Discussion of “Perceptual models of uncertainty for socio-hydrological systems: a flood risk change example”*

ABSTRACT

Dealing with uncertainty is key in socio-hydrological analysis. As such, thinking through what uncertainties mean for whom and when is key. This discussion contribution introduces three issues related to defining uncertainties. The first issue deals with the problem of defining uncertainty as a given external reality. The second issue deals with who decides about relevant uncertainties. The third issue deals with the issue whether coupled human-hydrological systems can be seen as existing on their own. Finally, the text provides two examples of hydrological research that try to be explicit about our dealing with multiple (interpretations of) realities.     

The “twin” storm surges during 21st–24th December, 1954. A case of resonance

Summary The actual sea level disturbances at Hoek van Holland during the two storms of 21^st–24^th December, 1954 are compared to disturbance heights that are computed on the assumption that the sea level disturbances at any place behave like the deviations of a damped harmonic oscillator from its equilibrium position. From this comparison the parameters of the model and, hence, the eigen-period and the resonance period of oscillation have been computed.Moreover, it is shown that in the present case, we can speak of resonance and, finally, that an assumed third storm of the same force and occurring after the same time interval would have given about the same maximum disturbance height.

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Die Sturmfluten der Nordsee vom 21. bis 24. Dezember 1954. Ein Fall von Resonanz

Zusammenfassung Die beobachteten Erhöhungen des Meeresspiegels in Hoek van Holland während zweier Stürme vom 21. bis 24. Dezember 1954 wurden mit Erhöhungen verglichen, die auf Grund der Voraussetzung berechnet wurden, daß die Wasserstandserhöhungen an einem beliebigen Ort sich wie die Ausschläge eines gedämpften harmonischen Oszillators verhalten. Aus diesem Vergleich ergeben sich Werte für die Parameter dieses mathematischen Modells, aus denen sich dann die Eigenschwingungsdauer und die Resonanzschwingungsdauer berechnen lassen.Außerdem ergibt sich, daß man es hier mit einem Fall von Resonanz zu tun hat.Schließlich wird gezeigt, daß ein hypothetischer, nach gleichem Zeitintervall folgender dritter Sturm derselben Stärke schon keine wesentlich größeren Erhöhungen mehr hervorgerufen haben würde.

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Les ondes des tempêtes du 21 au 24 décembre 1954 dans la mer du Nord. Un cas de résonance

Résumé On compare les élévations actuelles du niveau de la mer observées à Hoek van Holland pendant les deux tempêtes du 21 au 24 décembre 1954 avec des élévations calculées en supposant que l'élévation à une place quelconque peut être «expliquée» à l'aide du modèle mathématique d'un oscillateur harmonique amorti. On déduit de cette comparaison les paramètres du modèle et, conséquemment, la période propre et la période de résonance.En outre, on montre que dans le cas considéré il s'agit d'une résonance et, enfin, qu'une troisième tempête hypothétique de la même force qui se serait produite après un même interval de temps aurait causé environ la même élévation maximale.

     

Reply to Discussion of “Perceptual models of uncertainty for socio-hydrological systems: a flood risk change example”*

ABSTRACT

Ertsen discusses the representation of reality and uncertainty in our paper, raising three critical points. In response to the first, we agree that discussion of different interpretations of the concept of uncertainty is important when developing perceptual models – making different uncertainty interpretations explicit was a key motivation behind our method. Secondly, we do not, as Ertsen suggests, deny anyone who is not a “certified” scientist to have relevant knowledge. The elicitation of diverse views by discussing perceptual models is a basis for open discussion and decision making. Thirdly, Ertsen suggests that it is not useful to treat socio-hydrological systems as if they exist. We argue that we act as “pragmatic realists” in most practical applications by treating socio-hydrological systems as an external reality that can be known. But the uncertainty that arises from our knowledge limitations needs to be recognized, as it may impact on practical decision making and associated costs.     

Extreme flows and unusual water levels near a Caribbean coral reef: was this a case of a “perfect storm”?

Observations of currents aimed to study the flow near a spawning aggregation reef, Gladden Spit off the coast of Belize, reveal unusually strong currents on 19–20 October 2009 (the current speed was over 1?m?s^?1, when the mean and standard deviation are 0.2?±?0.12?m?s^?1). During this short time, the water level was raised by 60–70?cm above normal in one place, but lowered by 10–20?cm in another location just 2?km away. The temperature dropped by over 2°C within a few hours. Analyses of local and remote sensing data suggest that a rare combination of an offshore Caribbean cyclonic eddy, a short-lived local tropical storm, and a Spring tide, all occurred at the same time and creating a “perfect storm” condition that resulted in the unusual event. High-resolution simulations and momentum balance analysis demonstrate how the unique shape of the coral reef amplified the coastal current through nonlinear flow–topography interactions. The suggested mechanism for the water level change is different than the classical wind-driven storm surge process. The study has implications for the influence of external forcing on mixing processes and physical–biological interactions near coral reefs.     

