Distribution of seasonal rainfall in the East Asian monsoon region

Summary ?This study deals with the climatological aspect of seasonal rainfall distribution in the East Asian monsoon region, which includes China, Korea and Japan. Rainfall patterns in these three countries have been investigated, but little attention has been paid to the linkages between them. This paper has contributed to the understanding of the inter-linkage of various sub-regions. Three datasets are used. One consists of several hundred gauges from China and South Korea. The second is based on the Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP). The two sources of precipitation information are found to be consistent. The third dataset is the NCEP/NCAR reanalysis 850-hPa winds. The CMAP precipitation shows that the seasonal transition over East Asia from the boreal winter to the boreal summer monsoon component occurs abruptly in mid-May. From late March to early May, the spring rainy season usually appears over South China and the East China Sea, but it is not so pronounced in Japan. The summer monsoon rainy season over East Asia commonly begins from mid-May to late May along longitudes of eastern China, the Korean Peninsula, and Japan. A strong quasi-20-day sub-seasonal oscillation in the precipitation appears to be dominant during this rainy season. The end date of the summer monsoon rainy season in eastern China and Japan occurs in late July, while the end date in the Korean Peninsula is around early August. The autumn rainy season in the Korean Peninsula has a major range from mid-August to mid-September. In southern China, the autumn rainy season prevails from late August to mid-October but a short autumn rainy season from late August to early September is noted in the lower part of the Yangtze River. In Japan, the autumn rainy season is relatively longer from mid-September to late October. The sub-seasonal rainfall oscillation in Korea, eastern China and Japan are explained by, and comparable to, the 850-hPa circulation. The strong westerly frontal zone can control the location of the Meiyu, the Changma, and the Baiu in East Asia. The reason that the seasonal sea surface temperature change in the northwestern Pacific plays a critical role in the northward advance of the onset of the summer monsoon rainfall over East Asia is also discussed. Received October 5, 2001; revised April 23, 2002; accepted May 11, 2002     

Insolation estimates for the LITFASS area derived from high resolution satellite data

Summary ?For the LITFASS-98 experiment, from June 1 until June 30, 1998, the spatially resolved insolation at surface could be computed from NOAA-14 AVHRR data applying the modular analysis scheme SESAT (Strahlungs- und Energiebilanzen aus Satellitendaten). The satellite inferred insolation for this period shows for clear-sky regions a good agreement with surface based observations with a rms error of 76 Wm⁻². For cloudy conditions the insolation is overestimated with respect to ground based observations, with a rms error between 83 and 118 Wm⁻², depending on the cloud optical thickness. This overestimation can be explained by the surface heterogeneity, leading to underestimated cloud optical thickness, and also by a fixed relative humidity below clouds (55%, dry atmosphere) and a fixed horizontal visibility (50 km, clear atmosphere). A detailed study of comparable scales in space and time, considering the different observation geometries and sampling intervals, shows that a 30 min ground based observation can be compared with a 8 × 8 km² mean by the satellite data. Received July 12, 2001; revised April 29, 2002; accepted June 7, 2002     

Near-Wall Flow Development After A Step Change In Surface Roughness

An experimental study of the initial flow field downstream of a step change in surface roughness is presented. The roughness length of the downstream surface was approximately tenfold that of the upstream roughness and, unlike all previous studies, attention was concentrated on the roughness sublayer region beneath the inertial (log-law) region. The experiments were conducted at a boundary layer Reynolds number of about 6 × 10⁴ (based on layer thickness andfree-stream velocity) and around a longitudinal location where the (downstream) roughness length, z_o2, was about 1% of the boundary-layer thickness atthe roughness change point.The thickness of the roughness sublayer was found for the two roughness. It was observed that the vertical profiles of mean velocity and turbulence characteristics started to show similarity after about 160z₀₂ downstream of the roughness change. The presence of a shear stress overshoot is shown to depend strongly on the precise location (with respect to the roughness elements) at which the measurements are made and the thickness of the equilibrium layer is shown to be very sensitive to the way it is defined. It is demonstrated that the growing equilibrium layer has first to encompass the roughness sublayer before any thickness of inertial sublayer can be developed. It follows that, in somepractical cases, like flows across some urban environments, the latter(log-law) region may never exist at all.     

