the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 29 Feb 2024
The Paris Low-Level Jet During PANAME 2022 and its Impact on the Summertime Urban Heat Island
Abstract. The Low-Level Jet (LLJ) and the Urban Heat Island (UHI) are common nocturnal phenomena in the atmospheric boundary layer. While the canopy layer UHI has been studied extensively, interactions of the LLJ and the urban atmosphere in general (and the UHI in particular) have received less attention. In the framework of the PANAME initiative in the Paris region, continuous measurements of wind speed and vertical velocity profiles were recorded with two Doppler Wind Lidars (DWL) – for the first time allowing for a detailed investigation of the summertime LLJ characteristics in the region. Jets are detected for 70 % of the examined nights, often simultaneously at an urban and a suburban site highlighting the LLJ regional spatial extent. Emerging at around sunset, the mean LLJ duration is ∼10 h, the mean wind speed is ∼9 ms−1, and the average core height is 400 m above the city. For many jets, results show signatures in the temporal evolution that indicate the inertial oscillation mechanism plays a role in the jet development: a clockwise veering of the wind direction and a rapid acceleration followed by a slower deceleration. The LLJ core induces variance in the vertical velocity (σ2w) above the urban canopy layer. It is shown that σ2w is a powerful indicator for the air temperature spatial contrasts as UHI intensity decreases exponentially with increasing σ2w and strong values only occur when σ2w is very weak. This study demonstrates how DWL observations in cities provide valuable insights into near-surface processes relevant to human and environmental health.
- Preprint
(8674 KB) - Metadata XML
- BibTeX
- EndNote
Status: final response (author comments only)
-
RC1: 'Comment on egusphere-2024-520', Anonymous Referee #1, 27 Mar 2024
This manuscript describes a summertime period of nocturnal low-level jets (LLJs), observed with Doppler wind lidars, and the LLJ impact on the Urban Heat Island (UHI) intensity. The main finding of the study is that the LLJ vertical velocity variance observed at the urban site shows a clearer relation to the UHI intensity than the LLJ wind speed. The study is well-written and includes a very detailed description of methods, the observed LLJ characteristics and the conclusions. The abstract provides a clear and concise overview of the study. I have a few minor suggestions for improvement outlined below.
General comments
- The study concludes that the wind speed variance shows a clearer relation to the UHI intensity than the wind speed. I would like to make sure that the comparison is not impacted by the measurement location (near-surface wind speed at the rural site vs. wind speed variance at 238m agl at the urban site). Does the lowest-level wind speed derived from the urban site reveal the same trend (and is therefore more comparable to the variance)? Or, the other way around, are there near-surface wind speed variance measurements available from the rural site?
- Generally, the study provides a lot of details. It would be helpful for the reader to provide some guidance on how the details relate to the broader context. For example, in the methods section, start each sub-section with an introductory/ summary sentence, so that the reader can pay attention to the details of interest. (e.g. start Sect. 2.2 with something like “Two DWLs, one in the city and one in a suburban are, were used to obtain vertical profiles of horizontal wind speed and vertical wind speed variance.”
- The time periods used in this study are a bit confusing. It seems like the lidar analysis is based on the core period 15 June 2022 to 31 August 2022, but other time periods for validations are mentioned multiple times. Also, the PANAME initiative is not really introduced. Is the lidar study period part of this initiative or its Intensive Observation Period?
Specific comments
- The introduction is well-researched, but can be a little more concise and details not relevant to the study could be omitted.
- L. 160: Since the vertical velocity variance is highly sensitive to the averaging interval and sampling frequency, it might be worth pointing these out and giving an assessment about which parts of the turbulence spectrum are captured/ omitted with the used strategy.
- L. 210 "Note that the minimum below the core height may not be captured correctly by the observations because no information is available < 238m agl in the instrument’s blind zone.”: Are there near-surface wind measurements available at the urban site to close this gap? Especially, since QUALAIR-SU is referred to as a supersite.
- It seems like most of the LLJ statistics are based on the measurements from the urban lidar, and the suburban lidar serves only to show that the LLJs are a regional phenomenon. Maybe mention that at an early point in the paper, so that the reader does not expect a detailed analysis from the suburban lidar.
- L. 269 “Here we assume that the σ2w observations at the first range gate (238m agl) of the DWL at the urban site provides a representative proxy for vertical mixing in the nocturnal urban boundary layer.”: Is the aim getting a wind variance estimate as close as possible to the surface? I am a bit worried that the variance at a fixed height agl depends on the jet core height.
Technical corrections
- Please be more consistent with introducing and using abbreviations (IO, ABL, …)
- L. 65: access -> excess?
- L. 153: could you provide a reference for the hard target method?
Citation: https://doi.org/10.5194/egusphere-2024-520-RC1 - RC2: 'Comment on egusphere-2024-520', Anonymous Referee #2, 11 Apr 2024
Viewed
| HTML | XML | Total | BibTeX | EndNote | |
|---|---|---|---|---|---|
| 225 | 73 | 16 | 314 | 7 | 7 |
- HTML: 225
- PDF: 73
- XML: 16
- Total: 314
- BibTeX: 7
- EndNote: 7
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1
