patterns between 1875 and 2005 - Regional Climate Change in Progress?
Relationships between the soil moisture availability and the atmospheric circulation in Central Europe were analyzed for the period 1881-2005. The analysis was based on the Hess-Brezowsky catalogue of circulation types (CTs), and a series of weekly self-calibrated Palmer Z-index (scZ-index) and Palmer Drought Severity Index (scPDSI) values at seven stations where high-quality daily data has recently become available. The results show that the large-scale droughts during spring months (MAM) were associated with east (E), south (S), and south- east (SE) circulation types, whereas during summer (JJA) and the whole vegetation season, i.e., April-September (VEG), the Central Europe high pressure systems (HM) and east (E) circulation types were conducive to drought. Statistically significant drying trends were noted at a majority of the stations, especially during MAM and JJA, over the whole period for which the scPDSI and scZ-index series were available (1875-2005). Although almost no statistically significant tendencies were found prior to 1940, a significant tendency towards more intense drought was present at all sites after this year. The largest drying trend was noted during the VEG and AMJ seasons. The overall drying trend might be associated with shifts in the frequency of CTs, especially during AMJ. Although the aggregate frequency of occurrence of drought-conducive CTs (i.e. E, S and HM) remained stable at approximately 30% up to the 1940s, a steady increase to the present 55% frequency is observed afterwards. Higher frequencies of S and HM types drove the observed increase of drought-conducive CTs at the expense of N types that are associated with wet conditions. The long-term shifts in the frequency of circulation types conducive to drought explain more than 50% of the long-term variations of both scZ-index and PDSI values over the territory of the Czech Republic, and they are likely to affect whole central European region as well. Acknowledgement: This study was conducted with support of the 6th FP EU research project CECILIA (no GOCE 037005) and by the Research plan No. MSM6215648905 "Biological and technological aspects of sustainability of controlled ecosystems and their adaptability to climate change", which is financed by the Ministry of Education, Youth and Sports of the Czech Republic. The weekly drought indicators were parameterized with the help of KONTAKT project no. ME 844.
GC11A-0126
Multi-GCM Projections of Global Drought Conditions With Use of the Palmer Drought Indices
We use two Palmer Drought Indices (the PDSI and Z-index) to assess the drought conditions in future climates as projected by seven Global Climate Models (GCMs). Both indices are based on precipitation and temperature data (this makes them more suitable for climate change impact studies compared to the Standardized Precipitation Index, which is based only on precipitation) and the available water content of the soil. In contrast to the PDSI, the Z-index does not account for any persistence within the climate; rather, it characterizes the immediate (for a given week or month) conditions. The indices are calculated by computer programs available from the National Drought Mitigation Center and the Computer Science and Engineering Department, both located at the University of Nebraska-Lincoln. To allow for the assessment of climate change impacts, we modified the original computer code: the indices (which we named "relative" drought indices) are now calibrated using the present climate weather series and then applied to the future climate weather series. The resultant time series thus displays the drought conditions in terms of the present climate. The relative drought indices are applied to gridded (whole globe) GCM-simulated surface monthly weather series (available from the IPCC database), and the available water content is based on soil- texture-based water holding capacity global data developed by Webb et al. (1993, Global Biogeochem. Cycles 7: 97–108). The indices are calibrated with 1991-2020 period (considered to be the present climate) and then applied to two future periods: 2031-2060 and 2060-2099. To quantify impacts of climate change on the drought conditions, we analyze shifts in the grid-specific means of the drought indices and in the frequency of months belonging to drought spells. The drought spell is defined here as continuous periods in which the index does not exceed -1, and falls below -3 for at least one month. Results obtained by single GCMs related to North America and Europe were already discussed earlier (http://www.ufa.cas.cz/dub/impacts/2006-longmont-drought.pdf). Here, the emphasis will be put on (1) assessing drought conditions on the whole globe, and (2) aggregating results from the set of seven GCMs. We will identify (i) regions where the drought conditions (when averaged over all 7 GCMs) will change most significantly, and (ii) regions where the between-GCMs concordance in projected drought change is the greatest thus indicating the highest reliability of the projection. For example, for North America and Europe, the projection of the PDSI for the end of the 21st century indicate the following regions with the highest potential risk of drought exaggeration (indicated by high decreases in PDSI combined with a high between-GCM concordance): the belt along the Canadian-U.S. border, central Mexico, the Mediterranean, and central Europe. Acknowledgements: The study is supported by the AMVIS-KONTAKT project (ME 844) and the National Agency for the Agricultural Research (project QG60051).
