A simple coupled model of the wind-evaporation-SST feedback with a role for stability

The wind-evaporation-SST (WES) feedback describes a coupled mechanism by which an anomalous meridional sea surface temperature (SST) gradient in the tropics evolves over time. As commonly posed, the (positive) WES feedback depends critically on the atmospheric response to SST anomalies being governed by a process akin to that argued by Lindzen and Nigam (1987), and omits an alternative process by which SST anomalies modulate surface wind speed through vertical momentum mixing as proposed by Wallace et al. (1989) and Hayes et al. (1989). A simple model is developed that captures the essential coupled dynamics of the WES feedback as commonly posed, while also allowing for momentum entrainment in response to evolving SST anomalies. The evolution of the coupled system depends strongly on which effects are enabled in the model. When both effects are accounted for in idealized cases near the equator, the initial anomalous meridional SST gradient grows over a time scale of a few months, but is damped within one year. The sign and magnitude of the WES feedback depend on latitude within the tropics and exhibit hemispheric asymmetry. When constrained by realistic profiles of prevailing zonal wind, the model predicts that the WES feedback near the equator is stronger during boreal winter, while the domain over which it is positive is broader during boreal summer, and that low-frequency climate variability can also modulate the strength and structure of the WES feedback. These insights may aid in the interpretation of coupled climate behavior in observations and more complex models.

On the association between aerosol optical depths and surface meteorological conditions in a tropical coastal environment

Columnar aerosol spectral optical depth data, estimated using a ground based passive multi-wavelength solar radiometer at the tropical coastal station of Thumba, Thiruvananthapuram (Trivandrum) (8.55°N, 77°E) during the period November 1985 to May 1991, are examined to study the association of the seasonal variations in the optical depths and their association with the prevailing meteorological conditions. A systematic seasonal variation has been observed, with the optical depths maximising in the summer/pre-monsoon season and reaching a minimum in the winter season. Significant association has been observed between the seasonal variations of aerosol spectral optical depths with those of the (on-shore) surface wind speed and the rainfall. The implications of the findings are discussed.

Dirurnal variability of heat fluxes and heat content at A few locations off central east coast of India During April 1989

Diurnal variability of surface wind speed, net heat exchange, sea surface temperature, vertical thermal structure and heat content at three locations, viz., station A (17° 59'N, 83° 53.9'E), station B (17° OO'N , 82° 32.1'E) and station C (16° 31..3' N, 82° 21..8'E) off central east coast of India is described making use of the data collected on board R.V. Gaveshani during April 1989 .

Relationship between the Formation of PM2.5 and Meteorological Factors in Northern China: The Periodic Characteristics of Wavelet Analysis

China’s rapid urbanisation and industrialisation have led to frequent haze in China in recent years. Although many measures to control haze have been implemented, no significant improvement has been observed, and haze still exists. In this study, we used wavelet transform to investigate the changes in PM2.5 on the time scale, the relationship amongst meteorological factors, and the causes and changes in haze formation and take measures to prevent haze. Results indicated the following: (1) The peak of PM2.5 changes in winter in the past three years primarily occurred in the range from 11:00 to 13:00 and 20:00 to 22:00. (2) Multiple cycles of daily average PM2.5 concentrations existed in 3–5 d, 6–14 d, 6–21 d, and 16–27 d, with a significant oscillation in 6–14 d and stable cycle characteristics. (3) The meteorological factors promoted the formation of haze to a certain extent. When haze occurred, the near-surface wind speed was only 1 m/s, which was not conducive to the spread of pollutants. (4) The formation of haze was affected by the interaction of various factors; the photochemical reactions of NO2 and O3 also exacerbated the formation of pollutants. This study provided a clear direction for the prevention and prediction of haze. Furthermore, the government must take relevant measures to reduce pollutant emissions and ensure the air quality of cities in winter.

Recent trends and variations in surface meteorological parameters over Indian Antarctic station Maitri

Utilizing surface meteorological data of temperature, pressure and wind of recent two decades, 1991-2010 over Indian Antarctic research station Maitri the trends and variabilities in surface meteorological parameters have been discussed. The trend in mean air temperature at Maitri has been -0.4/decade during past two decades showing no impact of global warming on Maitri surface temperature. There is no trend in surface pressure at Maitri during last two decades. Mean surface wind speed has shown slight increasing trend during the decade 1991-2000 whereas it has shown slight decreasing trend during the decade 2001-2010. However, there are large interannual variations in surface parameters and the linear trends seem to be the result of such variations. During Jan-March of two contrasting monsoon years 2009 and 2010 interesting anomalies in surface pressure at Maitri have been observed. Surface pressure was anomalously lower at Matri in Jan-March of drought year 2009 whereas positive pressure anomalies prevailed during the same period of excess monsoon year 2010. Negative temperature anomalies during January-March over Maitri preceded drought monsoon 2009 and positive anomalies prevailed during the same period before good monsoon 2010.

Application of Bias Correction to Improve WRF Ensemble Wind Speed Forecast

Surface wind speed forecast from an operational WRF Ensemble Prediction System (WEPS) was verified, and the system-bias representations of the WEPS were investigated. Results indicated that error characteristics of the ensemble 10-m wind speed forecast were diurnally variated and clustered with the usage of the planetary boundary layer (PBL) scheme. To correct the error characteristics of the ensemble wind speed forecast, three system-bias representations with decaying average algorithms were studied. One of the three system-bias representations is represented by the forecast error of the ensemble mean (BC01), and others are assembled from each PBC group (BC03) as well as an independent member (BC20). System bias was calculated daily and updated within a 5-month duration, and the verification was conducted in the last month, including 316 gauges around Taiwan. Results show that the mean of the calibrated ensemble (BC03) was significantly improved as the calibrated ensemble (BC20), but both demonstrated insufficient ensemble spread. However, the calibrated ensemble, BC01, with the best dispersion relation could be extracted as a more valuable deterministic forecast via the probability matched mean method (PMM).

Unusual long & short spell of fog conditions over Delhi and northern plains of India during December-January, 2009-2010

Sheet of fog is a common scenario during December-January months, which not only occurs in northern India but also in Bangladesh and Pakistan. Maximum fog frequency is noticed in Delhi and adjoining areas. This also affected the maximum temperature during January 1969-88, it varied between 20.6-21.5° C and during 2005 it was 18.9° C and 19.03° C during 2010. Formation of fog and its sustainability mainly depends upon surface wind, relative humidity, minimum temperature and persistency. Maximum dense fog was 285, 190 and 176 hours in 1998-99, 2002-03 and in 2009-10 respectively. During the month of January 2010 there were 5 western disturbances which enhanced the moisture over northern India, surface wind speed remained around 5 kt or less. Inversion in lower levels and freezing level has also been taken into consideration. No significant relation was found between fog and freezing level. However, inversion layer played an important role in formation of fog. Visible satellite imageries have also been taken into consideration to monitor fog over vast area of northern India, Bangladesh and Pakistan.