8 research outputs found
Long-term variability of drought indices in the Czech Lands and effects of external forcings and large-scale climate variability modes
While a considerable number of records document the temporal variability of
droughts for central Europe, the understanding of its underlying causes remains
limited. In this contribution, time series of three drought indices (Standardized Precipitation Index –
SPI; Standardized Precipitation Evapotranspiration Index – SPEI; Palmer Drought Severity Index – PDSI) are analyzed with regard to mid- to long-term drought variability
in the Czech Lands and its potential links to external forcings and internal
climate variability modes over the 1501–2006 period. Employing instrumental
and proxy-based data characterizing the external climate forcings (solar and
volcanic activity, greenhouse gases) in parallel with series representing the
activity of selected climate variability modes (El Niño–Southern
Oscillation – ENSO; Atlantic Multidecadal Oscillation – AMO; Pacific
Decadal Oscillation – PDO; North Atlantic Oscillation – NAO), regression
and wavelet analyses were deployed to identify and quantify the temporal
variability patterns of drought indices and similarity between individual
signals. Aside from a strong connection to the NAO, temperatures in the AMO
and (particularly) PDO regions were disclosed as one of the possible drivers
of inter-decadal variability in the Czech drought regime. Colder and wetter
episodes were found to coincide with increased volcanic activity, especially
in summer, while no clear signature of solar activity was found. In addition
to identification of the links themselves, their temporal stability and
structure of their shared periodicities were investigated. The oscillations
at periods of approximately 60–100 years were found to be potentially
relevant in establishing the teleconnections affecting the long-term
variability of central European droughts.</p
Enhanced internal gravity wave activity and breaking over the northeastern Pacific–eastern Asian region
We have found a stratospheric area of anomalously low annual cycle amplitude
and specific dynamics in the stratosphere over the northeastern Pacific–eastern Asia coastal region. Using GPS radio occultation density profiles
from the Formosat Satellite
Mission 3/Constellation Observing System for Meteorology, Ionosphere, and
Climate (FORMOSAT-3/COSMIC), we have discovered an internal gravity wave (IGW) activity
and breaking hotspot in this region. Conditions supporting orographic wave
sourcing and propagation were found. Other possible sources of wave activity
in this region are listed.
<br><br>
The reasons why this particular IGW activity hotspot was not discovered
before as well as why the specific dynamics of this region have not been pointed out
are discussed together with the weaknesses of using the mean potential energy as a
wave activity proxy. Possible consequences of the specific dynamics in this
region on the middle atmospheric dynamics and transport are outlined
An analysis of the spatial distribution of approximate 8 years periodicity in NCEP/NCAR and ERA-40 temperature fields
In the presented paper, we describe an analysis of spatial patterns of the approximate 8 years cycle identified in the NCEP/NCAR and ERA-40 reanalyzed temperature series. The temperature series are examined by the pseudo-2D wavelet transform (p2D-WT) at 17 geopotential levels, ranging from 1000 hPa to 10 hPa. The results differ significantly for the NCEP/NCAR and for the ERA-40 temperature series. For the NCEP/NCAR dataset, oscillation of about 8 years is distinct over large areas at almost every analyzed level. At lower heights, the regions with significant presence of the periodicity are less compact and pronounced than at higher geopotential levels. The occurrence of the analyzed period is generally strongest in the equatorial and tropical areas. For the ERA-40 dataset, the approximate 8 years cycle is detected in substantially fewer grid points and the spatial patterns generally do not conform to the analysis of NCEP/NCAR series. These results indicate that a frequency analysis of the reanalysis datasets should only be interpreted after careful discussion and that the reliability (in the sense of frequency characteristics) of the reanalyzed temperature series still needs to be studied
Analysis of internal gravity waves with GPS RO density profiles
GPS radio occultation (RO) data have proved to be a great tool for
atmospheric monitoring and studies. In the past decade, they were frequently
used for analyses of the internal gravity waves in the upper troposphere and
lower stratosphere region. Atmospheric density is the first quantity of state
gained in the retrieval process and is not burdened by additional
assumptions. However, there are no studies elaborating in detail the
utilization of GPS RO density profiles for gravity wave analyses. In this
paper, we introduce a method for density background separation and a
methodology for internal gravity wave analysis using the density profiles.
