Estimation of Regional Evapotranspiration by Combining Aircraftand Ground-Based Measurements

Two methods are examined for combining measurements from instrumented aircraftand towers to estimate regional scale evapotranspiration. Aircraft data provided spatially averaged values of properties of the surface, the evaporative fraction and maximum stomatal conductance. These quantities are less sensitive to meteorological conditions than the turbulent fluxes of heat and water vapour themselves. The methods allowed aircraft data collected over several days to be averaged and thus to reduce the random error associated with the temporal under-sampling inherent in aircraft measurements. Evaporative fraction is estimated directly from the aircraft data, while maximum stomatal conductance is estimated by coupling the Penman–Monteith equation to a simple model relating surface conductance to the incoming shortwave radiation and specific humidity saturation deficit. The spatial averages of evaporative fraction and maximum stomatal conductance can then be used with routine tower data to estimate the regional scale evapotranspiration. Data from aircraft flights and six ground based sites during the OASIS field campaign in south–east New South Wales in 1995 have been used to check the methods. Both the evaporative fraction and the maximum stomatal conductance derived from the aircraft data give information on the spatial variability of the surface energy budget at scales from 10 to 100 km. Daily averaged latent heat fluxes estimated using these methods for the OASIS study region agree with the available observations in quasi-stationary conditions or in weakly non-stationary conditions when the data from several aircraft flights are averaged to reduce the impact of short term imbalances in the surface energy budget.     

Suburban-rural energy balance comparisons for Vancouver,B.C.: An extreme case?

Synchronous observations of the energy balances of a suburban and a rural area in the Vancouver region are used to investigate the impact of urbanization on energy exchange. Net radiation and rural soil heat flux density were directly measured, suburban heat storage was parameterized, and the turbulent heat flux densities were evaluated using the Bowen ratio-energy balance method. Most comparisons were conducted during a period of drying following an unusually wet early summer. These conditions produced atypical but very interesting results. With cloudless skies and high radiant input, suburban-rural differences of both net radiation and evapotranspiration were contrary to previous results and intuition. In both cases, suburban values were greater than their rural counterparts. In most respects the rural site behaved as expected, and the explanation for these findings is thought to be related to advective assistance of evapotranspiration from the suburban area. Under lesser radiant forcing the suburban budget acted in greater conformity with past experience and suburban-rural differences were similarly more in agreement with expectation.     

First Results of Measurements with a Newly-Designed Phased-Array Sodar with RASS

Summary At a special measuring site for boundary-layer studies as well as land-surface processes the Meteorological Observatory Lindenberg of the German Weather Service (DWD) has recently put into operation a newly-desi gned phased-array SODAR/RASS, which has been developed by METEK on behalf of the DWD. This system provides the vertical profiles of the three-di mensional wind vector in the boundary layer on an operational basis and is furthermore suitable for getting information on the profile of virtual temperature up to about 400 m in height based on the addition of RASS components. The following paper describes both the technique of this SODAR/RASS and the various modes of operation as well as the different options in managing the system. First evaluations on the data availability concerning the maximum height coverage will give an impression on the system’s capabilities. Finally, the accuracy of the derived profiles of winds and temperature will be investigated by means of comparisons of the SODAR/RASS data with measurements of a six-sonde tethered-balloon system as well as meteorological data of a 99 m tower in the vicinity of the system. Received November 27, 1998 Revised April 9, 1999     

Sensitivity of the Asian summer monsoon to an anomalous Eurasian snow cover within the Météo-France GCM

Both observational and numerical studies suggest that the Eurasian winter snow cover has a strong influence on the subsequent summer monsoon in Asia. An updated version of the ARPEGE climate model of Météo-France, including a simple but physically-based snow parameterization, is used to test the impact of an increased snow mass prescribed at the beginning of March on the simulated summer monsoon circulation and rainfall. The large-scale features of the Asian monsoon are reproduced in a realistic way in the control integration, which is a necessary premise of such a sensitivity test. In the heavy snow cover experiment, the anomalous persistence of the winter snow pack delays the springtime continental heating. This weakens the thermal low over northern India and Persia as well as the southwesterly winds over the monsoon area. There is also a significant decrease in the rainfall over western India and Bengal-Burma, which usually represent the centers of maximum precipitation. Radiative, turbulence transfer and hydrological processes seem to be involved in the snow-monsoon relationship. The changes in the monsoon precipitation are strongly related to changes in the atmospheric circulation and are not reinforced by a local evaporation/convection feedback in our experiment. Received: 17 May 1995 / Accepted: 27 November 1995     

