2015-04-20: Analysis of actinic flux profiles measured from an ozonesonde balloon

A green light sensor has been developed at KNMI to measure actinic flux profiles using an ozone sonde balloon.
A picture of the sensor is shown in Fig. 1. In total, 63 launches with ascending and descending profiles were performed between 2006 and 2010. The measured uncalibrated actinic flux profiles are analyzed using the Doubling Adding KNMI (DAK) radiative transfer model. Values of the cloud optical thickness (COT) along the flight track were taken from the Spinning Enhanced Visible and Infrared Imager (SEVIRI) Cloud Physical Properties (CPP) product.

The impact of clouds on the actinic flux profile is evaluated on the basis of the cloud modification factor (CMF) at the cloud top and cloud base, which is the ratio between the actinic fluxes for cloudy and clear-sky scenes. The impact of clouds on the actinic flux is clearly detected: the largest enhancement occurs at the cloud top due to multiple scattering. The actinic flux decreases almost linearly from cloud top to cloud base. Above the cloud top the actinic flux also increases compared to clear-sky scenes. We find that clouds can increase the actinic flux to 2.3 times of the clear-sky value at cloud top and decrease it to about 0.05 at cloud base. The relationship between CMF and COT agrees well with DAK simulations, except for a few outliers. Good agreement is found between the DAK simulated actinic flux profiles and the observations for single layer clouds in fully overcast scenes (see Fig. 2).

The instrument is suitable for operational balloon measurements because of its simplicity and low cost. It is worth for further developing the instrument and launch it together with atmospheric chemistry composition sensors.

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Figure 1. Photo of the light sensor: (a) data transmitter, (b) sphere, (c) green LED.
Figure 2. Cloudy-sky case on 5 September 2006. (a) SEVIRI cloud optical thickness image at 11:30 UTC (balloon launch) and (b) at 13:00 UTC (balloon maximum height). The ?ight track of balloon is indicated as a white line. The location of balloon at the time the SEVIRI image is taken is indicated as a white circle. (c) SEVIRI single-pixel cloud optical thickness values in 0.1°ree;×0.1°ree; (latitude × longitude) boxes along the trajectory of the balloon (dots) as a function of the height of the balloon. The COT values used in the simulations are connected by the black line. (d) Measured actinic ?ux pro?le and simulated actinic ?ux pro?le (both normalized at 30 km altitude, respectively). The actinic ?ux pro?le is also shown zoomed in at 0-10 km.