GC11A-0127
Analysis of trends in extreme hourly precipitation events in India
Variability and changes in the trends of extreme precipitation events, has been identified as one of the major results of global climate change. The latest report of the IPCC highlights the increasing trend in the frequency of extreme heavy precipitation events. In particular the report also highlights the increased frequency in the occurrence of intense precipitation events during the Indian summer monsoon at the regional scale. These findings are based on the analysis of daily precipitation data at both station level and modeling results for the Indian subcontinent. In view of the strong evidence indicating an increasing trend in extreme daily precipitation events, the present study focuses on the trends in extreme hourly precipitation events over the Indian subcontinent. Hourly precipitation data for over 80 stations were collected for the last 20 years (1980 to 2002) to examine the spatial patterns in overall trends in extreme heavy precipitation events at the seasonal scale. Unlike previous studies that only concentrate on the summer monsoon precipitation, the present study focuses on both summer and winter season precipitation events. In view of the existence of several different indices for measuring extreme precipitation events, some of the more effective indices have been used to determine the spatial and temporal variations in extreme hourly precipitation over the subcontinent. The trends in the following extreme precipitation indices for each station were analyzed: · Largest 1 hour event · Largest 3 hour total · Extreme frequency (90th percentile) · Extreme frequency (95th percentile) · Extreme frequency (97.5th percentile) Finally, the impact of global teleconnections in the form of ENSO, were also examined to reveal any significant role of teleconnections on the trends in hourly extreme precipitation events. The results of the present study have widespread hydrological implication especially for the areas along the foothills of Himalaya that are prone to intense flooding during the monsoon season. The spatial patterns of the trends in hourly extreme precipitation events will also be useful for arid and semi arid regions of western and central India for planning of flashflood mitigation measures.
GC11A-0128
More than Just Hurricanes: The Atlantic Multidecadal Oscillation and Extreme Precipitation over the US and Mexico from August to October
The tail of the distribution of daily precipitation for August-September-October was examined over the United States and Mexico in relation to the Atlantic Multidecadal Oscillation (AMO). As expected from previous studies linking the AMO to hurricane activity, Florida and the coastal Southeast U.S. showed an increase in precipitation intensity when the Atlantic was in a warm phase (AMO+). Also during AMO+ Northwest Mexico was dry and exhibited a reduction of extreme events and the Mid-Atlantic Appalachian Mountains showed evidence of an increase in heavy precipitation compared to when the Atlantic was cool. It is proposed that the aforementioned decadal variations in extreme rainfall are forced by changes in the large-scale surface winds and air temperature in conjunction with the AMO. Namely, an anomalous cyclonic circulation is observed off the Southeast coast, leading to a reduction of moisture flux into the decaying North American monsoon, and an increase in moisture flux into the Mid-Atlantic. Further, the Mid-Atlantic shows a relatively strong increase in the mid-tropospheric lapse rate. Thus, the unique combination of low-level humidity, potential instability, and elevated topography are consistent with an enhanced risk of intense rainfall during AMO+. http://dx.doi.org/10.1007/s00382- 007-0295-0
GC11A-0129
Understanding Regional Climate Change Consequences Through the Changes in Soil Moisture Regimes
Soils are an important control on water fluxes in the landscape and in many parts of the world act as the most important water reservoir mitigating the effects of rainfall variability. Soil moisture and temperature regimes are inherently more stable and quantifiable than their atmospheric counterparts and are essential in determining the environmental conditions of any region. They can also be used to demonstrate the impacts of climate change on a given region as they integrate not only the change of climate variables but also existing soil condition status and plant cover. In addition, the globally valid analysis of soil moisture and temperature regimes makes it possible to present a variety of consequences of climate change in terms of analogs. In order to easily estimate the soil moisture and temperature regime at a given site, or within a selected region, a software SoilClim was developed, tested, and applied in two markedly different regions of the Northern Hemisphere. SoilClim is based on an enhanced daily water balance model that incorporates interactions between the soil and atmosphere through a dynamic module of vegetation cover. SoilClim was developed by an international and interdisciplinary team of researchers and students. After an evaluation, SoilClim was run both in Central Europe and in Nebraska with the climatic data corresponding to the conditions expected under future climates taking into