Fluctuations of the relationship between ENSO and northeast Australian rainfall

It is well known that during an El Niño-Southern Oscillation (ENSO) warm event, drought occurs in regions of northeastern (NE) Australia, leading to anomalously low annual rainfall. The present study explores fluctuations of this ENSO-rainfall relationship. It is found that the relationship tends to weaken when the linearly detrended global mean temperature is rising or particularly high, as in the period of 1931–45 period and since the late 1970s. Prior to a weakening, a correlation pattern of increased rainfall during El Niño events is seen first in northwestern Australia, then in eastern and southeastern Australia, and eventually in NE Australia. The 1931–45 period was particularly intriguing, when in terms of rainfall variability over NE Australia, the interannual ENSO-rainfall relationship went through a process of weakening, reversal, and rapid recovery. Features associated with the reversal are therefore examined and these features are: (1) the global background anomaly pattern (upon which internnal ENSO events operate) is ENSO-like; (2) ENSO sea surface temperature (SST) anomalies in tropical Pacific are weaker compared with those averaged over all ENSO events, whereas SST anomalies in the mid- to-high latitude Pacific (which have opposing polarity to those in tropical Pacific) are larger; (3) there is strong coherence between ENSO and variability in northern mid- to high-latitudes; and (4) the relationship that an El Niño event contributes to a warming anomaly of global mean SST weakens. Possible interrelationship among these features are discussed.     

The Numerical Simulation on Atmospheric Transport and Dispersion of the Spray Atomized from Flood Discharging by Hydropower Station over Complex Terrain

Summary  A three-dimensional, nonhydrostatic numerical model with high spatial resolution, in which a simple energy closure scheme is employed, has been developed to simulate the spray dispersion over complex terrain. The evaporation, condensation, and dispersion of the spray and moisture are taken into account in model equations. The term of latent heat due to phase transformation is considered in detail to account for its effects on the temperature field and airflow. As an application of the model, the spray concentration and air relative humidity are calculated under neutral condition. The results indicate that under the neutral condition, the spray is transported to about 0.6 km downwind from the source, and its effects on the air humidity reach a further distance of 0.9 km downwind from the source. Attention is given to the dependence of the results upon the various factors influencing the simulation, such as the intensity of the source, the atmospheric stratification, and the dynamic factor of the terrain. Some numerical tests were carried out to provide extra insight to the effects of these factors. It has been demonstrated that the simulation results such as relative humidity and temperature are sensitive to these factors, especially to the thermal stratification. Under unstable conditions, the effects of the spray source increase significantly, and the variation extent of the temperature, relative humidity and flow field is larger than that under neutral condition. The effects of dynamic and thermal factors on the air flow field are discussed through the comparison of the modeling results over complex terrain and flat terrain. Received June 8, 1998 Revised April 17, 1999     

A note on estimating the effect of a limited fetch on micrometeorological evaporation measurements

A formula for the effective fetch of micrometeorological evaporation measurements is derived by application of diffusion theory, using Calder's approximation of a uniform wind field and neutral atmospheric stability. This simplification allows estimation of the likely sampling error, which would result from an upwind step-change in evaporation rate, without change in roughness.     

Accurate Derivation of Total and Stratospheric Vertical Columns of NO2 from Ground-Based Zenith-Sky Measurements

A new method for retrieving the vertical profile of NO2 from ground-based measurements is applied to four months of measurements made at Aberdeen (57°N) during part of SESAME from November 1994 to April 1995. The retrieval method is shown to be an invaluable tool both for deriving the true NO2 vertical column and for removing the tropospheric contribution to the vertical column. This dramatically reduces the effects of tropospheric pollution in the observations and enables a more appropriate comparison with stratospheric 3-D model results. The comparison confirms the accuracy of the model's transport and its reactive nitrogen photochemistry, although there are some detailed discrepancies.