P. Speth

Sensitivities of a cyclone detection and tracking algorithm: individual tracks and climatology

Northern Hemisphere cyclone activity is assessed by applying an algorithm for the detection and tracking of synoptic scale cyclones to mean sea level pressure data. The method, originally developed for the Southern Hemisphere, is adapted for application in the Northern Hemisphere winter season. NCEP-Reanalysis data from 1958/59 to 1997/98 are used as input. The sensitivities of the results to particular parameters of the algorithm are discussed for both case studies and from a climatological point of view. Results show that the choice of settings is of major relevance especially for the tracking of smaller scale and fast moving systems. With an appropriate setting the algorithm is capable of automatically tracking different types of cyclones at the same time: Both fast moving and developing systems over the large ocean basins and smaller scale cyclones over the Mediterranean basin can be assessed. The climatology of cyclone variables, e.g., cyclone track density, cyclone counts, intensification rates, propagation speeds and areas of cyclogenesis and -lysis gives detailed information on typical cyclone life cycles for different regions. The lowering of the spatial and temporal resolution of the input data from full resolution T62/06h to T42/12h decreases the cyclone track density and cyclone counts. Reducing the temporal resolution alone contributes to a decline in the number of fast moving systems, which is relevant for the cyclone track density. Lowering spatial resolution alone mainly reduces the number of weak cyclones.

Midwinter Suppression of Northern Hemisphere Storm Track Activity in the Real Atmosphere and in GCM Experiments

Abstract The seasonal cycle of the North Pacific and the North Atlantic storm track activity is investigated on the basis of daily National Meteorological Center (now known as NCEP) upper-air analyses (1946–89) and of data from the ECHAM3 T42 atmospheric general circulation model. Emphasis is put on the midwinter suppression of the Pacific storm track. This feature of seasonal variability is not sensitive to a particular definition of midlatitude synoptic wave activity, as is shown by applying a common definition of area mean storm track intensity. The suppression is reproduced by the atmospheric model with very similar characteristics. It is attributed to a negative correlation between the storm track intensity and the speed of the subtropical jet at 250 hPa for average zonal winds exceeding the threshold of approximately 45 m s−1, contrasting with a positive correlation below this value. The lack of an analogous behavior over the Atlantic may be assigned to the lower wind speeds there. In a 3·CO2 time-s...

Assessment of winter cyclone activity in a transient ECHAM4-OPYC3 GHG experiment

Winter cyclone activity over the Northern Hemisphere is investigated in an ECHAM4/OPYC3 greenhouse gas scenario simulation. The goal of this investigation is to identify changes in cyclone activity associated with increasing concentrations. To this aim, two 50-year time periods are analysed, one representing present day climate conditions and the other a perturbed climate when CO 2 concentrations exceed twice the present concentrations. Cyclone activity is assessed using an automatic algorithm, which identifies and tracks cyclones based on sea level pressure fields. The algorithm detects not only large and long living cyclones over the main ocean basins, but also their smaller counterparts in secondary storm track regions like the Mediterranean Basin. For the present climate, results show a good agreement with NCEP-reanalysis, provided that the spectral and time resolutions of the reanalysis are reduced to those available for the model. Several prominent changes in cyclone activity are observed for the scenario period in comparison to the present day climate, especially over the main ocean basins. A significant decrease of overall cyclone track density is found between 35 and 55 degrees North, together with a small increase polewards. These changes result from two different signals for deep and medium cyclones: for deep cyclones (core pressure below 990 hPa) there is a poleward shift in the greenhouse gas scenario, while for medium cyclones (core pressure between 990 and 1010 hPa) a general decrease in cyclone counts is found. The same kind of changes (a shift for intense cyclones and an overall decrease for the weaker ones) are detected when distinguishing cyclones from their intensity, quantified in terms of ∇ 2 p. Thus, the simulated changes can not solely be attributed to alterations in mean sea level pressure. Instead, corresponding increases in upper-tropospheric baroclinicity suggest more favourable conditions for the development of stronger systems at higher latitudes, especially at the delta regions of the North Atlantic and the North Pacific storm tracks.

The global energy cycle of stationary and transient atmospheric waves: Results from ECMWF analyses

SummaryThe role of stationary (monthly mean) and transient (departure from monthly mean) waves within the atmospheric energy cycle is examined using global analyses from the European Centre for Medium Range Weather Forecasts (ECMWF) for the period 1980–1987. Only January and July averages are considered.It is confirmed that planetary stationary waves are basically baroclinic. Their contribution to the globally averaged energy cycle of the atmosphere is comparable to that of the transient waves. In January they contribute about 40% to the baroclinic conversion (CA) from zonal mean to eddy available potential energy. Local values for the northern hemisphere even show a predominant role of the stationary wave conversions over those originating from transient waves. Part of the available potential energy of stationary waves (ASE) is converted to kinetic energy by warm air rising and cold air sinking. Nonlinear energy conversion, which can be interpreted as destruction of stationary temperature waves by transients, is the second sink forASE. The order of magnitude of these two processes is similar.Barotropic nonlinear conversions, though negligible in the global average, reveal large conversion rates between the mean positions of the polar and the subtropical jets. Their orientation is suggestive of a tendency to increase stationary wave kinetic energyKSE at its local minimum between the jets at the expense of the synoptic scale transients.While all terms of the energy cycle related to stationary waves reveal a predominance of the planetary scale (zonal wave numbers 1–3) transient waves are governed by synoptic scale waves (zonal wave numbers 4–9) only with respect to the baroclinic and barotropic conversions: a significant amount of transient wave energy (50% for the global average ofATE) is due to planetary scale waves.