account two Global Circulation Models (ECHAM and HadCM) and assuming the B1 and A2-SRES scenarios with low and high climate sensitivity for time slices of 2025, 2050 and 2100. It was found that under the present climate only a fraction of the territory of Central Europe is situated within the dry tempudic soil moisture regime, with high drought risk being confined to a small area. However, under a changing climate, a notable increase of the areas with a high probability of dry events was noted as well as sharp reduction of perudic (very-wet) mountainous areas that are essential for sustainable river flow. We found an especially alarming rate of these shifts in the soil climate characteristics taking place within decades rather than centuries. According to the SoilClim model, a new soil climate type that has not been recorded up to now at both case study areas might be expected at both regions between 2050 and 2100. The predicted changes in the soil climate regimes are closely related to drought impacts (e.g. decrease of crop yields, damage to forest stands, low streamflow and reservoir levels, etc.) or changes in the dynamics of key soil processes (e.g. rate of carbon sequestration or mineralization) and should be a part of a complex climate change impact assessment. Acknowledgement: The development of SoilClim and the international cooperation and data sharing was supported by the KONTAKT projects ME 844 and 17/2006. The Czech part of the study was supported by the Research plan No. MSM6215648905 . http://soils.usda.gov/use/worldsoils/mapindex/smr.html
GC11A-0130
Potential Predictability of Summertime Rainfall Extremes Over the Southwestern US
Many investigations of the North American monsoon system, particularly as it impacts northwestern Mexico and the southwestern United States, have focused upon summertime precipitation and its intraseasonal to interannual variations. While it is well known that in this region seasonal-mean rainfall anomalies and extreme- event occurrences - such as long-term drought and heavy rainfall events - have significant year-to-year variability, it is still unclear how much of the variability is actually due to climate-related shifts in the rainfall characteristics during anomalous years. Here we will show how stochastic models, in which the simulated interannual and intraseasonal rainfall variance is purely the result of the random evolution of daily rainfall events within a given year, can be used to identify and quantify those observed rainfall variations that are associated with systematic changes in the underlying rainfall characteristics and hence are "potentially predictable." This method can be used to isolate "hot spots" within the domain where potentially predictable signals occur more frequently than at others. In addition, though, the method can also be used to highlight potentially-predictable "hot events" at a given station or in a given region for use in analysis and model-evaluation studies. This presentation demonstrates how this technique has been used to: 1) identify summertime rainfall extremes over the southwestern US; and 2) analyze the large-scale and regional climate features related to these extremes.
GC11A-0131
Spatio-temporal analysis of extreme rainfall trends associated with the Indian monsoon
This study is a statistical assessment of the presence of linear trends in clearly defined extremes of daily precipitation data, and makes use of a (relatively) long time-series of gridded daily data for India to clarify a few points, including the presence of trends, at the grid level, the spatial signature of these trends. Much of literature conjectures an intensifying summer monsoonal precipitation, and in general, an intensification of the monsoonal patterns over the Indian peninsula; to date, however, convincing statistical evidence for increased extremes of precipitation have been rarely found. This study is an attempt to answer the first of the two important questions investigated in Climate literature, on the existence of trends in precipitation extremes, and the seasonal aspects of such a change (given that much of literature is focused on changes only during the summer monsoon), since without evidence of such trends, arguments of alterations in monsoonal precipitation following global warming must probably undergo some revisions, especially given the difficulty in consistently and accurately predicting even the average monsoonal rainfall using Climate models. This study attempts to separately estimate trends in two different aspects of precipitation extremes, intensity of events and frequency of events, unlike much of literature which focuses on one or the other, given the suspicion that the two maybe related (an increase in the one may, for instance, be easily conjectured a result of a decrease in the other); further, events in and out-of (monsoon) season are treated separately (as well as in the usual aggregated manner), thereby answering the question posed in as comprehensive a manner as possible. In-season, trends in both intensity and frequency appear to be not significant over most parts, and where significant, predominantly negative, with a few positive (significant) trends in frequency being evident; there is, further, no spatial signature in these (significant) trends.
