Precipitation:Modelingand DiagnosticStudies - ETH E

Diss. ETH No. 14861
InterannualVariabilityof Japan
Precipitation: Modeling and
Diagnostic Studies
A dissertation submitted to the
Swiss Federal Institute of Technology (ETH)
ZÜRICH
for the degree of
Doctor of Natural Science
presented by
Sophie A. Fukutome
Dipl. Phys.
ETH
born 22 October 1963
Citizen of the United States of America (USA)
accepted on the recommendationof
Prof. Dr. C Schär, examiner
Dr. I. N. James, co-examiner
Prof. Dr. A. Ohmura co-examiner
Dr. C
Frei, co-examiner
2002
Abstract
Boreal in the winter and subtropical in the summer, Japan's climate is characterized by
modest interannual variability in the winter season, and very large year-to-yearvariability
in summer. The purpose of the present thesis is to investigate the interannual variability
of precipitationover Japan in winter and summer months, using Statisticalmethods and
a regional climate model (RCM) as research tools. To assess the quality of the RCM employed, the model is, in the first part of the thesis, put to test for its ability to represent
natural interannual winter variability. In the second part of the thesis, Japan's observed
summer precipitation variability is related to sea surface temperature (SST) observed
variability by means of a Statistical analysis of their co-variability. In the third part of
this thesis, the RCM is used to bring to light the response of precipitationto summer sea
surface temperature anomalies.
An important tool for this study is a regional climate model (RCM), which simulates the physical laws governing the evolution of the atmosphere and the land-surface
Parameters within a given region of the globe. The model utilized is derived from the
operational numerical weather prediction model of the German Weather Service, and
is driven at its lateral boundaries by the European Centre for Medium-Range Weather
Forecasts (ECMWF) analysis. The computationaldomain extends over 5100 x 5100 km2,
covering Japan and part of East Asia with a horizontal grid-spacingof 0.5 degrees.
In the first part of this thesis, the regional climate model utilized in the experimentsis
validated for its ability to reproduce the observed interannual precipitation variability
during Japan's winters. The method used to test the model consists in separating the
systematic and unsystematic error contributions of the model. The results show that the
model can represent rather accurately the year-to-yearvariations in the evolution of the
dynamical variables and in the precipitationdistribution despite the small amplitude of
the interannual Variation of Japan's winter climate (which represent a severe test of the
model's Performance).
The second part of the thesis consists in an empirical Statisticalanalysis of the variability
of the observed Japan summerprecipitationand the sea surface temperature (SST) in the
Western North Pacific (120 160°W, 0 60°N). Singular value decomposition (SVD) is
used to isolate covariant patterns in the two variables. It is found that the first coupled
SVD mode accounts for 70% of the covariance of precipitation and SST. It is characterized
-
-
by a zonal, elongated SST dipole pattern straddling 30°N, and a precipitation anomaly
with its maximum in southern Japan. Precipitation is negatively correlated with SST
anomalies over the East China Sea, the Japan Sea, and the Kuroshio-Oyashioextension.
(dry) precipitation
reveals an enhanced
zonal
winds, roughly
(subdued) meridional temperature gradient and stronger (weaker)
estimation
of
the
radiative
and evapwith
the
SST
A
crude
underlying
gradient.
aligned
the
SST pattern merely
orative forcing of the ocean surface casts doubt on the idea that
Compositing with respect to
moist
scores
responds passively to atmosphericforcing. Thus, the results raise the question whether,
and how, the SST distributioncan affect Japan's precipitation.
The third part of the thesis tests the above hypothesis of a thermodynamicforcing from
the ocean surface to the atmosphere. The response of the regional climate and the associated precipitationwith respect to the SST anomalies is examined in a series of numerical
experiments. To this end, RCM simulations of two pairs of wet and dry Summers are
repeated with exchanged SST distributions. The results show that the response is non¬
linear, i.e. wet summers experience a decrease in precipitation of up to 25% over Japan
if driven by the SST of a dry summer, while the precipitation of a dry summer is hardly
affected by the SST distribution of a wet summer. Further analysis reveals that the latitudinal alignment of the low-level jet associated with the rain-belt with the sea surface
temperatureanomaly distribution is essential to a positive feedback ofthe SST anomalies
on precipitation.
In conclusion, the results of this thesis show that although the variability of Japan's sum¬
mer precipitation appears strongly determined by the large-scale circulation, it can be
efFectively modulated by the sea surface temperatureif the regions of active precipitation
processes are appropriately located.