Various background choices are discussed and the correspondence between
analytical forms of the density and temperature background profiles is
examined. In the stratosphere, a comparison between the power spectrum of
normalized density and normalized dry temperature fluctuations confirms the
suitability of the density profiles' utilization. In the height range of
8–40 km, results of the continuous wavelet transform are presented and
discussed. Finally, the limits of our approach are discussed and the
advantages of the density usage are listed
An analysis of the spatial distribution of approximate 8 years periodicity in NCEP/NCAR and ERA-40 temperature fields
Influence of the spatial distribution of gravity wave activity on the middle atmospheric dynamics
Analysing GPS radio occultation density profiles, we have
recently pointed out a localised area of enhanced gravity wave (GW) activity
and breaking in the lower stratosphere of the east Asian–northwestern
Pacific (EA/NP) region. With a mechanistic model of the middle and upper
atmosphere, experiments are performed to study the possible effect of such a
localised GW breaking region on large-scale circulation and transport
and, more generally, a possible influence of the spatial distribution of
gravity wave activity on middle atmospheric dynamics.<br><br>The results indicate the important role of the spatial distribution of GW
activity for polar vortex stability, formation of planetary waves and for
the strength and structure of zonal-mean residual circulation. Furthermore, a
possible effect of a zonally asymmetric GW breaking in the longitudinal
variability of the Brewer–Dobson circulation is analysed. Finally,
consequences of our results for a variety of research topics (e.g. sudden
stratospheric warming, atmospheric blocking, teleconnection patterns and a
compensation mechanism between resolved and unresolved drag) are discussed
The regional impact of urban emissions on climate over central Europe: present and future emission perspectives
The regional climate model RegCM4.2 was coupled to the chemistry transport
model CAMx, including two-way interactions, to evaluate the regional impact
of urban emission from central European cities on climate for present-day
(2001–2010) and future (2046–2055) periods, and for the future one only emission
changes are considered. Short-lived non-CO2 emissions are considered and,
for the future impact, only the emission changes are accounted for (the
climate is kept “fixed”). The urban impact on climate is calculated with the
annihilation approach in which two experiments are performed: one with all
emissions included and one without urban emissions. The radiative
impacts of non-CO2 primary and secondary formed pollutants are considered,
namely ozone (O3), sulfates (PSO4), nitrates (PNO3), primary organic
aerosol and primary elementary carbon (POA and PEC).The validation of the modelling system is limited to key climate parameters,
near-surface temperature and precipitation. It shows that the model, in general,
underestimates temperature and overestimates precipitation. We attribute
this behaviour to an excess of cloudiness/water vapour present in the model
atmosphere as a consequence of overpredicted evaporation from the surface.The impact on climate is characterised by statistically significant cooling
of up to −0.02 and −0.04 K in winter (DJF) and summer (JJA), mainly over
cities. We found that the main contributors to the cooling are the direct and
indirect effects of the aerosols, while the ozone titration, calculated
especially for DJF, plays rather a minor role. In accordance with the
vertical extent of the urban-emission-induced aerosol perturbation, cooling
dominates the first few model layers up to about 150 m in DJF and 1000 m in
JJA. We found a clear diurnal cycle of the radiative impacts with maximum
cooling just after noon (JJA) or later in afternoon (DJF). Furthermore,
statistically significant decreases of surface radiation are modelled in
accordance with the temperature decrease. The impact on the boundary layer
height is small but statistically significant and decreases by 1 and 6 m in
DJF and JJA respectively. We did not find any statistically significant
impact on precipitation and wind speed. Regarding future emissions, the
impacts are, in general, smaller as a consequence of smaller emissions,
resulting in smaller urban-induced chemical perturbations.In overall, the study suggest that the non-CO2 emissions play rather a
minor role in modulating regional climate over central Europe. Much more
important is the direct climate impact of urban surfaces via the urban canopy
meteorological effects as we showed earlier