Observed Variability of Cloud Frequency and Cloud-Base Height within 3600 m above the Surface over the Contiguous United States

Abstract

The geographic and temporal variability of the surface–3600-m cloud frequency and cloud-base height over the contiguous United States for a 5-yr period (2008–12) and the interannual variations for a 16-yr period (2000–15) are described using information from the Automated Surface Observing System (ASOS) observations. Clouds were separated into four categories by the cloud amount reported by ASOS: few (FEW), scattered (SCT), broken (BKN), and overcast (OVC). The geographic distributions and seasonal and diurnal cycles of the four categories of surface–3600-m cloud frequency have different patterns. Cloud frequency of FEW, SCT, and BKN peaks just after noon, whereas the frequency of OVC peaks in the early morning. However, the geographic distributions and seasonal and diurnal cycles of the four categories of the surface–3600-m cloud-base height are similar. The diurnal cycles of the cloud-base height within the surface–3600-m level present a minimum in the morning and peak in the late afternoon or early evening. Cloud frequency and cloud-base height within this range are closely related to surface air temperature and humidity conditions. From 2000 to 2015, the cloud frequency in the contiguous United States showed a positive trend of 0.28% yr−1 while the cloud-base height showed a negative trend of −4 m yr−1 for the surface–3600-m level, accompanied with a positive trend of precipitation days (0.14 days yr−1). Moreover, the increase of cloud frequency and the decrease of cloud-base height were most obvious in winter in the eastern half of the contiguous United States.

Determining Factors of Monthly to Decadal Variability in Surface Solar Radiation in China: Evidences From Current Reanalyses

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Evaluation of cloud base height in the North American Regional Reanalysis using ceilometer observations

“…The colour bar indicates the scatter density, which is defined as the number of data dots in 100 × 100 axis grids. The solid lines are the best fit lines calculated using least squares regression and fog of the coast (see figure 5 of An et al, 2017). The weak correlation between monthly mean CBH simulated by NARR and observed by ASOS in the West Coast region is primarily caused by the decoupling of clouds and surface conditions.…”

“…CBHs during precipitation are included because the ceilometer and micro-pulse lidar provide relatively accurate CBHs under large but sparse precipitation conditions (Clothiaux et al, 2000). When the intensity of precipitation is high enough that it obscures the CBH signal, the ASOS reports a vertical visibility (VV) calculated from the attenuation coefficient (Vaisala, 1989), and the proportion of VV cases is very low (less than 1%) over the CONUS (An et al, 2017). Besides, the precipitation identifications in the ASOS and NARR are different.…”

Biases in the Mesoscale Prediction of Ceiling and Visibility in Alaska and Their Reduction Using Quantile Matching

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