Resume
subtropical en ete, le climat du Japon se caracterise par une vari¬
abilite inter-annuelle modeste en hiver et tres importante en ete. Le but de la presente
these est d'examinerla variabilite inter-annuelle de la precipitation au Japon en ete et en
hiver, utilisant comme instrument de recherche des methodes statistiques et un modele
de climat regional (MCR). Afin d'evaluer la qualite du MCR employe, sa representation
de la variabilite inter-annuelle hivernale naturelle est d'abord mise ä l'epreuve dans la
premiere partie de la these. Dans la seconde partie de la these, la variabilite estivale
des precipitations observees au Japon est reliee ä la variabilite de la temperaturesuperficielle de la mer (TSM) observee au moyen d'une analyse statistique de leur co-variabilite.
Dans la troisieme partie de la these, le MCR est utilise pour etudier la reaction de la
precipitation aux anomalies estivales de la TSM.
Boreal
en
hiver et
Un instrument important pour cette etude est un modele de climat regional (MCR),
dont la täche est de simuler les lois physiques gouvernantl'evolution de Patmosphere et des
parametresde surface ä l'interieurd'une region donnee du globe. Le modele employe ici
derive du modele de prediction meteorologique operationnel du Service Meteorologique
Allemand. L'information aux limites laterales est fournie par l'analyse du Centre Eu¬
ropean de Prediction Meteorologique ä Moyen Terme. Le domaine de calcul s'etend sur
5100 x 5100 km2, couvrant le Japon et une partie de l'Asie de l'Est, avec une resolution
de 0.5 degres.
de cette these, l'habilete du modele climatique regional ä reproduire la variabilite observee du climat hivernal japonais est validee. La methode
utilisee consiste ä separer les contributions de l'erreur systematique et de l'erreur nonsystematique du modele. Les resultats montrent que le modele est en mesure de representer
les variations d'annee en annee de l'evolution des variables dynamiques et de la precipitation,
et ce malgre la faible Variation inter-annuelle du climat d'hiver japonais.
Dans la
premiere partie
La deuxieme partie est une analyse statistique empiriquede la variabilite des observations
de la precipitation estivale et de temperature superficielle de la mer (TSM) dans l'ouest
du Pacifique Nord (120 160°W, 0 60°N). La decompositionen valeurs singulieres est
utilisee pour extraire les distributions geographiques covariantes des deux variables. II
s'avere que le premier mode couple explique 70% de la covariance entre precipitationet
TSM. II se caracterise par un dipöle en TSM, allonge zonalement et chevauchant30°N, et
par une distribution de la precipitation augmentant vers le sud du Japon. La correlation
est negative entre la precipitation et la TSM de la Mer Jaune, la Mer de Chine de l'Est
-
-
et la Mer du
Japon ä l'extension
du
Kuroshio-Oyashio.
precipitations humides (secs) revelent un
gradient meridional de temperature accru (attenue) et des vents zonaux plus (moins)
prononces, et ce approximativement au-dessus du gradient en TSM. Une estimation
grossiere du. forgage exerce par la radiation et l'evaporationsur la surface de l'ocean met
en doute la possibilite que la distribution de TSM soit une reaction passive au forgage
de l'atmosphere. Ainsi, les resultats soulevent la question de savoir si, et comment, la
distributionde TSM est en mesure d'influencer la precipitation au Japon.
Des
composites
sur
la base des
scores
de
La troisieme partie de la these verifie cette derniere hypothese d'un forgage thermodynamiquede l'ocean sur l'atmosphere. La reactiondu climat regionalet de la precipitation
correspondante ä l'anomalie de TSM est examinee dans une serie d'experiences numeriques.
Pour ce faire, deux paires d'etes humides et secs sont simulees avec un MCR, puis simulees
ä nouveau apres avoir echange les evolutions de TSM. Les resultats montrent que la
reactionest non-lineaire, c'est-ä-dire que les etes humides voient leur precipitation decrotre
jusqu'ä 25% au Japon s'ils sont couples ä la TSM d'un ete sec, tandis que la precipitation
d'un ete sec se trouve ä peine modifiee par la TSM d'un ete humide. II s'avere que la latitude du jet ä bas niveau accompagnant la ceinture de pluies par rapport ä la distribution
de la TSM jouent un röle essentiel pour un forgage des precipitations par les anomalies
de TSM.
conclusion,les resultats de la presente these indiquent que, bien que la variabilite de la
precipitationestivale japonaise soit essentiellementdeterminee par la circulation ä large
echelle, eile peut etre modulee par la temperature superficielle de la mer si les processus
qui menent ä la precipitationsont places adequatement.
En