Metropolitan-scale Transport and Dispersion from the New York World Trade Center Following September 11, 2001. Part I: An Evaluation of the CALMET Meteorological Model

Following the collapse of the New York City World Trade Center towers on September 11, 2001, Local, State and Federal agencies initiated numerous air monitoring activities to better understand the impact of emissions from the disaster. A study of the estimated pathway that a potential plume of emissions would likely track was completed to support the U.S. EPA’s initial exposure assessments. The plume from the World Trade Center was estimated using the CALMET-CALPUFF dispersion modeling system. The following is the first of two reports that compares several meteorological models, including the CALMET diagnostic model, the Advanced Regional Prediction System (ARPS) and 5^th Generation Mesoscale Model (MM5) in the complex marine-influenced urban setting of NYC. Results indicate wind speed, in most cases, is greater in CALMET than the two mesoscale models because the CALMET micrometeorological processor does not properly adjust the wind field for surface roughness variations that exits in a major built-up urban area. Small-scale circulations, which were resolved by the mesoscale models, were not well simulated by CALMET. Independent wind observations in Lower Manhattan suggest that the wind direction estimates of CALMET possess a high degree of error because of the urban influence. Wind speed is on average 1.5 ms⁻¹ stronger in CALMET than what observations indicate. The wind direction downwind of the city is rotated 25–34 clockwise in CALMET, relative to what observations indicate.     

Skills of different mesoscale models over Indian region during monsoon season: Forecast errors

Performance of four mesoscale models namely, the MM5, ETA, RSM and WRF, run at NCMRWF for short range weather forecasting has been examined during monsoon-2006. Evaluation is carried out based upon comparisons between observations and day-1 and day-3 forecasts of wind, temperature, specific humidity, geopotential height, rainfall, systematic errors, root mean square errors and specific events like the monsoon depressions.It is very difficult to address the question of which model performs best over the Indian region? An honest answer is ‘none’. Perhaps an ensemble approach would be the best. However, if we must make a final verdict, it can be stated that in general, (i) the WRF is able to produce best All India rainfall prediction compared to observations in the day-1 forecast and, the MM5 is able to produce best All India rainfall forecasts in day-3, but ETA and RSM are able to depict the best distribution of rainfall maxima along the west coast of India, (ii) the MM5 is able to produce least RMSE of wind and geopotential fields at most of the time, and (iii) the RSM is able to produce least errors in the day-1 forecasts of the tracks, while the ETA model produces least errors in the day-3 forecasts.     

Diurnal variation of precipitation over the Carolina Sandhills region

The Carolina Sandhills are known to have an area of maximum precipitation on its western boundary during the summer mainly due to differences in soil types. Statistical analysis was performed on summer precipitation data from automated weather stations in the Carolinas, along the Sandhills for the years 2001 to 2006. Statistically significant difference was observed between the day and night precipitation amounts. A case study also revealed the diurnal pattern of convective precipitation.     

Monsoon rainfall simulations with the Kuo and Betts-Miller schemes

Summary Two numerical experiments are performed using a nested grid regional model to study the performance of the Kuo and the Betts-Miller cumulus parameterization schemes in simulating the rainfall during an active monsoon period. Results indicate that the monsoon circulation features, such as the Somali jet and monsoon depression are better simulated with the Kuo scheme. With the Kuo scheme, predicted intensity and associated rainfall of the monsoon depression are in good agreement with the observations. Uncertainty in the adjustment parameters in the Betts-Miller scheme appears to have lead to the poor prediction of rainfall. Also, the Betts-Miller scheme showed considerable sensitivity to the convergence in the lower troposphere in the initial conditions over the Arabian Sea, leading to a prediction of a spurious intense tropical cyclone. This cyclone replaced the normal heat-low over the desert region. Rainfall distribution and its maximum along the west coast of India were predicted better with the Kuo scheme. Area-averaged convective heating rates indicated that the cumulus convection is deeper and more intense with the Kuo scheme. Also, area averaged evaporation rates far exceeded the rainfall rates with the Betts-Miller scheme while with the Kuo scheme these rates are in balance after the spinup period. Forecast erros in the zonally averaged specific humidities indicate that the model atmosphere is more humid with the Betts-Miller scheme.With 14 Figures     

Episodic absorption in the outflow of the old nova V603 Aquilae

We report on the time-dependent behaviour of ultraviolet spectral lines in Hubble Space Telescope Goddard High-Resolution Spectrograph data of the classical nova V603 Aql. In particular, episodic blueshifted absorption (extending to ∼−2500 km s⁻¹) is present, with a variability time-scale down to ∼1 min. The data provide a rare opportunity to study the rapid evolution of absorption structures that may be associated with accretion-disc winds in cataclysmic variables. At least three absorption events are recorded (at blueward velocities only) over ∼5 h, each lasting ∼10–15 min. The derived velocity, acceleration and optical depth properties provide an empirical picture of stochastically variable structures in the outflow, with no evidence for short-term (less than ∼1 h) cyclic or modulated behaviour in the overall absorption properties. In contrast, the emission components of the ultraviolet resonance lines are very stable in velocity and strength in this low-inclination system. On at least two occasions there is an intriguing short-term 'flare' in the ultraviolet continuum flux (of up to ∼40 per cent). Though there is no clear one-to-one relation in these data between the continuum fluctuations and the occurrence of the absorption events, the time-scales for the two variable phenomena are essentially the same. The irregular absorption episodes in the ultraviolet data of V603 Aql presently defy a clear physical interpretation. Their overall characteristics are discussed in the context of instabilities in radiation-pressure-driven disc winds.     

