Improving Precipitation Forecasts over the Global Tropical Belt

Summary  This study explores the nowcasting and short-range forecasting (up to 3 days) skills of rainfall over the tropics using a high resolution global model. Since the model-predicted rainfall is very sensitive to model parameters, four key model parameters were first selected. They are the Asselin filter coefficient, the fourth order horizontal diffusion coefficient, the surface moisture flux coefficient, and the vertical diffusion coefficient. The optimal values were defined as those which contributed to the best one day rainfall forecasts in the present study. In order to demonstrate and improve the precipitation forecast skill, several numerical experiments were designed using the 14-level Florida State University Global Spectral Model (FSUGSM) at a resolution of T106. Comparisons were also made of the short-range forecasts obtained from a control experiment subjected to normal mode initialization (NMI) versus experiments based on physical initialization (PI). The latter experiments were integrated using the original FSUGSM and a modified version. This modified FSUGSM was developed here by applying a reverse cumulus parameterization alorithm to the regular forecast model, which restructures the vertical humidity distribution and constrains the large-scale model’s moisture error growth during the model integration. An improved short-range rainfall prediction skill was achieved from the modified FSUGSM in this study. The results showed a better agreement between model-based and observed rainfall intensity and pattern. Received January 18, 1999     

On the modification of structure of cometary nuclei by the transport of super-volatiles

Earlier, a study has been made of the transport mechanism of volatile molecules such as N₂ and CO through cometary nuclei as they are heated by radioactive elements. Coupled equations of heat and gas transport in the presence of gas sublimation and recondensation, as well as a heat source, were numerically solved. And it was shown that supervolatiles such as N₂ and CO are transported through the pores of the nucleus, and consequently the volatile molecules become more abundant near the surface than deep inside the nucleus. Here, the process is investigated for a wider range of paramaters such as porosity and nuclear radius. It is shown that provided the central temperature attains the sublimation point of the super-volatiles, they are transported toward the surface regardless of the values of the parameters.     

Evaluation of olivine-spinel geothermometry as an indicator of thermal history for peridotites

Olivine and spinel in peridotites from the Miyamori ultramafic complex and the Ichinogemata crater of Northeast Japan show a systematic variation in the Mg/ (Mg+Fe) ratio which is correlated mainly with the grain size of spinel. This correlation can be explained by a diffusion model assuming a semi-infinite composite sphere under cooling or heating conditions. In order to obtain absolute temperatures of thermal events, the olivine-spinel geothermometer is applied to pairs of spinel core and olivine core (average composition). The calculated temperatures range over two hundred degrees and have a systematic relationship with the grain size of spinel. In the Miyamori complex, the calculated temperatures decrease monotonically with decrease in grain size of spinel, whereas in the Ichinomegata lherzolite nodule those of spinel smaller than 0.2 mm increase as the grain size decreases and those of spinel larger than 0.2 mm remain constant regardless of further increase in grain size. These observations, in the light of the diffusion model, suggest that the Miyamori complex may have cooled from higher than 800° C to lower than 600° C and that the lherzolite nodule from the Ichinomegata crater may have been in equilibrium at 900° C before it was heated above 1,100° C for less than a few days. These two examples indicate that olivine-spinel pairs of peridotites do not always indicate an appropriate equilibration temperature. We cannot interpret the supposed equilibration temperatures until the existence of isothermal stages in the thermal history of peridotites is established by carefully checking the chemical heterogeneity.     

P-T history of a mantle diapir: the Horoman peridotite complex,Hokkaido, northern Japan

The Horoman peridotite complex, Hokkaido, Japan is divided into Lower and Upper zones on the basis of contrasting geological features. The complex recorded a consecutive decompression history in chemical zoning of pyroxenes and plagioclase in plagioclase lherzolite, which is interpreted to have been derived from garnet lherzolite by subsolidus decompression reactions. In the Lower Zone, and earlier decompression history is clearly preserved in large pyroxene porphyroclasts, which show marked M-shaped Al zoning characterized by low Al concentration at the core (Al=0.12/6 oxygens), gradual increase toward the marginal region, and rapid decrease toward the rim. The Ca content in the core is nearly constant (Ca=0.03/6 oxygens) with slight increase toward the margin followed by abrupt decrease toward the rim. The Al and Ca contents in the core of orthopyroxene in plagioclase lherzolite from the Upper Zone (Al=0.22, Ca=0.055/6 oxygens) are much higher than those for the Lower Zone, and the Al content typically decreases monotonously from the core to the rim with several exceptions that show poorly developed M-shaped zoning profiles. The earliest P-T conditions, inferable from the core compositions of pyroxenes are 900–950°C and 20 kbar for the Lower Zone and 1100–1150°C and 20 kbar for the Upper Zone. The increase of Al from the core to the margin is inferred to have resulted from nearly adiabatic decompression from these conditions into spinel peridotite facies. The complex experienced further decompression from the spinel stability field into the plagioclase stability field, which is inferred from plagioclase zoning in fine-grained aggregates composed mostly of plagioclase, chromite spinel, and olivine with minor pyroxenes. The Na-Ca ratio of each plagioclase grain decreases from the core to the rim, suggesting continuous decompression reaction producing olivine and plagioclase from pyroxenes and spinel. The sharp increase in Ca content toward the rim indicates that fairly rapid cooling associated with decompression is necessary to form and preserve the marked zoning. The sharp decrease in Al and Ca contents toward the rim of orthopyroxene was also formed during this final ascent of the complex. The systematic changes of the mineralogic and petrographic features that are gradational between the Lower and Upper zones suggest that the Horoman complex retains a temperature variation from the upper mantle. The Upper Zone is interpreted to have followed a higher temperature decompression path than the Lower Zone and probably represents a relatively hotter portion of a mantle diapir ascending from a depth greater than 60 km in the upper mantle.     

