California wildfire smoke contributes to a positive atmospheric temperature anomaly over the western United States

oleh: J. L. Gomez, R. J. Allen, K.-F. Li

Format: Article
Diterbitkan: Copernicus Publications 2024-06-01

Deskripsi

<p>Wildfires in the southwestern United States, particularly in northern California (nCA), have grown in size and severity in the past decade. As they have grown larger, they have been associated with large emissions of absorbing aerosols and heat into the troposphere. Utilizing satellite observations from MODIS, CERES, and AIRS as well as reanalysis from MERRA-2, the meteorology associated with fires during the wildfire season (June–October) was discerned over the nCA-NV (northern California and Nevada) region during the period 2003–2022. Wildfires in the region have a higher probability of occurring on days of positive temperature (<span class="inline-formula"><i>T</i></span>) anomalies and negative relative humidity (RH) anomalies, making it difficult to discern the radiative effects of aerosols that are concurrent with fires. To attempt to better isolate the effects of large fire emissions on meteorological variables, such as clouds and precipitation, variable anomalies on high fire emission days (90th percentile) were compared with low fire emission days (10th percentile) and were further stratified based on whether surface relative humidity (RH<span class="inline-formula"><sub>s</sub></span>) was anomalously high (75th percentile) or low (25th percentile) compared with typical fire season conditions. Comparing the simultaneously high fire emission and high RH<span class="inline-formula"><sub>s</sub></span> data with the simultaneously low fire emission and high RH<span class="inline-formula"><sub>s</sub></span> data, positive tropospheric <span class="inline-formula"><i>T</i></span> anomalies were found to be concurrent with positive AOD anomalies. Further investigation found that due to shortwave absorption, the aerosols heat the atmosphere at a rate of 0.041 <span class="inline-formula">±</span> 0.016 to 0.093 <span class="inline-formula">±</span> 0.019 K d<span class="inline-formula"><sup>−1</sup></span>, depending on whether RH conditions are anomalously positive or negative. The positive <span class="inline-formula"><i>T</i></span> anomalies were associated with significant negative 850–300 hPa RH anomalies during both 75th percentile RH<span class="inline-formula"><sub>s</sub></span> conditions. Furthermore, high fire emission days under high RH<span class="inline-formula"><sub>s</sub></span> conditions are associated with negative CF anomalies that are concurrent with the negative RH anomalies. This negative CF anomaly is associated with a significantly negative regional precipitation anomaly and a positive net top-of-atmosphere radiative flux anomaly (a warming effect) in certain areas. The <span class="inline-formula"><i>T</i></span>, RH, and CF anomalies under the simultaneously high fire emission and high RH<span class="inline-formula"><sub>s</sub></span> conditions compared with the simultaneously low fire emission and high RH<span class="inline-formula"><sub>s</sub></span> conditions have a significant spatial correlation with AOD anomalies. Additionally, the vertical profile of these variables under the same stratification is consistent with positive black carbon mass mixing ratio anomalies from MERRA-2. However, causality is difficult to discern, and further study is warranted to determine to what extent the aerosols are contributing to these anomalies.</p>