Matthias Zahn

Regional Climate Models Add Value to Global Model Data: A Review and Selected Examples

An important challenge in current climate modeling is to realistically describe small-scale weather statistics, such as topographic precipitation and coastal wind patterns, or regional phenomena like polar lows. Global climate models simulate atmospheric processes with increasingly higher resolutions, but still regional climate models have a lot of advantages. They consume less computation time because of their limited simulation area and thereby allow for higher resolution both in time and space as well as for longer integration times. Regional climate models can be used for dynamical down-scaling purposes because their output data can be processed to produce higher resolved atmospheric fields, allowing the representation of small-scale processes and a more detailed description of physiographic details (such as mountain ranges, coastal zones, and details of soil properties). However, does higher resolution add value when compared to global model results? Most studies implicitly assume that dynamical down...

Decreased frequency of North Atlantic polar lows associated with future climate warming

Every winter, the high-latitude oceans are struck by severe storms that are considerably smaller than the weather-dominating synoptic depressions. Accompanied by strong winds and heavy precipitation, these often explosively developing mesoscale cyclones—termed polar lows—constitute a threat to offshore activities such as shipping or oil and gas exploitation. Yet owing to their small scale, polar lows are poorly represented in the observational and global reanalysis data often used for climatological investigations of atmospheric features and cannot be assessed in coarse-resolution global simulations of possible future climates. Here we show that in a future anthropogenically warmed climate, the frequency of polar lows is projected to decline. We used a series of regional climate model simulations to downscale a set of global climate change scenarios from the Intergovernmental Panel of Climate Change. In this process, we first simulated the formation of polar low systems in the North Atlantic and then counted the individual cases. A previous study using NCEP/NCAR re-analysis data revealed that polar low frequency from 1948 to 2005 did not systematically change. Now, in projections for the end of the twenty-first century, we found a significantly lower number of polar lows and a northward shift of their mean genesis region in response to elevated atmospheric greenhouse gas concentration. This change can be related to changes in the North Atlantic sea surface temperature and mid-troposphere temperature; the latter is found to rise faster than the former so that the resulting stability is increased, hindering the formation or intensification of polar lows. Our results provide a rare example of a climate change effect in which a type of extreme weather is likely to decrease, rather than increase.

A comparison of two identification and tracking methods for polar lows

Abstract In this study, we compare two different cyclone-tracking algorithms to detect North Atlantic polar lows, which are very intense mesoscale cyclones. Both approaches include spatial filtering, detection, tracking and constraints specific to polar lows. The first method uses digital bandpass-filtered mean sea level pressure (MSLP) fields in the spatial range of 200–600 km and is especially designed for polar lows. The second method also uses a bandpass filter but is based on the discrete cosine transforms (DCT) and can be applied to MSLP and vorticity fields. The latter was originally designed for cyclones in general and has been adapted to polar lows for this study. Both algorithms are applied to the same regional climate model output fields from October 1993 to September 1995 produced from dynamical downscaling of the NCEP/NCAR reanalysis data. Comparisons between these two methods show that different filters lead to different numbers and locations of tracks. The DCT is more precise in scale separation than the digital filter and the results of this study suggest that it is more suited for the bandpass filtering of MSLP fields. The detection and tracking parts also influence the numbers of tracks although less critically. After a selection process that applies criteria to identify tracks of potential polar lows, differences between both methods are still visible though the major systems are identified in both.

The changing atmospheric water cycle in Polar Regions in a warmer climate

We have examined the atmospheric water cycle of both Polar Regions, polewards of 60◦N and 60◦S, using the ERAInterim reanalysis and high-resolution simulations with the ECHAM5 model for both the present and future climate based on the IPCC, A1B scenario. The annual precipitation in ERA-Interim amounts to ∼17000 km3 and is more or less the same in the Arctic and the Antarctic, but it is composed differently. In the Arctic the annual evaporation is ∼8000 km3 but ∼3000 km3 less in the Antarctica where the net horizontal transport is correspondingly larger. The net water transport of the model is more intense than in ERA-Interim, in the Arctic the difference is 2.5% and in the Antarctic it is 6.2%. Precipitation and net horizontal transport in the Arctic has a maximum in August and September. Evaporation peaks in June and July. The seasonal cycle is similar in Antarctica with the highest precipitation in the austral autumn. The largest net transport occurs at the end of the major extra-tropical storm tracks in the Northern Hemisphere such as the eastern Pacific and eastern north Atlantic. The variability of themodel is virtually identical to that of the re-analysis and there are no changes in variability between the present climate and the climate at the end of the 21st century when normalized with the higher level of moisture. The changes from year to year are substantial with the 20- and 30-year records being generally too short to identify robust trends in the hydrological cycle. In the A1B climate scenario the strength of the water cycle increases by some 25% in the Arctic and by 19% in the Antarctica, as measured by annual precipitation. The increase in the net horizontal transport is 29% and 22%, respectively, and the increase in evaporation correspondingly less. The net transport follows closely the Clausius—Clapeyron relation. There is a minor change in the annual cycle of the Arctic atmospheric water cycle with the maximum transport and precipitation occurring later in the year. There is a small imbalance of some 4–6% between the net transport and precipitation minus evaporation. We suggest that this is mainly due to the fact that the transport is calculated from instantaneous six hourly data while precipitation and evaporation is accumulated over a 6-h period. The residual difference is proportionally similar for all experiments and hardly varies from year to year.