GC11A-0132
Attribution Of Changes In Extreme Weather Risk
In 2003 the average summer temperature in continental Europe exceeded the 1961--1990 European summer mean by 2.3K. Many regions experienced a large number of deaths due to the elevated temperatures, and attribution studies have determined that human activity have at least doubled the risk of such a heatwave compared to pre-industrial times. However, other, non-linear processes could also have amplified summer 2003 temperatures: feedbacks between reduced cloud cover and precipitation and reduced soil-moisture may have prevented the usual convective disruption of the high pressure system. This study will build on previous attribution work by attempting to further isolate the change in risk of the heatwave due to anthropogenic influences. A large ensemble of the ECMWF IFS model will be performed at higher resolution than previous studies, with improved simulation of land surface processes.
GC11A-0133
Analysis of Temporal Changes in Daily Precipitation Intensities
We have compared an ensemble of regional climate modelling simulations from the European framework project PRUDENCE with observed daily precipitation data from the European Climate Assessment dataset by characterising precipitation in terms of probability density functions of various kinds. We compare distributions for wet days and consecutive dry days across 8 regions in Europe against 9 models. We find the models that best describe the observations in given regions as well as across regions. We show how to estimate robustness of the results for wet days and climatological drought days using bootstrapping on stations, with replacement.
GC11A-0134
Extreme Hydrometeorological Events and the Urban Environment: Dissecting the 7 July 2004 Thunderstorm over the Baltimore, MD Metropolitan Region.
Observational analyses and mesoscale modeling studies using the Weather Research and Forecasting (WRF) model are used to dissect the mechanisms associated with record lightning, rainfall and flooding over the Baltimore metropolitan region on 7 July 2004. Storm evolution on 7 July 2004 exhibited characteristic features for warm season thunderstorms producing flash flooding over the Baltimore - Washington DC Metropolitan region. The storm system was initiated along the Blue Ridge, with model simulations suggesting that convergence- induced spin-up of a mesolow was responsible for initial thunderstorm development. Observations and model analyses show that thermal effects associated with Chesapeake Bay had a pronounced impact on storm evolution and rainfall distribution. Analyses of radar reflectivity and lightning observations suggest that the urban environment played a significant role in storm evolution and heavy rainfall distribution. Model analyses show that urban canopy effects from both the Washington DC urban region and the Baltimore urban region play an important role in determining the storm environment associated with heavy rainfall. Urban Heat Island effects did not play a significant role in the storm evolution. Observations from a vertically-pointing Lidar and disdrometer observations of raindrop distributions suggest that hygroscopic growth of aerosols may have played an important role in stimulating efficient precipitation growth mechanisms and production of extreme rainfall rates.
GC11A-0135
The Impacts of the Urban Environment on Extreme Rainfall from Warm Season Thunderstorm Systems
The spatial variability of extreme rainfall over urban environments and its change in time is examined from both a climatological and a case-study perspective. The areas of focus are the Washington D.C-Baltimore and New York City Metropolitan areas. Climatological analyses utilize Cloud-to-Ground lightning data from the National Lightning Detection Network (NLDN) and radar-rainfall products from the HydroNEXRAD project. The regional distribution of heavy convection, time-trends, initiation locations and extreme thunderstorm lifecycle characteristics are presented for both areas. Case studies include a collection of extreme events over the two Metropolitan areas that combine detailed observations and mesoscale modeling using the Weather Research and Forecasting (WRF) model with chemistry and cloud-aerosol interaction (WRF-Chem) capabilities. The impacts of the urban environment on extreme thunderstorm evolution, in terms of the Urban Heat Island (UHI), the Urban Canopy Layer (UCL), and urban aerosols are examined for this collection of events. High-resolution urban data is incorporated in the modeling scheme with the use of the Urban Canopy Model capability of WRF. Key results of the study include large differences in rainfall accumulation when aerosols and chemistry are included in the model that hint to the role of hygroscopic aerosols on extreme warm season thunderstorm evolution. The urban environment itself impacts the rainfall distribution and movement of extreme thunderstorms, in the urban vicinity, mainly through the impacts of the urban canopy layer and aerosols.