Uncertainty in the Specification of Surface Characteristics, Part ii: Hierarchy of Interaction-Explicit Statistical Analysis

The uncertainty in the specification of surface characteristics in soil-vegetation- atmosphere-transfer (SVAT) schemes within planetary boundary-layer (PBL) or mesoscale models is addressed. The hypothesis to be tested is whether the errors in the specification of the individual parameters are accumulative or whether they tend to balance each other in the overall sense for the system. A hierarchy of statistical applications is developed: classical one-at-a-time (OAT) approach, level 1; linear analysis of variance (ANOVA), level 1.5; fractional factorial (FF), or level 2; two-factor interaction (TFI) technique, or level 2.5; and a non-linear response surface methodology (RSM), or level 3. Using the First ISLSCP Field Experiment (FIFE) observations for June 6, 1987 as the initial condition for a SVAT scheme dynamically coupled to a PBL model, the interactions between uncertainty errors are analyzed. A secondary objective addresses the temporal changes in the uncertainty pattern using data for morning, afternoon, and evening conditions. It is found that the outcome from the level 1 OAT-like studies can be considered as the limiting uncertainty values for the majority of mesoscale cases. From the higher-level analyses, it is concluded that for most of the moderate surface scenarios, the effective uncertainty from the individual parameters is balanced and thus lowered. However, for the extreme cases, such as near wilting or saturation soil moisture, the uncertainties add up synergistically and these effects can be even greater than those from the outcomes of the OAT-like studies. Thus, parameter uncertainty cannot be simply related to its deviation alone, but is also dependent on other parameter settings. Also, from the temporal changes in the interaction pattern studies, it is found that, for the morning case soil texture is the important parameter, for afternoon vegetation parameters are crucial, while for the evening case soil moisture is capable of propagating maximum uncertainty in the SVAT processes. Finally, a generic hypothesis is presented that an appropriate question for analysis has to be rephrased from the previous 'which parameters are significant?’ to 'what scenarios make a particular parameter significant?’     

Solar X-Ray Spectral Irradiance Variability

Solar spectral irradiance at X-ray wavelengths show large variations over a period of solar cycle. We use X-ray irradiance data in three narrow spectral regimes deduced from Yohkoh SXT measurements to study coronal irradiance and their possible association with the activity in the lower atmosphere. Time variation of the X-ray irradiance is important in understanding the emergence of magnetic flux and the effects of such variation on the upper atmosphere of the Earth. We note that about 66% of the total (2 – 30 Å) X-ray irradiance arise from 10 to 20 Å spectral range, while 2 – 10 Å contribute only about 3% of the total. The time variation in 2 – 10 and 10 – 20 Å ranges follow each other closely. Further they follow closely the solar indices such as sunspot, F _10.7, and plage indices, although similarity in the variation of 10 – 20 Å is quite apparent. However, the variation in the other spectral band (20 – 30 Å) differ to a large extent except for the solar cycle dependent variation. We infer that in addition to the active regions, the remnants of active regions contribute considerably to the emission in this spectral range.     

On the reliability of C iv λ1549 as an abundance indicator for high-redshift star-forming galaxies

We reconsider the use of the equivalent width of C  iv  λ1549, EW(C  iv ), as an indicator of the oxygen abundance in star-forming galaxies, as proposed by Heckman et al. for nearby starbursts. We refine the local calibration of EW(C  iv ) versus log (O/H) by using a restricted wavelength window which minimizes blending with interstellar absorption lines. When applied to the stellar component only of the complex C  iv  λ1549 features in two high-redshift galaxies with good quality spectra,  MS 1512−cB58 ( z = 2.7268)  and  Q1307−BM1163 ( z = 1.4105)  , the local calibration gives values of the oxygen abundance which are in good agreement with other metallicity determinations based on nebular emission and interstellar absorption lines. Our main conclusion is that for this method to give reliable results at high redshifts, it should only be used on data of sufficiently high spectral resolution  ( R ≳ 1000)  for stellar and interstellar C  iv components to be clearly separated. Oxygen abundances will be systematically overestimated if the local calibration is applied to spectra of high-redshift galaxies obtained with the low resolving powers  ( R ≃ 200–300)  of many current wide-field surveys. It will also be necessary to understand better the causes of the scatter in the local relation, before we can be confident of inferences from it at high z .