Evaluation of Seasonal Sea Level Variation at Syowa Station, Antarctica, Using GPS Observations

Global Positioning System (GPS) measurements of the vertical displacement of fast ice near Syowa Station, Antarctica, were conducted between April and December 1998 to evaluate measurements of sea level variation derived with a conventional bottom pressure gauge (BPG). The BPG-derived sea level revealed a seasonal variation of about 0.13 m, with a high in April–June and a low in November–December. The GPS-derived sea level, combined with observed sea ice thickness, supported the BPG result, with an RMS error of 0.007 m. Our result also demonstrates that GPS is a powerful technique for monitoring sea level variations. This revised version was published online in July 2006 with corrections to the Cover Date.     

Influence of food quality on egg production and viability of the marine planktonic copepod Acartia omorii

Egg production, egg viability and fecal pellet production were determined for individual Acartia omorii, which were fed diets of two species of diatoms (Skeletonema costatum and Phaeodactylum tricornutum) and three species of dinoflagellates (Scrippsiella trochoidea, Heterocapsa triquetra and Cochlodinium polykrikoides). Diets were analyzed for fatty acid content as an indicator of food quality. Depending on the diet, egg production of A. omorii varied over time, diminishing with some diets (S. trochoidea, C. polykrikoides, P. tricornutum). This rate of reduction was much more rapid for a diet of C. polykrikoides, which caused egg production to decrease to ca. 2.4 eggs f⁻¹ d⁻¹ in only four days. As for all diets, egg viability was high at the beginning but with the C. polykrikoides and P. tricornutum diets, it rapidly decreased with time. Fecal pellet production also varied with time, depending on the diet. Egg production rate was closely correlated with fecal pellet production. There was no direct relationship between egg viability and egg production rate, but both egg production and viability were affected by the nutritional quality of food. Egg viability was also highly dependent on the composition of fatty acids in the eggs. Egg viability showed positive correlation with the ratio of ω3:ω6 groups among egg fatty acids, and negative correlation with the ratio of 20:5 (n−3) : 22:6 (n−3). While comparing several diets, egg production rate was higher on diets (H. triquetra and S. trochoidea) containing ample amounts of essential fatty acids such as 18:4 (n−3) and 22:6 (n−3). The results suggest that fertility of A. omorii was dependent upon the quality of the food, and dinoflagellate diets, with the exception of C. polykrikoides, were preferable to diatom diets.     

Characteristics of alkenones synthesized by a bloom of emiliania huxleyi in the Bering Sea

We investigated the characteristics of the alkenones produced by a bloom of Emiliania huxleyi in the eastern Bering Sea in 2000. Alkenones were detected in surface waters between 57°N and 63°N, where phosphate concentrations were low and the ammonium/nitrate ratio was high. The total alkenone content (C_37:2, C_37:3, and C_37:4) ranged from 22.0 to 349 μg g⁻¹ in suspended particles and from 0.109 to 1.42 μg g⁻¹ in surface sediments. This suggests that a large proportion of the particulate alkenones synthesized in the surface water rapidly degraded within the water column and/or at the water-sediment interface of the Bering Shelf. The change in the stable carbon isotopic composition (δ¹³C) of C_37:3 alkenone could not be explained only by variation in [CO₂(aq)] in the surface water but also depended on the growth rate of E. huxleyi. The alkenone unsaturation index (U^K′₃₇) was converted into an alkenone “temperature” with three equations [Prahl et al 1988], [Sikes et al 1997] and [Müller et al 1998]; Sikes et al.’s (1997) equation gave the best correlation with the observed sea surface temperature (SST) in the eastern Bering Sea. However, some temperatures estimated by Sikes et al.’s (1997) equation from the U^K′₃₇ varied from the observed SST, possibly because of the rapidly changing rate of alkenone synthesis in the logarithmic growth stage or the low rate of alkenone synthesis when nutrients were limiting. Temperatures estimated from U^K′₃₇ in the surface sediments (6.8-8.2°C) matched the observed SST in September (7-8°C) but differed from the annual average SST of 4 to 5°C, suggesting that most of the alkenone in the eastern Bering Sea was synthesized during limited periods, for instance, in September. The relative amounts of C_37:4 alkenone as proportions of the total alkenones (referred to as C_37:4%) were high, ranging from 18.3 to 41.4%. Low-salinity water (<32 psu) within the study area would have contributed to the high C_37:4% because a negative linear relationship between C_37:4% and salinity was found in this study.