GC11A-0136
Past and Future Changes in Water Availability
A steadily increasing population and water demand in the Western United States, an area historically prone to severe droughts, coupled with increased aridity due to warming may lead to increased drought severity and consistency over the next hundred years. A global time series of future drought indices created from output of multiple GCMs is examined and compared with historic data. Potential physical mechanisms for changes in the conformity of future droughts to historic droughts, associated with an altered hydrologic cycle, are explored, and the potential impacts are reviewed.
GC11A-0137
Investigating Changes in Large-Scale Indicators of Extreme Weather
Previous studies have shown a link between concurrently high values of convective available potential energy (cape) and vertical shear (0-6 km wind vector magnitude difference) and smaller scale severe weather. Here, we study a global reanalysis data set containing these two indicators, and explore how modeled extreme return levels have changed over a 42-year period. Data are fit to extreme-value distributions with temporal covariates, and return level estimates are compared. It is found that some parts of the world have seen a decrease in these large-scale indicators (e.g., Southern S. America), while other areas (e.g., Eastern China) have experienced a marked increase in these return levels, suggesting a likelihood of increases in the frequency of severe weather events. Future work will include an application of this methodology to the output of climate prediction models.
GC11A-0138
Changes in Global and Regional Drought: Retrospective Analysis and Future Projections
Recent and potential future increases in global temperatures are likely to be associated with impacts on the hydrologic cycle, including changes to precipitation and increases in extreme events such as droughts. We summarize recent analyses of historic reconstructions of drought, including trends over the second half of the 20th century, and projections of the future drought occurrence from IPCC AR4 multi-model, multi-scenario experiments. Global and regional trends in drought for 1950-2000 are analyzed using a soil moisture based drought index derived from a simulation of the terrestrial hydrologic cycle driven by a hybrid reanalysis-observation forcing dataset. There is an overall small wetting trend in global soil moisture, forced by increasing precipitation, which is weighted by positive trends over the western hemisphere. Regional variation is nevertheless apparent and significant drying over West Africa stands out. Trends in drought duration, intensity and severity are predominantly decreasing but statistically significant changes are limited in areal extent, depending on the variable and drought threshold. Concurrent changes in drought spatial extent are evident. Despite the overall wetting trend there is a switch since the 1970s to a drying trend, globally and in many regions, especially in high northern latitudes. This is shown to be caused, in part, by concurrent increasing temperatures. Although drought is driven primarily by variability in precipitation, projected continuation of temperature increases during the 21st century indicate the potential for enhanced drought occurrence. We analyze changes in future drought occurrence using soil moisture data for the SRES B1, A1B and A2 future climate scenarios from eight AOGCMs that participated in the IPCC AR4. The models show decreases in soil moisture globally for all scenarios with a corresponding doubling of the spatial extent of severe soil moisture deficits and frequency of short-term (4-6 month duration) droughts from the mid 20th century to the end of the 21st. Long-term (more than 12 month duration) droughts become three times more common. Regionally, the Mediterranean, west African, central Asian and central American regions show large increases most notably for long term frequencies as do mid latitude North American regions but with larger variation between scenarios. In general, changes under the higher emission scenarios, A1B and A2 are the greatest, and despite following a reduced emissions pathway relative to the present day, the B1 scenario shows smaller but still substantial increases in drought, globally and for most regions. Increases in drought are driven primarily by reductions in precipitation with increased evaporation from higher temperatures modulating the changes. In some regions, increases in precipitation are offset by increased evaporation.
