download report - Sapienza
download report - Sapienza
download report - Sapienza
You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
Scientific Report 2007-2009<br />
Geophysics<br />
G1. Theory and observations of climate and its changes<br />
The main objective in Climate Dynamics is the understanding<br />
of the origin of the atmospheric general circulation,<br />
i.e. climatic zones. Aristotele devoted a volume to<br />
the description of atmospheric phenomena and climatic<br />
zones, while Galileo was the first who studied the origin<br />
of trade winds. Today, there are rational bases for an<br />
explanation of climatic zones, but the nonlinear character<br />
of the involved physical processes makes difficult the<br />
development of a comprehensive and established theory.<br />
Furthermore, nowadays the problem of future changes of<br />
the Earth’s climate is attracting an increasing interest.<br />
However, the complex nature of the physical problem<br />
necessitates a major scientific effort to make possible assessments<br />
of likely future climate changes, regardless of<br />
whether these may be natural or man-made.<br />
In this framework, the Climate Dynamics group of<br />
Roma carried out theoretical studies and numerical simulations<br />
to investigate the origin of the observed atmospheric<br />
general circulation features in relation to the<br />
role of the stratosphere, of the baroclinic eddies through<br />
their heat and momentum transports, and of changes<br />
in the imposed meridional temperature gradient in the<br />
troposphere. The formation and variability of tropospheric<br />
double-jet patterns observed in the Southern<br />
Hemispheres during the transition seasons (Fig. 1) has<br />
been investigated using a quasigeostrophic and a simplified<br />
general circulation model (GCM) [1].<br />
NCEP reanalysis data for the last 50 years [3]. The analysis<br />
suggested the importance of baroclinic adjustment<br />
processes for midlatitude tropopause dynamics.<br />
Using both NCEP reanalysis and observations space<br />
and time variability of drought and wetness at large-scale<br />
has been investigated also in relation to a changing climate.<br />
An updated analysis for the European area has<br />
been carried out computing the Standardized Precipitation<br />
Index (SPI) and applying the Principal Component<br />
Analysis (PCA) to the SPI field [4]. Linear and nonlinear<br />
trends of drought and wetness were compared for<br />
two time sections: 1949–1997 and 1949–2009 (Fig. 2).<br />
a)<br />
April 2000<br />
b)<br />
30<br />
35<br />
pressure (mb)<br />
100<br />
200<br />
300<br />
400<br />
500<br />
600<br />
700<br />
800<br />
900<br />
1000<br />
−80 −60 −40 −20 0<br />
latitude<br />
time (day)<br />
25<br />
20<br />
15<br />
10<br />
5<br />
−80 −60 −40 −20 0<br />
latitude<br />
Figure 1: a) Latitude-pressure cross-section of the monthly<br />
mean zonal wind for April 2000, and b) latitude-time diagram<br />
of the zonal mean zonal wind at 200 mb [1].<br />
The role of eddy heat fluxes in generating the observed<br />
double-jet pattern was ascertained using an analytical<br />
Eady model with stratospheric easterlies. Sensitivity of<br />
the results to the meridional temperature gradient in<br />
the troposphere showed a regime change from a prevailing<br />
subtropical jet to a midlatitude one. The intermittent<br />
nature of the tropospheric double jets has been also<br />
studied for the Northern Hemisphere winter and Southern<br />
Hemisphere summer [2]. The impact of baroclinic<br />
eddies on the mean tropopause height (a key parameter<br />
in climate change detection) has been assessed using<br />
30<br />
25<br />
20<br />
15<br />
10<br />
5<br />
Figure 2: a) First loading of SPI field and b) first principal<br />
component score time series with the fitting linear and<br />
nonlinear trends for the whole period and the shorter period<br />
[4].<br />
The study showed that the SPI time series are not<br />
stationary and have multi-year fluctuations. Linear<br />
trends highly depend on the time section considered and<br />
classical statistics, commonly used in hydrology, should<br />
be revised under the hypothesis of a varying climate.<br />
Finally, in the last 3 years the group is participating<br />
to the development of the ground segment of ASI<br />
ROSA satellite mission (launch occurred in September<br />
2009 on board of OCEANSAT-2) that uses the radio<br />
occultation technique for sounding the atmosphere<br />
(http://www.asi.it/Rosa/RosaIT/ROSA.htm).<br />
References<br />
1. I. Bordi et al., J. Atmos. Sci. 66, 1366 (2009).<br />
2. I. Bordi et al., Mon. Wea. Rev. 135, 3118 (2007).<br />
3. A. Dell’Aquila et al., Clim. Dyn. 28, 325 (2007).<br />
4. I. Bordi et al., Hydrol. Earth Syst. Sci. 13, 1519 (2009).<br />
Authors<br />
I. Bordi, A. Sutera<br />
http://romatm13.phys.uniroma1.it/<br />
<strong>Sapienza</strong> Università di Roma 166 Dipartimento di Fisica