Characteristics of climate change in the “significant impact zone” affected by aerosols over eastern China in warm seasons

Through analysis of the distribution pattern and changing characteristics of atmospheric aerosols over the East Asia region during warm seasons in recent 20 a and beyond as well as their possible interactive relationship with a variety of meteorological elements, it is found that the high-value zone of aerosol optical depth derived from the Total Ozone Mapping Spectrometer (TOMS), its significant negative correlation zones in terms of sunshine duration (SD) and surface air temperature (SAT) and its significant positive correlation zones with low-level cloud amount (LCC) are co-located in the South China region during warm periods. Based on this finding, the region is referred to as a “significant impact zone” (SI zone) affected by aerosols. Then, a comparative analysis is made on variation differences of observed SAT, SD and LCC, etc. in different regions. It is also found that the LCC is increased and the SD is decreased within the “SI zone” over eastern China during the warm season. These characteristics are more evident than those beyond the zone, while the warming trend within the zone is evidently weaker than that outside it. Studies show that since recent 20 a, under the influence of aerosols, the LCC tend to increase substantially with a clear decrease of SD and an unnoticeable warming trend within the “SI zone”. Comparing with the climate change beyond the zone, the difference is significant. Therefore, the effects of atmospheric aerosols on climate is possibly one of the contributions to the difference of climate change existed between the southern and northern parts of the Eastern China during a warm season. Supported by the International Sci-Tech Cooperative Project under the auspices of the Ministry of Science and Technology of the People’s Republic of China (No. 2004DFA06100)     

Characteristics of climate change in the “significant impact zone” affected by aerosols over eastern China in warm seasons

Through analysis of the distribution pattern and changing characteristics of atmospheric aerosols over the East Asia region during warm seasons in recent 20 a and beyond as well as their possible interac- tive relationship with a variety of meteorological elements, it is found that the high-value zone of aerosol optical depth derived from the Total Ozone Mapping Spectrometer (TOMS), its significant negative correlation zones in terms of sunshine duration (SD) and surface air temperature (SAT) and its significant positive correlation zones with low-level cloud amount (LCC) are co-located in the South China region during warm periods. Based on this finding, the region is referred to as a "significant impact zone" (SI zone) affected by aerosols. Then, a comparative analysis is made on variation differ- ences of observed SAT, SD and LCC, etc. in different regions. It is also found that the LCC is increased and the SD is decreased within the "SI zone" over eastern China during the warm season. These characteristics are more evident than those beyond the zone, while the warming trend within the zone is evidently weaker than that outside it. Studies show that since recent 20 a, under the influence of aerosols, the LCC tend to increase substantially with a clear decrease of SD and an unnoticeable warming trend within the "SI zone". Comparing with the climate change beyond the zone, the difference is significant. Therefore, the effects of atmospheric aerosols on climate is possibly one of the contri- butions to the difference of climate change existed between the southern and northern parts of the Eastern China during a warm season.     

GCM-related uncertainty in river flow projections at the threshold for “dangerous” climate change: the Kalu Ganga river,Sri Lanka

The Kalu Ganga catchment is one of the largest in Sri Lanka, and is home to 5% of the national population. A first assessment is provided here of the sensitivity of Kalu Ganga runoff to a 2°C increase in global mean temperature – the supposed threshold for “dangerous” climate change. Runoff is simulated using the HBV-Light hydrological model and scenario data from seven general circulation models (GCMs). Precipitation is the strongest cause of change in runoff. Substantial inter-GCM differences in scenario precipitation lead to uncertainty in the direction of change in mean annual runoff from the baseline (range ?25% to +19%). Scenario monthly runoff ranges from ?41% to +124% of the baseline values at its most extreme (March); June is the only month with a consistent direction of change (range ?17% to ?65%) – thus indicating that climate change may lead to a substantially different hydrological regime in the Kalu Ganga catchment.     

Role of magnetospheric convection and precipitation in the formation of the “dusk effect” during the main phase of a magnetic storm

This paper studies the role of magnetospheric factors, such as convection and energetic electron precipitation during the formation of positive disturbances in the total electron content under the conditions of the summer evening ionosphere. à numerical model of the ionosphere and plasmasphere, where time variations in the magnetospheric convection velocity and electron precipitation parameters correspond to the main phase of a magnetic storm, has been used for this purpose. It has been indicated that the total electron content sharply increases (the “dusk effect”) in the eastern and western sectors at approximately the same geomagnetic latitudes corresponding to the subauroral zone provided that a sudden storm commencement is registered in the morning hours. local time. This peak of the total electron content is formed as a result of joint reconstruction of the magnetospheric convection pattern and energetic electron precipitation during the main phase of a storm. In this case, magnetospheric convection plays the main role, raising the F2 layer by 40–80 km into the region with a lower recombination rate.