GC11A-0139
The Effect of Temperature and Precipitation Trends on U.S. Drought
Drought in the continental United States (U.S.) is a recurring phenomenon that has shown large natural variability on both the instrumental and paleoclimatic time scales. Recent research examining global changes in drought suggest that drought conditions have increased globally in the last 50 years (Dai et al. 2004). In the U.S. as a whole there is no indication that drought has become more frequent. In some regions, such as the Southwestern U.S. there is evidence that drought has become more prevalent, while in other areas drought has become less frequent. Over the 20th century the U.S. experienced statistically significant increases in both the annually averaged mean temperature and the total annual precipitation. Here we examine the possibility that with the observed increase in temperature and without the observed increase in precipitation, drought conditions since 1950, as defined by the PDSI, over the United States would have been more prevalent. The starting point of 1950 is chosen to include the major droughts of the early 1950s and includes the period in the U.S. temperature record that is roughly the start of a slight temperature decline until the 1970s when U.S. temperatures, like global temperatures, began a strong increase. Results indicate that without the increase in precipitation, the percentage of U.S area in drought would increase substantially, in some years by as much as a 25 percent increase in the area in severe and extreme drought.
GC11A-0140
Regional Probablistic RCM-Estimates of Extreme Rainfall for the UK
Widespread flooding in the summer of 2007 and previously in the autumn/winter 2000/01 caused significant damage to the built and natural environment of the UK, highlighting the vulnerability of UK-infrastructure to extreme rainfall events. There is public concern that these events may increase in the future, possibly due to global warming. Projected rainfall fields from regional climate models are used to better understand how regional rainfall extremes may change in a climate of enhanced greenhouse conditions. However, due to their complex nature, RCMs are associated with a number of sources of uncertainty. By using a large number of RCMs some of this model uncertainty can be quantified. Here, changes to future rainfall extremes for the UK are investigated using 13 RCMs provided by the PRUDENCE experiments. Of particular interest is model performance in the spatial domain, and model results are analysed for each of the nine commonly accepted UK rainfall regions. The PRUDENCE RCMs were nested with 4 different global climate models, although only 2 RCMs were driven by more than one GCM. For each region and model, probability densities are estimated for the 1, 2, 5 and 10-day RCM rainfall amounts associated with the 5, 10 and 25-year return period for a control period (1961-90) and for a future period (following the SRES A2 2071-2100 scenario). Return period magnitudes are estimated using a combination of Regional Frequency Analysis and Extreme Value Analysis, where a Generalized Extreme Value (GEV) distribution is fitted using the method of L-moments to annual maxima series of the rainfall totals. Probability densities are generated using 10 000 samples of return period estimates, derived from boot strap samples of regionally pooled annual maxima series. Probability densities based on RCM rainfall for the control period are validated using observed rainfall, and changes in magnitude between control and future experiments are calculated. This will be presented and results discussed.
GC11A-0141
Analysis of trends in extreme daily precipitation from observation and ERA-40 driven ENSEMBLES RCM simulations over the UK.
The aim of this work is the detection of trends in the tail of the daily precipitation distribution, described in the framework of the Extreme Value Analysis, for the period 1960-2000 over the UK and to study the dependency on relevant physical quantities and circulation indices. The distributions are those obtained from observed precipitation, aggregated over the RCM ENSEMBLES UK grid-boxes (at 25km horizontal resolution), and simulated by the ENSEMBLES RT3 regional climate models driven by ERA-40 reanalysis boundary conditions over the same area. Trends from the observations and an assessment of the ability of the models to reproduce these trends and the relationship with other covariates will be presented at the conference.
