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California heat waves: their spatial evolution, variation, and coastal modulation by low clouds


We examine the spatial and temporal evolution of heat waves through California and consider one of the key modulating factors of summertime coastal climate—coastal low cloudiness (CLC). Heat waves are defined relative to daytime maximum temperature (Tmax) anomalies after removing local seasonality and capture unseasonably warm events during May—September. California is home to several diverse climate regions and characteristics of extreme heat events are also variable throughout these regions. Heat wave events tend to be shorter, but more anomalously intense along the coast. Heat waves typically impact both coastal and inland regions, although there is more propensity towards coastally trapped events. Most heat waves with a strong impact across regions start at the coast, proceed inland, and weaken at the coast before letting up inland. Typically, the beginning of coastal heat waves are associated with a loss of CLC, followed by a strong rebound of CLC starting close to the peak in heat wave intensity. The degree to which an inland heat wave is expressed at the coast is associated with the presence of these low clouds. Inland heat waves that have very little expression at the coast tend to have CLC present and an elevated inversion base height compared with other heat waves.

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We are thankful for support from NOAA Coastal and Ocean Climate Applications (COCA) program grant NA15OAR4310114. This study supports the climate science education efforts of Climate Education Partners, a project funded by a National Science Foundation Grant #DUE-1239797. The study also contributes to DOI’s Southwest Climate Science Center activities and to NOAA’s California and Nevada Applications Program. NCEP Reanalysis data are provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their website at Radiosonde Database at raobs/.

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Correspondence to Rachel E. S. Clemesha.

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Fig. S1

As in Fig. 3 showing the May 2008 case study but showing maps (top) using HWI_raw. Also, CLC for north (N) and South (S) are shown in the regional time series. Supplementary material 5 (PDF 204 KB)

Fig. S2

1960 Case Study. 2nd strongest Central Valley heat wave event, but cloudy and protected from the heat wave at the South Coast. Traces show ZBASE (black), CLC (gray), and regional HWI (colors). Supplementary material 6 (PDF 103 KB)

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Clemesha, R.E.S., Guirguis, K., Gershunov, A. et al. California heat waves: their spatial evolution, variation, and coastal modulation by low clouds. Clim Dyn 50, 4285–4301 (2018).

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  • Marine layer
  • Heat waves
  • Coastal climate
  • Low clouds