Tracking Polar Lows in CLM

Polar lows are severe cyclones in sub-polar oceans sized beyond the resolved scale of existing global reanalysis products. We used the NCEP/NCAR reanalyses data to drive a regional climate model (CLM) in order to reproduce finer resolved atmospheric fields over the North Atlantic over a two year period. In these fields we detected polar lows by means of a detection algorithm based on a spatial digital bandpass filter. CLM was run in two different ways, the conventional way and with additionally prescribing the analysed large scale situation. The resulting temporal and spatial distributions of polar lows between the different simulations are compared. A reasonable seasonal cycle and spatial distribution was found for all simulations. A lower number of polar lows in the spectral nudged simulation indicates a closer vicinity to reality. Higher temporal and spatial variability between the conventional simulations suggest a more random generation of polar lows. Frequency distributions of track-lengths reveal shorter tracks when nudging is applied. Maximum wind speeds reveal only minor, insignificant differences between all runs and are higher in conventional mode.

Changes in water vapor transports of the ascending branch of the tropical circulation

[2] Recent and future changes in the statistical characteristics of the tropical atmospheric circulation (TAC) pattern and associated changes in the tropical hydrological cycle do not only locally affect the weather properties in the tropical areas but they have an influence on the extratropical regions. The general TAC pattern consists of convective regions of upward, ascending air movement (ASC) and of regions of downward, descending air motion (DESC), with low‐level flow into ASC and midlevel outflow into DESC commonly referred to as the Hadley Cell circulation. Along with the upward air motion, ASC contains most of the tropical precipitation and in its annual cycle by and large follows the Sun’s zenith. Part of the water vapor originates from evaporation in areas close by, but additionally large amounts of water are transported by atmospheric movements from DESC, causing a largely positive tropical water balance (precipitation‐evaporation) at the cost of the dry subtropics. A number of recent studies have addressed past and possible future changes in the moisture budget or in other components of the hydrological cycle over the tropics or over parts of the tropics. An increase in low‐level atmospheric water vapor of 7% per degree of warming is derived from theoretical considerations (Clausius‐Clapeyron relation, e.g., Wentz and Schabel[2000]; Trenberth et al. [2003]; Held and Soden

Climate Warming–Related Strengthening of the Tropical Hydrological Cycle

Theauthorsestimateclimatewarming‐relatedtwenty-first-centurychangesofmoisturetransportsfromthe descending into the ascending regions in the tropics. Unlike previous studies that employ time and space averaging, here homogeneous high horizontal and vertical resolution data from an Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4) climate model are used. This allows for estimating changes in much greater detail (e.g., the estimation of the distribution of ascending and descending regions, changes in the vertical profile, and separating changes of the inward and outward transports). Lowlevel inward and midlevel outward moisture transports of the convective regions in the tropics are found to increase in a simulated anthropogenically warmed climate as compared to a simulated twentieth-century atmosphere, indicating an intensification of the hydrological cycle. Since an increase of absolute inward transport exceeds the absolute increase of outward transport, the resulting budget is positive, meaning that more water is projected to converge in the moist tropics. The intensification is found mainly to be due to the higher amount of water in the atmosphere, while the contribution of weakening wind counteracts this response marginally. In addition the changing statistical properties of the vertical profile of the moisture transport are investigated and the importance of the substantial outflow of moisture from the moist tropics at midlevels is demonstrated.