GC11A-0142
On the Verification and Comparison of Extreme Rainfall Indices From Climate Models
The interpretation of model precipitation output (e.g., as a grid-point estimate vs. as an areal mean) has a large impact on the evaluation and comparison of simulated daily extreme rainfall indices from climate models. We first argue that interpretation as a grid-point estimate (i.e., corresponding to station data) is incorrect. We then illustrate impacts of this interpretation vs. the areal-mean interpretation in the context of rainfall extremes. A high resolution (0.25°x0.25° grid) daily observed precipitation dataset for the U.S. (from CPC) is used as idealized perfect model gridded data. Both 30-year return levels of daily precipitation (P30) and a simple daily intensity index are substantially reduced in this data when estimated at coarser resolution compared to the estimation at finer resolution. The reduction of P30 averaged over the conterminous US is about 9, 15, 28, 33, and 43% when the data were first interpolated to 0.5°x0.5°, 1°x1°, 2°x2°, 3°x3° and 4°x4° grid boxes, respectively, before the calculation of extremes. The differences resulting from the point estimate vs. areal mean interpretation are sensitive to both the data grid size and to the particular extreme rainfall index analyzed. The differences are not as sensitive to the magnitude and regional distribution of the indices. Almost all IPCC AR4 models underestimate US mean P30 if it is compared directly with P30 estimated from the high resolution CPC daily rainfall observation. On the other hand, if CPC daily data is first interpolated to various model resolutions before calculating the P30 (a more correct procedure in our view), about half of the models show good agreement with observations while most of the remaining models tend to overestimate the mean intensity of heavy rainfall events. A further implication of interpreting model precipitation output as an areal mean is that use of either simple multimodel ensemble averages of extreme rainfall or of inter-model variability measures of extreme rainfall to assess the common characteristics and range of uncertainties in current climate models is not appropriate if simulated extreme rainfall is analyzed at a model?|s native resolution. Owing to the large sensitivity to the assumption used, we recommend that for analysis of precipitation extremes, investigators interpret model precipitation output as an area average as opposed to a point estimate, and then ensure that various analysis steps remain consistent with that interpretation.
GC11A-0143
Tropical Cyclone Weather and Storminess
This study investigates the frequency of occurrence of tropical cyclones and land-falling hurricanes in the North Atlantic and the Gulf of Mexico over the five telescoping periods of 1851-2006, 1888-2006, 1900-2006, 1950-2006 and 1970-2006. An effort is made to assess the intrinsic modes of variability and trends buried in the data but also to identify periods of greater to lesser storminess. The question as to whether or not the past decadal increases in overall numbers and intensities of events has past analogues or presents a new paradigm is addressed in the context of the several modes of variability that emerge from the data and of the periods of increased numbers of storm events. Multiple modes are identified in the time series ranging from inter-annual to decadal to multi-decadal years. Clear and possible relationships with climate factors will be presented and or speculated upon. A "storminess" perspective (or the lack thereof) is introduced which further partitions and segregates the information buried in the time series and offers additional insights into this complex issue.
GC11A-0144
Hurricane Wave Power Extremes Along the U.S. Atlantic and Gulf Coasts
Extremes in wave power generated by tropical cyclones (TCs) will have an increasingly greater coastal impact as mean sea level rises. The Gulf 98th percentile (3 m) deep-water significant wave height, HS, measured at four open ocean NOAA buoys along the U.S. Atlantic coast and three Gulf buoys identifies extreme TC-generated wave events during the June-November hurricane season. Since 1978, there were substantially more significant HS events along the Atlantic coast than in the Gulf, with almost three times as many extreme wave events during September. The monthly distribution along both coasts peaks in September, with an equally likely chance of a significant TC wave event occurring during October as during August over the 1978-2006 data record. However, no clear trend in TC-generated extreme wave heights is observed. In general, the Atlantic buoys show a significant increase in seasonal wave power, PW, since 1995. PW during six of the hurricane seasons since 1995 exceeds all prior years at at least one of the Atlantic group buoys. In contrast to the Atlantic buoys, the Gulf buoys show exceptional seasonal PW levels only during the 2005 hurricane season when major Hurricanes Dennis, Emily, Katrina, Rita, and Wilma tracked trough the Gulf. The exceptional PW levels observed in the Gulf during 2005 were exceeded in the Atlantic during 1999, and approached during 1995 and 1996, attesting to a greater frequency of extreme TC-associated extreme wave events along the East Coast compared to the Gulf during the last four decades. A TC wave power index (WPI) increases significantly in the Atlantic during the mid-1990s, resulting largely from an increase in mid-to-late hurricane season TCs. The WPI is related to TC strength, size, duration, and frequency, and is highly correlated with the TC power dissipation index (PDI, Emanuel 2005). The close association of the WPI to hurricane activity implies that significant coastal impacts will increase as the PDI increases, regardless of TC landfall frequency. Differences between the Atlantic and Gulf WPI reflect systematic changes in TC genesis regions and subsequent tracks, characterized by their relationship with the regional circulation patterns described by the Atlantic Meridional Mode (Kossin and Vimont 2007).