Groundwater recharge in Northrhine-Westfalia predicted by a statistical model for greenhouse gas scenarios

A statistical model that generates regional climate scenarios on the basis of observed time series is applied for precipitation calculations for Northrhine-Westfalia (NRW, Germany). The method consists of three steps and can be used for any climate station. In the first step, 30–50 years observed daily values of Tmax, Tmin, Tmean, spread between Tmin and Tmax and the duration of daylight are clustered by a non hierarchical cluster analysis. The resulting clusters are considered to be typical weather situations with respect to these five “relevant parameters”. Secondly, artificial time series for a future climate state are generated. The time series are designed to reproduce the observed statistical characteristics for the “relevant parameters”. Additionally, a transient increase of the mean temperature of 1.5 K / 50 years is prescribed, which is chosen as an estimate for temperature change by doubling the CO2. In step three every element of the generated time series can be assigned to one of the clusters estimated in step one. Thus it is possible to associate an observed day to each artificially generated situation. Other parameters like precipitation can then be taken from the observed day. In this way scenario temperature and precipitation values are produced for each of the 48 stations. Annual groundwater recharge is calculated from these values employing the Turc formula. Finally, a kriging interpolation estimates the distribution of groundwater recharge for the complete area. With the prescribed rise in mean temperature the statistical model simulates decreasing precipitation. Both effects contribute to a reduction of groundwater recharge. While in the mountainous parts of NRW the groundwater recharge change is small, in the planes a reduction of up to 30% of the current groundwater recharge is predicted. A comparison to direct model output from GCM integrations for a standard greenhouse gas scenario produces a similar reduction for that region.

Tropical cyclones, 6–25 day oscillations, and tropical-extratropical interaction over the northwestern Pacific

SummaryThis paper investigates tropical-extratropical interactions over the northwestern Pacific Ocean that involve tropical cyclones and subtropical jet streaks. Another aspect of this study is to examine the relation between 6–25 day convective variability and tropical cyclones. This investigation is conducted for the fall and early winter season, with a focus on the months, October through December (OND). In addition to outgoing longwave radiation (OLR) data, we use 10 years (1985–1994) of WCRP/TOGA archive II analyses produced by ECMWF to compute equivalent temperature,θe, precipitable water, W, and kinematic and kinetic energy transfer variables. These variables are composited for two classes of tropical cyclones, recurving cyclones (RCs) and non-recurving cyclones (NCRs), in order to examine the influence of tropical cyclones and baroclinic processes on changes in the jet streak intensity.We found that RCs interacted with extratropical regions during all composite days. A strong baroclinic zone developed throughout the troposphere on the north side of the composite cyclone as it propagated poleward. Between the day of recurvature, DR, and the day after recurvature, DR+1, the main band of convection shifted from the RC to a frontal band within the baroclinic zone indicating a transformation of the tropical cyclone into an extratropical one. An eastward propagating jet streak at 200 hPa, located north of the RC and in the vicinity of the baroclinic zone, increased its speed from 57 ms−1 to 79 ms−1 on DR+1. Although we could not measure the role of baroclinic processes in this regard, we were able to infer that upper-level outflow from the RC did supply momentum and energy to the jet streak.Whereas we expected tropical-extratropical interactions for the RCs, we also found evidence that NRCs that stay south of 20° N throughout their lifetime and that dissipate over Indo-China have an influence on the subtropical jet by their upper-level outflow, especially in the late OND season. The tropical (i.e., momentum) forcing did appear to cause increases in the speed of the jet after the composited storm crossed the Phillippines on the fourth day of its life cycle, D4. Concurrently, a baroclinic zone developed along the coast of southern China by about D4, but it was confined to the lower troposphere.Finally, our spectral analysis investigations for the northwestern Pacific showed significant peaks at 6–10 days and 15–20 days from late September to early December. The first peak is well known and is associated with typhoon activity. In several of the investigated autumn seasons (1987, 1989, 1992, and 1993), the second peak was clearly related to the recurrence interval of northwestern Pacific tropical cyclones. This result is in accordance with the findings of Hartmann et al. (1992). For some years of the investigation period (1985, 1986, and 1988), however, our results showed that westward propagating convective disturbances that fail to reach tropical depression strength also contribute to the power in the 15–25 day band, whereas in a few years (1990 and 1991), no OLR peak between 15 and 20 days could be found at all. Therefore, it appears that further work needs to be done with regard to the relationship between convective systems and their accompanying relationships on time scales ranging between 10 and 25 days.

Moroccan Climate in the Present and Future: Combined View from Observational Data and Regional Climate Scenarios

The impact of climate change on water availability in the Middle East and the Upper Jordan catchment (UJC) is investigated by dynamic downscaling of ECHAM4 time slices and subsequent hydrological modelling. Two time slices (1961–90 and 2070–99) of the global climate scenario B2 of ECHAM4 were dynamically downscaled with the meteorological model MM5 in two nesting steps of 54 km and 18 km resolution. The meteorological fields were used to drive a physically based hydrological model, computing in detail the surface and subsurface water flow and water balance of the UJC.