Climatology of Polar Lows over the Sea of Japan Using the JRA-55 Reanalysis

AbstractPolar lows are intense meso-α-scale cyclones that develop over the oceans poleward of the main baroclinic zone. A number of previous studies have reported polar low formation over the Sea of Japan within the East Asian winter monsoon. To understand the climatology of polar lows over the Sea of Japan, a tracking algorithm for polar lows is applied to the recent JRA-55 reanalysis. The polar low tracking is applied to 36 cold seasons (October–March) from October 1979 to March 2015. The polar lows over the Sea of Japan reach their maximum intensity on the southeastern side of the midline between the Japanese islands and the Asian continent. Consistent with previous case studies, composite analysis demonstrates that the polar low development is associated with the enhanced northerly flow on the western side of a synoptic-scale extratropical cyclone, with the cold trough in the midtroposphere and with increased heat fluxes from the sea surface. Furthermore, the present climatological study has revealed ...

Hurricane Gonzalo and its Extratropical Transition to a Strong European Storm

Introduction. Recent studies simulating continued anthropogenic climate change provide evidence that extratropically transitioning tropical cyclones (TCs) will become more frequent and will hit western Europe more often (Baatsen et al. 2015; Haarsma et al. 2013). Mokhov et al. (2014) asserted, “Under the tendency towards global warming, we can expect an increase in the number of intensive cyclones in the warmer and more humid troposphere.” We saw Hurricane Gonzalo of 2014 as an occasion to assess if these aforementioned properties of extratropically transitioned storms—frequency, intensity, and tracks—have changed. East of the Leeward Islands a tropical depression formed on 12 October 2014. On its way it passed through the northern Leeward Islands and intensified to a category 4 hurricane (Saffir-Simpson hurricane wind scale) on 16 October, known as “Gonzalo”. After changing its direction to northeast, Gonzalo weakened and crossed Bermuda with gusts of more than 200 km h−1 and heavy rains of about 70 mm within 24 hours. On 19 October, the storm transitioned to an extratropical cyclone off the coast of Newfoundland (Brown 2015). While continuing its path across the North Atlantic towards northwestern Europe, the cyclone was absorbed by a cold front and strengthened again. Afterwards, it hit the northern part of the United Kingdom on 21 Octber. It crossed the North Sea and then central parts of Europe, and went down to the Balkans. On 23 October ex-Gonzalo merged with another low pressure system that led to heavy precipitation for several days in this region. Maximum wind gusts between 100 and 180 km h−1, causing North Sea storm surges, were reported from several countries1,2,3. In addition, ex-Gonzalo triggered regional precipitation amounts of 50–100 mm in 24 hours, while the advection of cold air led to a sudden temperature drop with snowfall in some areas. Gonzalo and its remnants caused several fatalities, storm surges, structural damage, and power outages on both sides of the Atlantic4,5. Gonzalo attracted strong media attention as it affected many countries along its path6. There is no general definition of extratropical transition (ET) of TCs (Malmquist 1999). Basically, it is a gradual transformation of a TC into a system with extratropical characteristics while moving poleward into a more baroclinic environment with higher wind shear, a larger Coriolis parameter, and lower sea surface temperatures (Jones et al. 2003). The ET storm may interact with upper-level troughs or extratropical low pressure systems. Evans and Hart (2003) describe ET as the transition of a warm-core TC that interacts with a baroclinic midlatitude environment and then develops a cold core. Forty-six percent of the Atlantic TCs transitioned into extratropical cyclones between 1950 and 1996 (Hart and Evans 2001). This result was supported by Jones et al. (2003) for 1970–99 and Mokhov et al. (2014) for 1970–2012 who found 45% of North Atlantic TCs underwent ET. But only very few

The Changing Energy Balance of the Polar Regions in a Warmer Climate

AbstractEnergy fluxes for polar regions are examined for two 30-yr periods, representing the end of the twentieth and twenty-first centuries, using data from high-resolution simulations with the ECHAM5 climate model. The net radiation to space for the present climate agrees well with data from the Clouds and the Earth’s Radiant Energy System (CERES) over the northern polar region but shows an underestimation in planetary albedo for the southern polar region. This suggests there are systematic errors in the atmospheric circulation or in the net surface energy fluxes in the southern polar region. The simulation of the future climate is based on the Intergovernmental Panel on Climate Change (IPCC) A1B scenario. The total energy transport is broadly the same for the two 30-yr periods, but there is an increase in the moist energy transport on the order of 6 W m−2 and a corresponding reduction in the dry static energy. For the southern polar region the proportion of moist energy transport is larger and the dry ...