Estimating surface visibility at Hong Kong from ground-based LIDAR, sun photometer and operational MODIS products

Figures & data

Figure 1. Locations of the visibility monitoring stations in Hong Kong (HKO and HKPU are 0.5 km apart).

Table 1.  Summary statistics for hourly averages of visibility, LIDAR AOD, AERONET AOD, extinction coefficient, and ML heights presenting sample size (N), mean, median, standard deviation (SD), and maximum/minimum (Max/Min)

Parameter	N	Mean	Median	SD	Max	Min
V HKPU at 875 nm (km)	489	21.77	20.82	10.41	46.17	3.965
V HKIA at 875 nm (km)	492	24.28	25.79	11.39	44.03	4.28
at 550 nm	183	0.28	0.23	0.24	1.41	0.01
at 550 nm	492	0.39	0.28	0.30	1.80	0.04
at 335 nm (km^−1)	181	0.22	0.19	0.16	0.72	0.008
Z H (km)	216	1.56	1.55	0.72	2.82	0.36
Z L (km)	216	0.97	0.76	0.64	2.51	0.23
Notes: Z H = ML-High; Z L = ML-Low.

Figure 2. Frequency distribution hourly averages of (a) ML-Low, (b) ML-High, (c) AOD from AERONET, (d) AOD from ALS, (e) visibility at HKIA, (f) visibility at HKPU, and (g) extinction coefficient from ALS for height between 75 and 150 m.

Figure 3. Relationship between visibility at HKPU derived from visibility meter at the 875 nm wavelength and (a) AOD from AERONET, (b) AOD from ALS, (c) ML-Low, and (d) extinction coefficient from ALS for height between 75 and 150 m at HKPU. Here R is correlation and N is the number of data points.

Table 2.  Proposed models for estimating extinction coefficient of MODIS at HKIA (σ_MA) and V_HKIA

Model 1
Model 2
Model 3
Model 4 (a and b)
Model 5
Model 6
Model 7
Notes: = MODIS extinction coefficient of at HKIA
= MODIS AOD at HKIA
= MODIS AOD at HKPU
= AERONET AOD at HKPU
= LIDAR AOD at HKPU
= Low- and High-ML heights at HKPU
= VR from visibility meter at HKPU
= VR from visibility meter at HKIA.

Figure 4. Nonlinear regression fit for scaled extinction coefficient and visibility at HKPU. Estimated values of the regression coefficients a and b and N are reported along with root mean square error (RMSE), mean absolute deviation (MAD), and coefficient of determination (R ²) for 165 measured values of visibility at HKPU and scaled surface extinction coefficient.

Figure 5. MODIS derived modeled visibility at HKIA from the proposed models listed in Table 1 and actual visibility (histogram bars) from visibility meter at HKIA. In addition to MODIS AOD, Model 1–4 uses AOD, ML heights, as well as extinction coefficients from ground-based instruments at HKPU, whereas Models 5 and 6 use visibility readings from visibility meter at HKPU. Error bars are ±20% of the visibility at HKIA from the manual of visibility meter.

Figure 6. Scatter plot of V _HKIA and MODIS derived visibility at HKIA for each proposed model (Table 1). Dashed line displays the 1:1 line and dashed rectangle encompasses the data for 26 May 2011 and 31 May 2011. R ², RMSE, and MAD are described in Figure 4.

Table 3.  Comparison of model's performances before and after removal of data for 26 May 2011 and 31 May 2011

	Before Removal				After Removal
Model	R ^2	RMSE	R	MAD	R ^2	RMSE	R	MAD
Model 1	0.46	11.24	−0.68	8.91	0.28	11.45	−0.53	8.80
Model 2	0.09	5.78	0.30	4.08	0.49	3.35	0.70	2.65
Model 3	0.17	4.75	0.41	3.59	0.44	2.96	0.66	2.48
Model 4a	0.00	13.05	−0.07	12.12	0.22	13.29	0.46	12.25
Model 4b	0.03	10.50	−0.17	9.16	0.05	9.85	0.22	8.40
Model 5	0.42	4.63	0.65	3.79	0.59	4.77	0.77	3.95
Model 6	0.48	5.22	0.69	4.47	0.49	5.61	0.63	5.03
Model 7	0.29	4.73	0.54	4.06	0.59	4.67	0.77	3.89
Note: Italicized values represent those of the best model.

Figure 7. Scatter plot of V _HKIA and MODIS derived visibility at HKIA for Models 4–6 with extended validation data sets. Dashed line displays the 1:1 line. R ², RMSE, and MAD are described in Figure 4 and N is the available number of data points for validation.

Figure 8. Pattern statistics (Taylor diagram; Taylor, 2001) describing the visibilities from the six models compared with observed visibilities at HKIA. The radial distance from the origin at “0.0” represents the normalized standard deviation. “Obs” represents the statistics of observed visibilities at HKIA. RMS difference for the modeled visibilities is proportional to the radial distances from origin at “Obs” (units same as normalized standard deviation). Normalized Pearson's correlation between observed and modeled visibilities is represented along the azimuthal position along the outer hemisphere. Color bar scales the normalized bias (%) in each model (color figure available online).

Taylor , E.K. 2001 . Summarizing multiple aspects of model performance in a single diagram . J. Geophys. Res. , 106 : 7183 – 7192 . doi: 10.1029/2000JD900719

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