Turbulent Characteristics of the Atmospheric Surface Layer in the Coastal Region of Qatar

Abstract

The atmospheric turbulence characteristics in the Persian Gulf are of great interest due to the strategic geographical location of the Gulf as a waterway of major oil transportation and its in-creasing regional economic and geopolitical importance. However, long-term and continuous turbulence observations in this region are rare. The present study explores the near-surface atmospheric boundary layer (ABL) characteristics in the coastal region of Qatar (26.08 N, 51.36 E). High-resolution micrometeorological data are collected and analyzed in this work, from August 2015 to September 2016, using sonic anemometers (20 Hz) at three heights on the top of a 9 m tower. The local climate, atmospheric stability, and turbulence mixing process in ABL are mainly discussed in this dissertation.

The results in this work show that there are two typical wind patterns in the Qatar peninsula, which are the shamal wind and the land-sea breeze. The shamal days (SD), non-shamal days (NSD), and sea breeze days (SBD) are identified and analyzed based on corresponding criteria. This study has identified that the summer shamal prevents the development of the sea breeze. The wind in the southern part of the gulf is dominant from the northwest with a diurnal average speed of 4.7 m/s. During the test year, the diurnal average temperature and relative humidity were 27◦C and 70%, respectively. The turbulence characteristics are studied within the framework of the Monin-Obukhov similarity theory (MOST) in the investigated coastal region. Two different atmospheric daily stability patterns, ‘orderly’ and ‘disheveled’, are identified based on the wind conditions. The orderly stability pattern shows a daily descending and ascending trend during the sunrise and sunset periods, respectively, while the disheveled days follow a random pattern with no clear order. The two patterns are then related to the wind continuity and direction relative to the shoreline.

The turbulence characteristics under shamal are analyzed and focus on the summer months (May, June, and July, as MJJ) 2016. Overall, both SD and NSD indicate a similar diurnal trend in that the turbulent dissipation rate, ε, is higher during the daytime than at nighttime, with an increasing trend in the morning before reaching its peak at noon in summer. On average, the TKE dissipation rate for SD is up to 13 times larger than that of NSD during MJJ. A lognormal distribution for PDFs of ϵ under NSD and a Weibull distribution is observed for PDFs of ϵ under SD in log scale for both stable and unstable conditions in summer. The combined distribution of ϵ for MJJ 2016 displays a bimodal pattern. The turbulent characteristics during winter SD are similar to those of summer SD. The temperature decreases faster during winter SD than NSD since the winter shamal usually comes after a cold front.

The investigation of sea breeze has shown that the ABL is mostly unstable during the sea breeze mainly because the sea breeze development is mostly during the daytime and partially because of the inhomogeneity between the land and sea. The ABL heights during SBDs are slightly lower than the annual value. Both the normalized vertical wind spectra and the integral length scales illustrate the eddy sizes decrease as approach the surface. The corresponding frequency of the spectra peaks of the sea breeze (unstable) is also much smaller than that of the land breeze (stable). The turbulence cascading process from larger eddies to the small eddies at the lower levels is also slower. The height of the convective internal boundary layer (CIBL) is estimated based on the vertical velocity spectra and the distribution of the TKE dissipation rate.

Additionally, the current and wave were measured off Al Ghariyah, Qatar from 18 August 2016 to 3 January 2017 to better understand the wind-wave correlation. The reaction of the current and wave to the different wind conditions, especially under the shamal winds is analyzed. Waves in the Persian Gulf are broadly studied and dominated by the northwesterly wind during the winter. However, the waves near the shore may show different features due to the sheltering effect, re-fraction, and fetch limitation. The waves in this measurement during the shamal days and most of the non-shamal days are from the northeast (45◦) and approach the more extensive shoreline. Refraction greatly impacts the wave height and direction that would cause them to deviate from the direction of the wind even on heavy shamal days. The ocean depth is less than half of the estimated wavelength which indicates the detected wave here is a primarily intermediate wave.