| 1. |
- Bian, Jianpu, et al.
(författare)
-
Mechanisms for African easterly wave changes in simulations of the mid-Holocene
- 2023
-
Ingår i: Climate Dynamics. - : Springer Science and Business Media LLC. - 0930-7575 .- 1432-0894. ; 61:7-8, s. 3165-3178
-
Tidskriftsartikel (refereegranskat)abstract
- The mid-Holocene was a warm period with significantly amplified precipitation in North Africa, and a northward shifted Western African Monsoon during boreal summer. We conduct simulations for the pre-industrial and mid-Holocene periods to investigate the connection between summer rainfall variability and changes of African easterly waves (AEWs) during the mid-Holocene. Summer rainfall increases and migrates northward during the mid-Holocene, but the magnitude of change fails to reconcile the discrepancy with mid-Holocene proxy evidence, possibly due to no prescribed vegetation change in our simulations. The spectrum of summer rainfall over the Sahel and West Africa reveals enhanced synoptic time scale (3-to-6 days) variability during the mid-Holocene, which is consistent with the enhanced AEW activity influence. Specifically, the southern AEW track strengthens and migrates poleward during the mid-Holocene period, which modulates summer rainfall over the Sahel and West Africa. By comparison, the northern AEW track changes less and produces a minor contribution to rainfall changes in those regions. We find enhanced baroclinic and barotropic instabilities to promote the AEW activity during the mid-Holocene, with a doubling of the eddy kinetic energy of the meridional wind from that in PI, and baroclinic energy conversion plays a more important role. Stronger low-level meridional thermal gradients increase moisture flux from the Atlantic Ocean to inland.The amplified AEW activity, together with promoted moist convection and increased precipitation, results in a northern shift of the summer rainfall band during the mid-Holocene.
|
|
| 2. |
- Lewinschal, Anna, 1983-
(författare)
-
Interactions between aerosols and large-scale circulation systems in the atmosphere
- 2013
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Anthropogenic aerosol emissions have increased during the last century. The higher atmospheric aerosol burden is believed to partly have masked the enhanced greenhouse gas warming during the same period. However, the many different types of aerosols, and the uncertainties regarding their effect on clouds, makes it difficult to estimate their total climate impact. With their strong effect on atmospheric radiation and their varying spatial and temporal distribution, aerosols may also affect the atmospheric circulation. This thesis focuses on aspects of aerosol-induced circulation changes as represented in general circulations models.Anthropogenic aerosol forcing is believed to generally cool the earth system, but model simulations show that the strongest cooling is not necessarily co-located with the strongest aerosol radiative forcing. It is shown that aerosol forcing can cause anomalies in the stationary wave pattern, which affects surface temperatures far from the region of aerosol forcing. In absence of a substantial global mean aerosol-induced cooling, the anomalous stationary wave pattern has a large influence on the simulated temperature-response pattern. The waves are primarily generated by aerosol-induced precipitation changes in the tropics, showing an important connection between aerosol emissions at low latitudes and surface temperate changes in the extra-tropics.It is also demonstrated that the aerosol climate response differs depending on how the ocean surface is represented in a model, i.e. if a sea surface temperature response is permitted or not. The anthropogenic aerosol forcing generates a stronger cooling of the northern hemisphere when the sea surface temperatures can change compared to when they are fixed. The stronger inter-hemispheric temperature gradient affects both the tropical and extra-tropical zonal mean circulation. Thus, aerosol-induced circulation changes are dependent on the simulated surface temperature response.
|
|
| 3. |
|
|
| 4. |
- Welling, Irma, et al.
(författare)
-
Contaminant dispersion in the vicinity of a worker in a uniform velocity field
- 2000
-
Ingår i: Annals of Occupational Hygiene. - 0003-4878 .- 1475-3162. ; 44:3, s. 219-225
-
Tidskriftsartikel (refereegranskat)abstract
- The transportation of gaseous contaminant from a low and moderate low impulse (momentum<1 m s-1) source to the breathing zone was studied in a uniform air stream flow. Results of the effects of the direction and the velocity of principal air flow, convection due to a human body, arm movement of a human being and the type of source on the concentration profiles are presented. Three important results were obtained. Firstly, for a given low and moderate impulse low impulse contaminant source in the near field of a worker, his/her orientation relative to the principal air flow direction is the most important factor in reducing occupational exposure, with an air velocity of about 0.3 m s-1. Secondly, the effect of convection resulting from body heat on air flow was lower than expected. Thirdly, arm movements influence contaminant dispersion, and should be included when models assessing exposure are developed. The present data can also be used to validate existing computational fluid dynamic (CFD) models.
|
|
| 5. |
|
|