Effective and Equitable Dissemination of Seasonal-to-Interannual Climate Forecasts: Policy Implications from the Peruvian Fishery during El Niño 1997–98

The development of seasonal-to-interannual climate predictions has spurred widespread claims that the dissemination of such forecasts will yield benefits for society. Based on the use as well as non-use of forecasts in the Peruvian fishery during the 1997–98 El Niño event, weidentify: (1) potential constraints on the realization of benefits, such aslimited access to and understanding of information, and unintended reactions; (2) theneed for an appropriately detailed definition of societal benefit, considering whose welfare counts as a benefit among groups such as labor, industry, consumers, citizens of different regions, and future generations.We argue that consideration of who benefits, and an understanding of potential socioeconomic constraints and how they might be addressed, should be brought to bear on forecast dissemination choices. We conclude with examples of relevant dissemination choices made using this process.     

Determination of Area-Averaged Water Vapour Fluxes with Large Aperture and Radio Wave Scintillometers over a Heterogeneous Surface – Flevoland Field Experiment

A large aperture scintillometer (LAS) andradio wave scintillometer (RWS)were installed over a heterogeneous areato test the applicability of the scintillation method.The heterogeneity in the area, whichconsisted of many plots, was mainly caused bydifferences in thermal properties ofthe crops; the variations in theaerodynamic roughness lengthwere small. The water vapour fluxesderived from the combined LAS-RWSsystem, also known as the two-wavelengthmethod, agreed fairly well with the aggregatedwater vapour fluxes derived from in-situeddy covariance measurements. The water vapourfluxes derived from a stand-alone LASare also presented. It was found that a single LASand an estimate of the area averagedavailable energy (using a simple parameterisationscheme) can provide also reasonablearea-averaged water vapour fluxes.     

On the mechanism of winter cyclogenesis in relation to vertical axis tilt

Summary ?This work is concerned with the behavior of the vertical trough axis tilt during the development of a Mediterranean cyclone. The period covering stages of the development of the depression is from 1200 UTC 18 January to 1200 UTC 25 January 1981. Two procedures have been used to illustrate the behavior of the vertical axis tilt with the development of our case study. In the first direct procedure we trace the centers of the cyclone for each level for each day and determine its longitude. In the second indirect procedure we compute the values and signs of the product of the perturbation of temperature and meridional velocity over the computational domain. The energetics of the depression is studied and the partition of available potential energy and kinetic energy into zonal and eddy component is adopted. The energy contents and their changes in different atmospheric levels are discussed in the course of the cyclone’s development. Received April 27, 2001; Revised December 12, 2001     

Electrical aerosol spectrometer of Tartu University

The electrical aerosol spectrometer (EAS) of the parallel measuring principle at Tartu University is an efficient instrument for rapid measurement of the unstable size spectrum of aerosol particles. The measuring range from 10 nm to 10 μm is achieved by simultaneously using a pair of differential mobility analyzers with two different particle chargers. The particle spectrum is calculated and measurement errors are estimated in real time by using a least-squares method. Experimental calibration ensures reliability of measurement. The instrument is well suited for continuous monitoring of atmospheric aerosol.     

Advective, orographic and radiation fog in the Tarapacá region, Chile

A project in northern Chile was undertaken to determine the origin and behaviour of fog in the coastal and inland locations of the Tarapacá Region. In the Pampa del Tamarugal, 50 km from the sea, conditions exist for the formation of radiation fog. Advective fog has been studied on the coast and orographic fog was observed at a few coastal sites near mountain ranges with elevations above 1000 m. Fog water collected by two standard fog collectors (SFC) for 3 1/2 years showed an average flux of 8.5 l m⁻² day⁻¹ on the coast and 1.1 l m⁻² day⁻¹ inland 12 km from the coastline. On only a few days in 10 months was water collected at the inland site of Pampa del Tamarugal. GOES satellite images are shown to illustrate the pattern of formation of the stratocumuli cloud over the sea, its approach to the coastline, the entrance of fog by corridors through the coastal range and the presence of radiation fog inland. The results are important for the understanding of fog formation and dissipation along the coastal mountain range and for the recognition of potential sites for the installation of fog water collectors, which can be used as a water source in the Atacama Desert. The results also provide vital information for use in the preservation of the unique ecosystems of the most arid desert of the world.